High Temperature, Oxygen, and Performance: Insights from Reptiles and Amphibians

High Temperature, Oxygen, and Performance: Insights from Reptiles and Amphibians Synopsis Much recent theoretical and empirical work has sought to describe the physiological mechanisms underlying thermal tolerance in animals. Leading hypotheses can be broadly divided into two categories that primarily differ in organizational scale: 1) high temperature directly reduces the function of subcellular machinery, such as enzymes and cell membranes, or 2) high temperature disrupts system-level interactions, such as mismatches in the supply and demand of oxygen, prior to having any direct negative effect on the subcellular machinery. Nonetheless, a general framework describing the contexts under which either subcellular component or organ system failure limits organisms at high temperatures remains elusive. With this commentary, we leverage decades of research on the physiology of ectothermic tetrapods (amphibians and non-avian reptiles) to address these hypotheses. Available data suggest both mechanisms are important. Thus, we expand previous work and propose the Hierarchical Mechanisms of Thermal Limitation (HMTL) hypothesis, which explains how subcellular and organ system failures interact to limit performance and set tolerance limits at high temperatures. We further integrate this framework with the thermal performance curve paradigm commonly used to predict the effects of thermal environments on performance and fitness. The HMTL framework appears to successfully explain diverse observations in reptiles and amphibians and makes numerous predictions that remain untested. We hope that this framework spurs further research in diverse taxa and facilitates mechanistic forecasts of biological responses to climate change. Introduction For most animals, the proximate mechanisms that underlie reduced performance and eventual death at high temperatures, and how such mechanisms might change with ontogeny and context, are uncertain (reviewed in Angilletta 2009; Clark et al. 2013; Pörtner et al. 2017). Such a mechanistic understanding would greatly enhance our ability to predict the effects of realistic high-temperature exposures on individuals or populations, which are difficult to forecast with traditional methods because responses can be highly variable (e.g., Gunderson and Stillman 2015; Seebacher et al. 2015; Kingsolver and Woods 2016; Sheldon and Dillon 2016; Williams et al. 2016). Even so, much progress has been made toward developing mechanistic models that use knowledge of individual physiology to predict the effects of thermal environments on populations, largely inspired by growing concern over the impacts of global climate change (Buckley 2008; Kearney and Porter 2009; Huey et al. 2012; Kearney 2012; Levy et al. 2015; Malishev et al. 2018). Commonly, such models employ empirically-derived thermal performance curves (TPC, Huey and Stevenson 1979, all terms and abbreviations are defined in Box 1) to predict the effects of thermal environments on organisms and populations (e.g. Colwell et al. 2008; Deutsch et al. 2008; Vasseur et al. 2014; Buckley and Huey 2016; Dillon et al. 2016). TPCs describe how performance varies with temperature— typically performance increases with temperature above a critical minimum (CTMIN) until an optimum is reached (TOPT), then rapidly drops to zero at the critical maximum (CTMAX, see Fig. 1 for examples). Pejus (getting worse) temperatures above and below TOPT are described by breakpoints in physiological function (e.g., ventilation rate, heart rate, PO2) indicative of rapid declines in whole-organism performance (TPEJUS, Frederich and Pörtner 2000; Pörtner 2002; Pörtner et al. 2017). Typically, TPCs are estimated under controlled laboratory conditions for a single trait and time, but TPC shape can vary with season, ontogeny, trait, and prior experience in ways that are difficult to predict (Rezende et al. 2014; Telemeco 2014; Kingsolver and Woods 2016; Williams et al. 2016). Thus, using empirically-derived TPCs to predict the effects of natural environments on performance is problematic because it requires extrapolating from the traits or environments originally used to estimate TPCs, thereby ignoring probable context-dependency. A mechanistic understanding of the processes that underlie thermal performance is needed to predict the effects of variable or novel environments on TPC shape, which will greatly improve models relying on TPCs to predict population responses. In particular, knowledge of the mechanisms that result in loss of function at high temperatures is needed to predict the rate at which performance will drop in response to thermal challenge, the capacity for animals to recover from sub-lethal thermal exposure, and the capacity for thermal tolerance to change via plasticity or evolution (Helmuth et al. 2005; Buckley and Huey 2016; Williams et al. 2016). Box 1 List of terms and abbreviations ATP  Adenosine triphosphate  active-aerobic TCRIT  Critical temperature where aerobic respiration is maximized for active individuals (i.e., V˙O2MAX is maximized, ˚C)  resting-aerobic TCRIT  Critical temperature where aerobic respiration would be maximized for resting individuals, assuming they survive to such high temperatures (i.e., V˙O2REST is maximized, ˚C)  subcellular TCRIT  Critical temperature for subcellular function (˚C)  CTMAX  Critical thermal maximum (˚C)  HMTL  Hierarchical Mechanisms of Thermal Limitation hypothesis  HSP  Heat shock protein  MPMO  Multiple Performances – Multiple Optima hypothesis (sensu Clark et al. 2013)  OCLTT  Oxygen- and Capacity-Limited Thermal Tolerance hypothesis (sensu Pörtner 2002 and Pörtner et al. 2017)  TGAPE  Gaping temperature: Temperature at which animals gape to promote evaporative cooling (˚C)  TLETHAL  Lethal temperature: Temperature at which an organism dies under acute exposure (˚C)  TOPT  Optimal temperature for performance (˚C)  TPANT  Panting temperature: Temperature at which animals pant to promote evaporative cooling (˚C)  TPEJUS  Pejus (i.e. getting worse) temperature (˚C, sensu Frederich and Pörtner 2000)  TPC  Thermal performance curve (sensu Huey and Stevenson 1979)  PBT  Preferred body temperature (˚C)  PCTMAX  Oxygen partial pressure below which CTMAX is reduced (kPa) (sensu Ern et al. 2016)  PO2  Partial pressure of oxygen (kPa)  PCO2  Partial pressure of carbon dioxide (kPa)  V˙O2  Oxygen consumption rate (generally mL O2 min−1)  ATP  Adenosine triphosphate  active-aerobic TCRIT  Critical temperature where aerobic respiration is maximized for active individuals (i.e., V˙O2MAX is maximized, ˚C)  resting-aerobic TCRIT  Critical temperature where aerobic respiration would be maximized for resting individuals, assuming they survive to such high temperatures (i.e., V˙O2REST is maximized, ˚C)  subcellular TCRIT  Critical temperature for subcellular function (˚C)  CTMAX  Critical thermal maximum (˚C)  HMTL  Hierarchical Mechanisms of Thermal Limitation hypothesis  HSP  Heat shock protein  MPMO  Multiple Performances – Multiple Optima hypothesis (sensu Clark et al. 2013)  OCLTT  Oxygen- and Capacity-Limited Thermal Tolerance hypothesis (sensu Pörtner 2002 and Pörtner et al. 2017)  TGAPE  Gaping temperature: Temperature at which animals gape to promote evaporative cooling (˚C)  TLETHAL  Lethal temperature: Temperature at which an organism dies under acute exposure (˚C)  TOPT  Optimal temperature for performance (˚C)  TPANT  Panting temperature: Temperature at which animals pant to promote evaporative cooling (˚C)  TPEJUS  Pejus (i.e. getting worse) temperature (˚C, sensu Frederich and Pörtner 2000)  TPC  Thermal performance curve (sensu Huey and Stevenson 1979)  PBT  Preferred body temperature (˚C)  PCTMAX  Oxygen partial pressure below which CTMAX is reduced (kPa) (sensu Ern et al. 2016)  PO2  Partial pressure of oxygen (kPa)  PCO2  Partial pressure of carbon dioxide (kPa)  V˙O2  Oxygen consumption rate (generally mL O2 min−1)  Box 1 List of terms and abbreviations ATP  Adenosine triphosphate  active-aerobic TCRIT  Critical temperature where aerobic respiration is maximized for active individuals (i.e., V˙O2MAX is maximized, ˚C)  resting-aerobic TCRIT  Critical temperature where aerobic respiration would be maximized for resting individuals, assuming they survive to such high temperatures (i.e., V˙O2REST is maximized, ˚C)  subcellular TCRIT  Critical temperature for subcellular function (˚C)  CTMAX  Critical thermal maximum (˚C)  HMTL  Hierarchical Mechanisms of Thermal Limitation hypothesis  HSP  Heat shock protein  MPMO  Multiple Performances – Multiple Optima hypothesis (sensu Clark et al. 2013)  OCLTT  Oxygen- and Capacity-Limited Thermal Tolerance hypothesis (sensu Pörtner 2002 and Pörtner et al. 2017)  TGAPE  Gaping temperature: Temperature at which animals gape to promote evaporative cooling (˚C)  TLETHAL  Lethal temperature: Temperature at which an organism dies under acute exposure (˚C)  TOPT  Optimal temperature for performance (˚C)  TPANT  Panting temperature: Temperature at which animals pant to promote evaporative cooling (˚C)  TPEJUS  Pejus (i.e. getting worse) temperature (˚C, sensu Frederich and Pörtner 2000)  TPC  Thermal performance curve (sensu Huey and Stevenson 1979)  PBT  Preferred body temperature (˚C)  PCTMAX  Oxygen partial pressure below which CTMAX is reduced (kPa) (sensu Ern et al. 2016)  PO2  Partial pressure of oxygen (kPa)  PCO2  Partial pressure of carbon dioxide (kPa)  V˙O2  Oxygen consumption rate (generally mL O2 min−1)  ATP  Adenosine triphosphate  active-aerobic TCRIT  Critical temperature where aerobic respiration is maximized for active individuals (i.e., V˙O2MAX is maximized, ˚C)  resting-aerobic TCRIT  Critical temperature where aerobic respiration would be maximized for resting individuals, assuming they survive to such high temperatures (i.e., V˙O2REST is maximized, ˚C)  subcellular TCRIT  Critical temperature for subcellular function (˚C)  CTMAX  Critical thermal maximum (˚C)  HMTL  Hierarchical Mechanisms of Thermal Limitation hypothesis  HSP  Heat shock protein  MPMO  Multiple Performances – Multiple Optima hypothesis (sensu Clark et al. 2013)  OCLTT  Oxygen- and Capacity-Limited Thermal Tolerance hypothesis (sensu Pörtner 2002 and Pörtner et al. 2017)  TGAPE  Gaping temperature: Temperature at which animals gape to promote evaporative cooling (˚C)  TLETHAL  Lethal temperature: Temperature at which an organism dies under acute exposure (˚C)  TOPT  Optimal temperature for performance (˚C)  TPANT  Panting temperature: Temperature at which animals pant to promote evaporative cooling (˚C)  TPEJUS  Pejus (i.e. getting worse) temperature (˚C, sensu Frederich and Pörtner 2000)  TPC  Thermal performance curve (sensu Huey and Stevenson 1979)  PBT  Preferred body temperature (˚C)  PCTMAX  Oxygen partial pressure below which CTMAX is reduced (kPa) (sensu Ern et al. 2016)  PO2  Partial pressure of oxygen (kPa)  PCO2  Partial pressure of carbon dioxide (kPa)  V˙O2  Oxygen consumption rate (generally mL O2 min−1)  Fig. 1 View largeDownload slide Schematic of the HMTL framework illustrating proposed relationships between body temperature, oxygen environment, metabolism, thermal limits, and performance. The top row of plots displays resting and active metabolic rates as a function of temperature within thermal and physiological limits, and the bottom row displays predicted thermal performance curves. Panel (A) displays these relationships in normoxic environments. Data are means ± s.e.m. for the snake, P. regius, from Fobian et al. (2014). These data were used to fit curves for resting and active metabolic rate, which were then used to estimate TOPT (maximal aerobic scope), resting-aerobic TCRIT, active-aerobic TCRIT, and thermal performance curve shape. The critical thermal maximum is 42˚C (Fobian et al. 2014). Panel (B) Moderate hypoxia (10–20 kPa) is predicted to have no effect on CTMAX or resting metabolism because resting aerobic TCRIT is above subcellular TCRIT. Even so, active-aerobic TCRIT will be reduced thereby lowering aerobic scope, maximal performance, and TOPT. Panel (C) Exposure to extreme hypoxia (< 10 kPa) will reduce CTMAX because resting-aerobic TCRIT drops below subcellular TCRIT. Active-aerobic TCRIT will be strongly affected by such hypoxic environments as well, with large reductions in aerobic scope, maximal performance, and TOPT. In Panels B) and C), gray lines illustrate values predicted for normoxia and are included to aid interpretation of changes predicted to result from reduced environmental oxygen availability. Similarly, arrows in Panels B) and C) indicate predicted displacement of maximum oxygen capacity, resting-aerobic TCRIT, TOPT, and CTMAX values given the oxygen environment. Subcellular TCRIT is not predicted to be affected by oxygen environment and therefore is never displaced. Fig. 1 View largeDownload slide Schematic of the HMTL framework illustrating proposed relationships between body temperature, oxygen environment, metabolism, thermal limits, and performance. The top row of plots displays resting and active metabolic rates as a function of temperature within thermal and physiological limits, and the bottom row displays predicted thermal performance curves. Panel (A) displays these relationships in normoxic environments. Data are means ± s.e.m. for the snake, P. regius, from Fobian et al. (2014). These data were used to fit curves for resting and active metabolic rate, which were then used to estimate TOPT (maximal aerobic scope), resting-aerobic TCRIT, active-aerobic TCRIT, and thermal performance curve shape. The critical thermal maximum is 42˚C (Fobian et al. 2014). Panel (B) Moderate hypoxia (10–20 kPa) is predicted to have no effect on CTMAX or resting metabolism because resting aerobic TCRIT is above subcellular TCRIT. Even so, active-aerobic TCRIT will be reduced thereby lowering aerobic scope, maximal performance, and TOPT. Panel (C) Exposure to extreme hypoxia (< 10 kPa) will reduce CTMAX because resting-aerobic TCRIT drops below subcellular TCRIT. Active-aerobic TCRIT will be strongly affected by such hypoxic environments as well, with large reductions in aerobic scope, maximal performance, and TOPT. In Panels B) and C), gray lines illustrate values predicted for normoxia and are included to aid interpretation of changes predicted to result from reduced environmental oxygen availability. Similarly, arrows in Panels B) and C) indicate predicted displacement of maximum oxygen capacity, resting-aerobic TCRIT, TOPT, and CTMAX values given the oxygen environment. Subcellular TCRIT is not predicted to be affected by oxygen environment and therefore is never displaced. Potential mechanisms explaining why animals lose function at high temperatures can be divided into two major categories differing in the level of organization first affected. At lower levels of organization, subcellular components could be critically impaired when animals are exposed to temperatures above the optimum. Subcellular impairment results from either denaturation of key molecules, such as proteins and cell membranes, or reduced efficiency of these molecules to perform their biological functions (reviewed in Hochachka and Somero 2002; Angilletta 2009; Schulte 2015). Impairment of subcellular components would result in the breakdown of higher levels of organization and lead to rapid performance loss. Alternatively, higher-levels of organization, such as organ systems, could be impaired at temperatures below those that directly affect the performance of their subcellular components if high temperatures disrupt subcellular interactions or pathways necessary for organ system function. A recent mechanistic model explaining organ-system impairment at high temperatures is the oxygen and capacity limited thermal tolerance (OCLTT) hypothesis, which proposes that oxygen demand for aerobic metabolism at high temperatures outpaces the ability of the cardiovascular and respiratory systems to provide sufficient amounts of oxygen to tissues (Pörtner 2002; Pörtner and Knust 2007; Verberk et al. 2016; Pörtner et al. 2017). Under this hypothesis, oxygen diffusion and transport capacity limit metabolic rates such that resting rates converge with potential maximum rates at high temperatures thereby reducing aerobic scope to zero (i.e., zero aerobic power budget sensu Pörtner et al. 2017). This mismatch in supply and demand will first reduce aerobic performance at TPEJUS but eventually result in basal oxygen demands not being met and thus collapse of organismal systems at CTMAX. Proponents of OCLTT argue that this mechanism integrates across levels of organization from systemic to molecular mechanisms and can explain diverse evolutionary and ecological phenomena (Pörtner 2002; Pörtner et al. 2017). However, the temporal window, activity range, magnitude, and biological relevancy of reduced aerobic scope at high temperature are debated (Clark et al. 2013; Gräns et al. 2014; Jutfelt et al. 2014; Pörtner 2014; Verberk et al. 2016; Jutfelt et al. 2018). Identifying the level of organization first impaired by high temperature is necessary to understand how temperature affects whole-organism function. Recent work exploring the mechanisms underlying thermal tolerance in ectotherms focuses on aquatic taxa (e.g., Frederich and Pörtner 2000; Lucassen et al. 2006; Verberk et al. 2013; Gräns et al. 2014; Ern et al. 2016; Pörtner and Gutt 2016; Verberk et al. 2018) and terrestrial arthropods, particularly insects (reviewed in McCue and De Los Santos 2013; Verberk et al. 2016). These observations suggest that subcellular-level mechanisms are more important determinants of thermal tolerance than oxygen capacity limitations in terrestrial taxa, perhaps because oxygen is more abundant in air than in water. However, recent work includes few studies of terrestrial vertebrate ectotherms: non-avian reptiles (hereafter, reptiles) and amphibians. Reptiles and amphibians display varied ecology, life-history, and thermal tolerance (Vitt and Caldwell 2009; Sunday et al. 2011) and are important for a broad understanding of thermal tolerance. Additionally, these animals frequently contend with challenging temperatures, with some species inhabiting the hottest terrestrial environments (Cowles and Bogert 1944; Vitt and Caldwell 2009; Sunday et al. 2014). Moreover, both the thermal and oxygen environment can vary throughout ontogeny (e.g., aquatic to terrestrial transition in amphibians), and modes of respiration can vary (e.g., cutaneous respiration in many amphibians and some reptiles, such as turtles; Hutchison et al. 1968; Glass et al. 1981; Wang 2011). Because of these traits, reptiles and amphibians have been important models for thermal physiology and ecology for the last century (e.g., Cowles and Bogert 1944; Snyder and Weathers 1975; Huey and Stevenson 1979; Huey 1982; Huey and Berrigan 2001; Vitt and Caldwell 2009; Kearney 2012). This historic work can be leveraged to address modern ideas, such as the OCLTT hypothesis. Furthermore, these taxa are at risk of high-temperature induced extinction resulting from global climate change (Thomas et al. 2004; Huey et al. 2010; Rohr and Raffel 2010; Sinervo et al. 2010; Kearney 2013). Here, we apply the rich history of physiological studies in reptiles and amphibians to modern hypotheses for mechanisms underlying thermal performance and tolerance. This will complete taxonomic coverage of recent reviews, and, more importantly, further understanding of how organismal performance and survival are restricted by high temperatures. In this commentary, we begin by synthesizing evidence for subcellular- and organ-system-level failures underlying high temperature tolerance and performance in reptiles and amphibians. Because neither framework satisfactorily explains observed patterns individually, we next build upon work in other taxa to present an integrative framework combining both subcellular and organ-system sensitivity to high temperatures and the thermal performance curve paradigm. Using data for reptiles and amphibians, we illustrate how this combined framework appears to predict a broad array of physiological and behavioral observations. We conclude by identifying future research directions to test this framework and discuss how it might be applied to other systems to predict consequences of future high-temperature exposure for organisms and populations. Evidence for mechanisms underlying thermal tolerance and performance in reptiles and amphibians Subcellular mechanisms High temperatures compromise the structure and function of subcellular components, such as enzymes and cell membranes (Fields 2001; Hochachka and Somero 2002) but such damage will only underlie organismal thermal tolerance if it occurs at temperatures below those affecting function at higher levels of organization, such as organ systems (Pörtner 2002). The complexities of enzymatic interactions make extending observations for individual reactions to the whole cell or organism potentially problematic (Schulte 2015). Nonetheless, the observation that subcellular components are frequently stable to temperatures above the critical and lethal limits of animals (hereafter, CTMAX and TLETHAL), commonly maintaining function to temperatures >50˚C and capable of evolving stability to 120˚C (Fields 2001), was an important motivation for the development of hypotheses such as OCLTT (Pörtner 2001; 2002; Pörtner et al. 2017). For example, CTMAX in reptiles and amphibians generally ranges from mid-30s to low 40s ˚C (Brattstrom 1965; Sunday et al. 2011, 2014), although a few warm-adapted reptiles tolerate acute exposure to 47.5˚C such as the desert iguana (Dipsosaurus dorsalis) (Cowles and Bogert 1944; Brattstrom 1965). Even so, most reptile and amphibian proteins do not lose function until they experience temperatures well above CTMAX. For example, ribonucleases from three frog species were stable up to 85˚C and maintained high activity at temperatures >50˚C (Irie et al. 1998), lactate dehydrogenase in Agama stellio lizards maintained both stability and function up to 70˚C (Al-Jassabi 2002), and alkaline phosphatase maintained high function up to 50˚C in four lizard species (Licht 1964). Similarly, acute exposure to CTMAX did not cause tissue damage or reduce function of serum glutamic-oxaloacetic or glutamic-pyruvic transaminases in Great Plains toads (Anaxyrus cognatus, Paulson and Hutchinson 1987) and 2.5-h exposure to temperatures just below CTMAX had no effect on mitochondrial respiration or free-radical production in alligator lizards (Elgaria coerulea and E. multicarinata; Telemeco et al. 2017), implying no subcellular damage. That said, only one key component needs to lose function for the entire organism to become compromised. For example, the activity of myosin ATPase, a key enzyme for organismal muscle function, closely resembled whole-organism thermal performance curves and denatured at relatively low temperatures in eight lizard species corresponding closely to their respective CTMAX (20% denatured between 37˚C and 45.2˚C; Licht 1964). Subcellular components such as ATPase could underlie thermal tolerance even though most components maintain function to higher temperature. Given the complexities of subcellular interactions and the paucity of data for the thermal performance of subcellular components in reptiles and amphibians, heat shock protein (HSP) production might better indicate whether subcellular-level components are challenged at sub-critical high temperatures. HSPs commonly act as molecular chaperones, maintaining protein structure and preventing aggregations of denatured proteins, and inducible variants are produced in response to cellular stress or damage (Fernando and Heikkila 2000; Kregel 2002; Daugaard et al. 2007). In diverse reptiles and amphibians, HSPs (particularly HSPA family members) are produced in response to sub-critical high temperatures, and production of HSPs can allow acclimation for increased thermal tolerance (Ulmasov et al. 1992; Fernando and Heikkila 2000; Zatespina et al. 2000; McMillan et al. 2011; Gao et al. 2014; Simoniello et al. 2016; Tedeschi et al. 2016). Moreover, patterns of HSP expression are correlated with thermal tolerance in lizards: warm-adapted species display higher constitutive HSP concentrations, and both initiate and maintain synthesis of HSPs to higher temperatures than more cold-adapted species (Ulmasov et al. 1992; Zatespina et al. 2000). This pattern of increased HSP production correlating with increases in thermal tolerance in reptiles and amphibians, along with observations for myosin ATPase in lizards, provides compelling evidence for subcellular-function loss playing a role in setting thermal limits, despite other subcellular components displaying little loss of function at relevant temperatures. Organ-system mechanisms Even though subcellular-level traits will be compromised at sufficiently high temperatures, higher-order systems might break down at lower temperatures and thus be proximally responsible for setting thermal tolerances (Pörtner 2001; 2002; Storch et al. 2014, but see Clark et al. 2013). The OCLTT hypothesis proposes that the highest organizational level in animals is the integrated cardiovascular and respiratory system because all tissues will be limited by their ability to acquire oxygen for respiration, and that this system is compromised by high temperatures prior to other systems (Pörtner 2001; 2002; Storch et al. 2014). However, similar to other terrestrial species (Klok et al. 2004; McCue and De Los Santos 2013; Verberk et al. 2016) and many fish (Clark et al. 2013; Gräns et al. 2014; Norin et al. 2014; Wang et al. 2014; Ern et al. 2016), evidence for the OCLTT mechanism underlying thermal tolerance in reptiles and amphibians is limited. Under the OCLTT hypothesis, maximal and resting rates of oxygen consumption are expected to converge as animals reach their physiological limits at high temperatures, thereby reducing aerobic scope and potentially inducing a short-term reliance on anaerobic respiration (Frederich and Pörtner 2000; Pörtner and Knust 2007; Elaison et al. 2011; Verberk et al. 2013, 2016; Table 1). Some evidence points to such a mechanism playing an important role in early animal evolution, notably in the transition to air breathing (Berner et al. 2007; Giomi et al. 2014; Teague et al. 2017). However, the few studies exposing animals to high temperatures and measuring indicators of aerobic and anaerobic respiration fail to find evidence for oxygen limitation in adult reptiles and amphibians (Carey 1979; Overgaard et al. 2012; Fobian et al. 2014; Gangloff et al. 2016; Telemeco et al. 2017). For example, oxygen consumption ( V˙O2) by pythons (Python regius) did not plateau at temperatures approaching CTMAX either when at rest or during periods of high metabolic demand (Fobian et al. 2014, Fig. 1), and resting oxygen consumption in garter snakes (Thamnophis elegans) increased with temperature with no apparent limit when animals experienced near-lethal temperatures (Gangloff et al. 2016). Moreover, neither garter snakes (T. elegans) nor alligator lizards (E. coerulea and E. multicarinata) transitioned to anaerobic respiration when exposed to near-critical temperatures (Gangloff et al. 2016; Telemeco et al. 2017), despite snakes and lizards rapidly transitioning when oxygen availability is limited during exercise (reviewed in Gleeson 1991). Observations in amphibians are similar to those for reptiles. For example, oxygen consumption, arterial oxygen saturation, and the proportion of saturated hemoglobin did not plateau at high temperatures in active or resting cane toads (Rhinella marina), thus providing evidence for these toads’ ability to maintain a positive aerobic power budget at near-critical temperatures (Seebacher and Franklin 2011; Overgaard et al. 2012; Winwood-Smith et al. 2015). In both the boreal toad (Anaxyrus boreas) and leopard frog (Lithobates pipiens), aerobic scope increased with temperature up to 30°C (Carey 1979). Whole-organism lactate concentration also increased with temperature, but there is no indication that either species becomes oxygen limited up to at least 30°C (Carey 1979). These results are in line with previous work showing that amphibians can maintain substantial aerobic scope at temperatures above active and preferred temperatures, although not necessarily at temperatures approaching TLETHAL (Whitford 1973). While such studies provide strong evidence for adult reptiles and amphibians maintaining aerobic scope at high temperatures, we currently lack data on tissue and cellular oxygen supply, such as venous PO2, which would more directly test the mechanisms described by the OCLTT hypothesis (Pörtner et al. 2017, 2018). Table 1 Experimental designs for testing aspects of the HMTL hypothesis in reptiles and amphibians with example studies Experiment type: Factor manipulated  Manipulation  Dependent variable  What it demonstrates  Examples  Temperature  Temperature treatments or ramp in lab  Oxygen capacity parameters ( V˙O2, Active V˙O2, alveolar/arterial PO2, heart rate), lactate production  Maintenance of aerobic scope at high temperatures; No evidence of transition to anaerobic respiration  Carey (1979), Seebacher and Franklin (2011), Overgaard et al. (2012), Fobian et al. (2014), Gangloff et al. (2016)  Temperature treatments in lab  Skeletal muscle metabolome, mitochondrial function  No transition from aerobic to anaerobic metabolism or subcellular damage at high temperatures  Telemeco et al. (2017)  Ex vivo temperature treatments  Enzyme activity  Temperature where subcellular components lose function  Licht (1964), Paulson and Hutchinson (1987), Irie et al. (1998), Al-Jassabi (2002)  Temperature treatments in lab  HSP induction  Temperature that induces a subcellular-protection response  Ulmasov et al. (1992), Fernando and Heikkila (2000), Zatespina et al. (2000), McMillan et al. (2011), Gao et al. (2014), Simoniello et al. (2016), Tedeschi et al. (2016)  Ambient oxygen  Oxygen treatments in lab  Oxygen capacity parameters ( V˙O2, heart rate, alveolar/arterial PO2, ventilation rate)  Aerobic capacity is maintained under conditions of mild hypoxia, but is limited under extreme hypoxia  Boyer (1963, 1966), Withers and Hillman (1983), Pörtner et al. (1991), Branco et al. (1993), Wang et al. (1994)  Oxygen treatments in lab  PBT  PBT is unaffected by mild hypoxia, but is reduced under extreme hypoxia  Hicks and Wood (1985), Branco et al. (1993), Cadena and Tattersall (2009)  Both ambient oxygen levels and temperature  Temperature and oxygen treatments in lab  Embryo development and survival  Hyperoxia increases survivorship while hypoxia reduces survivorship at high temperatures; Hypoxia reduces development, growth, and hatchling performance at high temperatures  Flewelling and Parker (2015), Liang et al. (2015), Smith et al. (2015)  Temperature gradient and/or ramp and oxygen treatments in lab  CTMAX, TGAPE, TPANT  Behavioral responses to high temperatures depend on ambient O2, but only under extreme hypoxia  Dupre et al. (1986), Tattersall and Gerlach (2005), Shea et al. (2016)  Transplant across altitudinal gradients within species’ ranges; Manipulation of ambient O2 in field; Temperature ramp  CTMAX, TGAPE, TPANT  Behavioral responses to high temperatures depend on ambient O2, but only under extreme hypoxia  DuBois et al. (2017)  Oxygen capacity  Hematocrit reduction  PBT  Reduced oxygen capacity affects temperature perception and animals choose lower temperatures  Wood (1990), Hicks and Wood (1985)  Blood volume reduction  Oxygen capacity parameters (heart rate, alveolar/arterial PO2, ventilation rate)  Heart rate increases while ventilation rate is unchanged by reduced oxygen carrying capacity  Wang et al. (1994)  Observational  –  Quantification of CTMAX across life stages  Oxygen capacity limits thermal tolerance at some developmental stages in larval anurans  Cupp (1980), Sherman (1980), Floyd (1983)  –  Comparison of PBT, resting V˙O2, Active V˙O2,  PBT matches temperature of maximal aerobic scope in lizards  Wilson (1974)  Experiment type: Factor manipulated  Manipulation  Dependent variable  What it demonstrates  Examples  Temperature  Temperature treatments or ramp in lab  Oxygen capacity parameters ( V˙O2, Active V˙O2, alveolar/arterial PO2, heart rate), lactate production  Maintenance of aerobic scope at high temperatures; No evidence of transition to anaerobic respiration  Carey (1979), Seebacher and Franklin (2011), Overgaard et al. (2012), Fobian et al. (2014), Gangloff et al. (2016)  Temperature treatments in lab  Skeletal muscle metabolome, mitochondrial function  No transition from aerobic to anaerobic metabolism or subcellular damage at high temperatures  Telemeco et al. (2017)  Ex vivo temperature treatments  Enzyme activity  Temperature where subcellular components lose function  Licht (1964), Paulson and Hutchinson (1987), Irie et al. (1998), Al-Jassabi (2002)  Temperature treatments in lab  HSP induction  Temperature that induces a subcellular-protection response  Ulmasov et al. (1992), Fernando and Heikkila (2000), Zatespina et al. (2000), McMillan et al. (2011), Gao et al. (2014), Simoniello et al. (2016), Tedeschi et al. (2016)  Ambient oxygen  Oxygen treatments in lab  Oxygen capacity parameters ( V˙O2, heart rate, alveolar/arterial PO2, ventilation rate)  Aerobic capacity is maintained under conditions of mild hypoxia, but is limited under extreme hypoxia  Boyer (1963, 1966), Withers and Hillman (1983), Pörtner et al. (1991), Branco et al. (1993), Wang et al. (1994)  Oxygen treatments in lab  PBT  PBT is unaffected by mild hypoxia, but is reduced under extreme hypoxia  Hicks and Wood (1985), Branco et al. (1993), Cadena and Tattersall (2009)  Both ambient oxygen levels and temperature  Temperature and oxygen treatments in lab  Embryo development and survival  Hyperoxia increases survivorship while hypoxia reduces survivorship at high temperatures; Hypoxia reduces development, growth, and hatchling performance at high temperatures  Flewelling and Parker (2015), Liang et al. (2015), Smith et al. (2015)  Temperature gradient and/or ramp and oxygen treatments in lab  CTMAX, TGAPE, TPANT  Behavioral responses to high temperatures depend on ambient O2, but only under extreme hypoxia  Dupre et al. (1986), Tattersall and Gerlach (2005), Shea et al. (2016)  Transplant across altitudinal gradients within species’ ranges; Manipulation of ambient O2 in field; Temperature ramp  CTMAX, TGAPE, TPANT  Behavioral responses to high temperatures depend on ambient O2, but only under extreme hypoxia  DuBois et al. (2017)  Oxygen capacity  Hematocrit reduction  PBT  Reduced oxygen capacity affects temperature perception and animals choose lower temperatures  Wood (1990), Hicks and Wood (1985)  Blood volume reduction  Oxygen capacity parameters (heart rate, alveolar/arterial PO2, ventilation rate)  Heart rate increases while ventilation rate is unchanged by reduced oxygen carrying capacity  Wang et al. (1994)  Observational  –  Quantification of CTMAX across life stages  Oxygen capacity limits thermal tolerance at some developmental stages in larval anurans  Cupp (1980), Sherman (1980), Floyd (1983)  –  Comparison of PBT, resting V˙O2, Active V˙O2,  PBT matches temperature of maximal aerobic scope in lizards  Wilson (1974)  Table 1 Experimental designs for testing aspects of the HMTL hypothesis in reptiles and amphibians with example studies Experiment type: Factor manipulated  Manipulation  Dependent variable  What it demonstrates  Examples  Temperature  Temperature treatments or ramp in lab  Oxygen capacity parameters ( V˙O2, Active V˙O2, alveolar/arterial PO2, heart rate), lactate production  Maintenance of aerobic scope at high temperatures; No evidence of transition to anaerobic respiration  Carey (1979), Seebacher and Franklin (2011), Overgaard et al. (2012), Fobian et al. (2014), Gangloff et al. (2016)  Temperature treatments in lab  Skeletal muscle metabolome, mitochondrial function  No transition from aerobic to anaerobic metabolism or subcellular damage at high temperatures  Telemeco et al. (2017)  Ex vivo temperature treatments  Enzyme activity  Temperature where subcellular components lose function  Licht (1964), Paulson and Hutchinson (1987), Irie et al. (1998), Al-Jassabi (2002)  Temperature treatments in lab  HSP induction  Temperature that induces a subcellular-protection response  Ulmasov et al. (1992), Fernando and Heikkila (2000), Zatespina et al. (2000), McMillan et al. (2011), Gao et al. (2014), Simoniello et al. (2016), Tedeschi et al. (2016)  Ambient oxygen  Oxygen treatments in lab  Oxygen capacity parameters ( V˙O2, heart rate, alveolar/arterial PO2, ventilation rate)  Aerobic capacity is maintained under conditions of mild hypoxia, but is limited under extreme hypoxia  Boyer (1963, 1966), Withers and Hillman (1983), Pörtner et al. (1991), Branco et al. (1993), Wang et al. (1994)  Oxygen treatments in lab  PBT  PBT is unaffected by mild hypoxia, but is reduced under extreme hypoxia  Hicks and Wood (1985), Branco et al. (1993), Cadena and Tattersall (2009)  Both ambient oxygen levels and temperature  Temperature and oxygen treatments in lab  Embryo development and survival  Hyperoxia increases survivorship while hypoxia reduces survivorship at high temperatures; Hypoxia reduces development, growth, and hatchling performance at high temperatures  Flewelling and Parker (2015), Liang et al. (2015), Smith et al. (2015)  Temperature gradient and/or ramp and oxygen treatments in lab  CTMAX, TGAPE, TPANT  Behavioral responses to high temperatures depend on ambient O2, but only under extreme hypoxia  Dupre et al. (1986), Tattersall and Gerlach (2005), Shea et al. (2016)  Transplant across altitudinal gradients within species’ ranges; Manipulation of ambient O2 in field; Temperature ramp  CTMAX, TGAPE, TPANT  Behavioral responses to high temperatures depend on ambient O2, but only under extreme hypoxia  DuBois et al. (2017)  Oxygen capacity  Hematocrit reduction  PBT  Reduced oxygen capacity affects temperature perception and animals choose lower temperatures  Wood (1990), Hicks and Wood (1985)  Blood volume reduction  Oxygen capacity parameters (heart rate, alveolar/arterial PO2, ventilation rate)  Heart rate increases while ventilation rate is unchanged by reduced oxygen carrying capacity  Wang et al. (1994)  Observational  –  Quantification of CTMAX across life stages  Oxygen capacity limits thermal tolerance at some developmental stages in larval anurans  Cupp (1980), Sherman (1980), Floyd (1983)  –  Comparison of PBT, resting V˙O2, Active V˙O2,  PBT matches temperature of maximal aerobic scope in lizards  Wilson (1974)  Experiment type: Factor manipulated  Manipulation  Dependent variable  What it demonstrates  Examples  Temperature  Temperature treatments or ramp in lab  Oxygen capacity parameters ( V˙O2, Active V˙O2, alveolar/arterial PO2, heart rate), lactate production  Maintenance of aerobic scope at high temperatures; No evidence of transition to anaerobic respiration  Carey (1979), Seebacher and Franklin (2011), Overgaard et al. (2012), Fobian et al. (2014), Gangloff et al. (2016)  Temperature treatments in lab  Skeletal muscle metabolome, mitochondrial function  No transition from aerobic to anaerobic metabolism or subcellular damage at high temperatures  Telemeco et al. (2017)  Ex vivo temperature treatments  Enzyme activity  Temperature where subcellular components lose function  Licht (1964), Paulson and Hutchinson (1987), Irie et al. (1998), Al-Jassabi (2002)  Temperature treatments in lab  HSP induction  Temperature that induces a subcellular-protection response  Ulmasov et al. (1992), Fernando and Heikkila (2000), Zatespina et al. (2000), McMillan et al. (2011), Gao et al. (2014), Simoniello et al. (2016), Tedeschi et al. (2016)  Ambient oxygen  Oxygen treatments in lab  Oxygen capacity parameters ( V˙O2, heart rate, alveolar/arterial PO2, ventilation rate)  Aerobic capacity is maintained under conditions of mild hypoxia, but is limited under extreme hypoxia  Boyer (1963, 1966), Withers and Hillman (1983), Pörtner et al. (1991), Branco et al. (1993), Wang et al. (1994)  Oxygen treatments in lab  PBT  PBT is unaffected by mild hypoxia, but is reduced under extreme hypoxia  Hicks and Wood (1985), Branco et al. (1993), Cadena and Tattersall (2009)  Both ambient oxygen levels and temperature  Temperature and oxygen treatments in lab  Embryo development and survival  Hyperoxia increases survivorship while hypoxia reduces survivorship at high temperatures; Hypoxia reduces development, growth, and hatchling performance at high temperatures  Flewelling and Parker (2015), Liang et al. (2015), Smith et al. (2015)  Temperature gradient and/or ramp and oxygen treatments in lab  CTMAX, TGAPE, TPANT  Behavioral responses to high temperatures depend on ambient O2, but only under extreme hypoxia  Dupre et al. (1986), Tattersall and Gerlach (2005), Shea et al. (2016)  Transplant across altitudinal gradients within species’ ranges; Manipulation of ambient O2 in field; Temperature ramp  CTMAX, TGAPE, TPANT  Behavioral responses to high temperatures depend on ambient O2, but only under extreme hypoxia  DuBois et al. (2017)  Oxygen capacity  Hematocrit reduction  PBT  Reduced oxygen capacity affects temperature perception and animals choose lower temperatures  Wood (1990), Hicks and Wood (1985)  Blood volume reduction  Oxygen capacity parameters (heart rate, alveolar/arterial PO2, ventilation rate)  Heart rate increases while ventilation rate is unchanged by reduced oxygen carrying capacity  Wang et al. (1994)  Observational  –  Quantification of CTMAX across life stages  Oxygen capacity limits thermal tolerance at some developmental stages in larval anurans  Cupp (1980), Sherman (1980), Floyd (1983)  –  Comparison of PBT, resting V˙O2, Active V˙O2,  PBT matches temperature of maximal aerobic scope in lizards  Wilson (1974)  A second approach for testing the influence of oxygen capacity on thermal constraints is to manipulate oxygen availability and examine changes in thermal behavior or tolerance (Table 1). Under the OCLTT hypothesis, hypoxia is predicted to reduce thermal optima and tolerance limits (Smith et al. 2015; Verberk et al. 2016; DuBois et al. 2017), and this prediction is somewhat supported in reptiles and amphibians. For example, CTMAX, preferred body temperature (PBT), panting temperature (TPANT), and gaping temperature (TGAPE) are reduced when diverse species are exposed to very-low oxygen environments (<10 kPa; mostly lizards examined; Hicks and Wood 1985; Dupre et al. 1986; Branco et al. 1993; Cadena and Tattersall 2009; Shea et al. 2016; DuBois et al. 2017; Fig. 2) and when hematocrit is experimentally reduced (only PBT examined; Hicks and Wood 1985; Wood 1990). Hypoxia-induced PBT reduction is a well-described phenomenon in ectotherms, termed “behavioral anapyrexia,” that allows adaptive reduction of metabolic demand when oxygen is limited (Hicks and Wood 1985; Wood and Gonzales 1996; Steiner and Branco 2002; Hicks and Wang 2004), supporting the hypothesis that oxygen limitation influences thermal preference and possibly tolerance. Along with CTMAX, reductions in TGAPE and TPANT, which provide an indication of perceived heat stress (Heatwole et al. 1973; Tattersall et al. 2006; DuBois et al. 2017), imply reduced thermal tolerance under hypoxia. Hypoxia also induces elevated heart rates and reduces both resting and active oxygen consumption ( V˙O2) in diverse species (Fig. 3), suggesting observed shifts in thermal tolerance and behavior are related to physiological limits of oxygen capacity (including diffusion and transport). Interestingly, only extreme hypoxia had the predicted effects on thermal tolerance and behavior, with levels of hypoxia within the range generally found in terrestrial environments having no effect (Figs. 2 and 3). Thus, it is not clear that the OCLTT mechanism will be generally relevant in nature. Fig. 2 View largeDownload slide Thermal preference and tolerance are unaffected by moderate hypoxia but reduced in extremely hypoxic environments (<10 kPa). This “broken-stick” pattern is apparent for (A) PBT (i.e. behavioral anapyrexia), (B) critical thermal maximum, (C) gaping temperature, and (D) panting temperature across species. Data are mostly available for lizards (solid lines) but PBT data are also available for alligator (dashed line). Lines between points are included to aid visualization, but connections between values at normoxia and hypoxia are likely nonlinear. Colors (online only) and initials denote species: A.m., Alligator mississippiensis; B.v., Basiliscus vittatus; C.p., Ctenosaura pectinata; I.i., Iguana iguana; P.v., Pogona vitticeps; S.g., Sceloporus graciosus; S.o., Sceloporus occidentalis; V.e., Varanus exanthematicus; V.v., Varanus varius. Data are means ± s.e.m. derived from the literature. See Supplementary Table S1 for data and citations. Fig. 2 View largeDownload slide Thermal preference and tolerance are unaffected by moderate hypoxia but reduced in extremely hypoxic environments (<10 kPa). This “broken-stick” pattern is apparent for (A) PBT (i.e. behavioral anapyrexia), (B) critical thermal maximum, (C) gaping temperature, and (D) panting temperature across species. Data are mostly available for lizards (solid lines) but PBT data are also available for alligator (dashed line). Lines between points are included to aid visualization, but connections between values at normoxia and hypoxia are likely nonlinear. Colors (online only) and initials denote species: A.m., Alligator mississippiensis; B.v., Basiliscus vittatus; C.p., Ctenosaura pectinata; I.i., Iguana iguana; P.v., Pogona vitticeps; S.g., Sceloporus graciosus; S.o., Sceloporus occidentalis; V.e., Varanus exanthematicus; V.v., Varanus varius. Data are means ± s.e.m. derived from the literature. See Supplementary Table S1 for data and citations. Fig. 3 View largeDownload slide As the environment becomes increasingly hypoxic, (A) arterial PO2 decreases, whereas (B) resting metabolic rate, (C) heart rate, and (D) maximal metabolic rate are relatively unaffected until the environment becomes extremely hypoxic (<10 kPa), similar to thermal preference and tolerance (see Fig. 2 for comparison). Colors (online only) denote taxonomic group whereas initials denote species: A.c., Anaxyrus cognatus (formerly Bufo cognatus); A.m., Alligator mississippiensis; C.s., Chelydra serpentina; D.d, Dipsosaurus dorsalis; L.p., Lithobates pipiens (formerly Rana pipiens); P.m., Pituophis melanoleucus; R.m., Rhinella marina (formerly Bufo marinus); R.s., Rhinella schneideri (formerly Bufo paracnemis). A.c., L.p., R.s., and R.m. are frogs, D.d. is a lizard, P.m. is a snake, C.s. is a turtle, and A.m. is an alligator. Experimental temperature is given to the right of the species’ initials. Data are means ± s.e.m. derived from the literature. See Supplementary Table S1 for data and citations. Fig. 3 View largeDownload slide As the environment becomes increasingly hypoxic, (A) arterial PO2 decreases, whereas (B) resting metabolic rate, (C) heart rate, and (D) maximal metabolic rate are relatively unaffected until the environment becomes extremely hypoxic (<10 kPa), similar to thermal preference and tolerance (see Fig. 2 for comparison). Colors (online only) denote taxonomic group whereas initials denote species: A.c., Anaxyrus cognatus (formerly Bufo cognatus); A.m., Alligator mississippiensis; C.s., Chelydra serpentina; D.d, Dipsosaurus dorsalis; L.p., Lithobates pipiens (formerly Rana pipiens); P.m., Pituophis melanoleucus; R.m., Rhinella marina (formerly Bufo marinus); R.s., Rhinella schneideri (formerly Bufo paracnemis). A.c., L.p., R.s., and R.m. are frogs, D.d. is a lizard, P.m. is a snake, C.s. is a turtle, and A.m. is an alligator. Experimental temperature is given to the right of the species’ initials. Data are means ± s.e.m. derived from the literature. See Supplementary Table S1 for data and citations. In contrast to adult stages, naturalistic hypoxia reduces thermal performance and tolerance in eggs and larvae of reptiles and amphibians. Because amphibian eggs and larvae inhabit aqueous environments, environmental oxygen availability is reduced relative to terrestrial stages, thereby increasing the potential importance of the OCLTT mechanism similar to some fully aquatic ectotherms (e.g., Pörtner and Knust 2007; Verberk et al. 2013; Pörtner and Gutt 2016). Oxygen limitation is further exacerbated when species develop in small water bodies that can rapidly lose dissolved oxygen, such as ephemeral pools (Seymour and Bradford 1995; Sacerdote and King 2009), and because amphibian eggs are commonly produced within large gelatinous masses with low oxygen diffusion potential (Pinder and Friet 1994; Woods 1999; Sacerdote and King 2009). Although data are limited, these constraints appear to affect early stage amphibians congruent with oxygen limiting thermal tolerances. For example, amphibians that nest in warm water produce smaller eggs and egg masses to facilitate oxygen diffusion to embryos (Woods 1999; Sacerdote and King 2009), and aquatic larvae born into warm water (from aquatic or terrestrial eggs) are smaller and have lower metabolic demands (Kuramoto 1975; Rollinson and Rowe 2018). In oviparous reptiles, embryonic gas exchange occurs via passive diffusion across the shell into the chorioallantoic membrane, which is much less efficient than adult respiration (Vitt and Caldwell 2009). As predicted by the OCLTT hypothesis, thermal tolerance is reduced when developing embryos experience modest to extreme hypoxia (Flewelling and Parker 2015; Liang et al. 2015; Smith et al. 2015). For example, lower environmental PO2 experienced at high elevations reduces TLETHAL in plateau fence lizard (Sceloporus tristichus) embryos compared to PO2 at sea level (Smith et al. 2015), although such modest hypoxia does not affect the thermal tolerance of adult congeners (S. occidentalis and S. graciosus; Shea et al. 2016; DuBois et al. 2017). Thus, available data suggest that reptiles and amphibians experience ontogenetic shifts in the proximate mechanism underlying thermal tolerance: early stages appear more affected by organ-system-level failures, such as the OCLTT mechanism, whereas later stages are more impacted by subcellular mechanisms. An integrative framework: HMTL Available evidence implies that neither failure of subcellular components nor oxygen and capacity limitation are solely responsible for loss of performance at high temperatures in reptiles and amphibians. Rather, mechanisms across levels of organization appear to play partial, context-dependent roles. We propose a unified framework that combines subcellular mechanisms, the oxygen and capacity limited thermal tolerance (OCLTT) hypothesis, and the thermal performance curve (TPC) paradigm. We call this integrative framework “Hierarchical Mechanisms of Thermal Limitation” (HMTL, Fig. 1). Like the OCLTT hypothesis, we propose that oxygen diffusion and transport capacity largely shape thermal performance at sub-critical temperatures via effects of temperature on aerobic scope. However, the HMTL framework explicitly recognizes the importance of subcellular-level mechanisms and allows them to underlie absolute thermal limits, such as CTMAX and TLETHAL. Under this hypothesis, the relative importance of subcellular- and organ-system-level mechanisms on thermal tolerance is context dependent, but predictable. We propose that the HMTL framework explains diverse empirical results and can bring together decades of physiological studies in reptiles and amphibians. Given that numerous models of thermal performance and tolerance have been proposed, we think that it is useful to state their similarities and differences with HMTL. The HMTL framework builds upon ideas developed for the OCLTT hypothesis proposed by Pörtner and colleagues (recently reviewed in Pörtner et al. 2017) and is similar to that put forth by Ern et al. (2016) for fishes (see Fig. 1 in Ern et al. 2016), but more formally incorporates the TPC paradigm and explicitly describes mechanism-dependent critical temperatures. Importantly, this hypothesis predicts conditions under which subcellular and organ-system failure will set thermal tolerance limits and when transitions are likely to occur. Our framework also differs importantly from the Multiple Performances—Multiple Optima (MPMO) framework put forth by Clark et al. (2013) for fishes. The MPMO suggests a single thermal limit set by an undefined mechanism but multiple, trait-specific optima that can dramatically differ from the temperature that maximizes aerobic scope (summarized in Fig. 7 of Clark et al. 2013). By contrast, we propose multiple potential mechanisms of thermal limitation but a single whole-organism thermal optimum for performance correlated with the temperature that maximizes aerobic scope (although this will be an integration across traits). Figure 1 illustrates the HMTL framework. In the top row of panels, solid blue and red lines represent the effect of temperature on oxygen consumption during rest and activity, respectively. Assuming activity is maximal, aerobic scope is the difference between these lines. To illustrate this concept with empirical data, we derived these lines by fitting a generalized logistic function to data from Fobian et al. (2014) for pythons (P.regius) assuming the shape of the response curve is the same during rest and activity. Based on available data, the general shape of these curves is qualitatively similar across diverse taxa (e.g., Carey 1979; Frederich and Pörtner 2000; Overgaard 2012; Fobian et al. 2014; Ern et al. 2016) and we think that the predictions of the HMTL framework can be generalized beyond the specific data used to generate this illustration. The dotted blue and red lines depict exponential functions fit to these data and illustrate predicted oxygen demand/use if organisms were unconstrained by capacity limitations. The columns illustrate predicted effects of variable oxygen environments, and the bottom row displays TPCs predicted to result given relationships in the top row. A central assumption of the HMTL framework is that critical thermal limits exist for both subcellular mechanisms (subcellular TCRIT) and aerobic respiration (aerobic TCRIT), and that organismal thermal limits are proximally caused by the lower TCRIT (Fig. 1). The environment and oxygen-handling capacity of the organism will co-determine the hierarchy of these critical limits and thus their relative importance. The subcellular TCRIT is the temperature at which key subcellular components begin to lose function, and should be insensitive to activity state or oxygen environment. By contrast, aerobic TCRIT is the temperature at which oxygen capacity is maximized, and is affected by activity state and oxygen environment. For example, aerobic TCRIT during rest will be higher than aerobic TCRIT during activity because elevated O2 demand during activity causes capacity limits to be reached more readily (Fig. 1). Moreover, both active- and resting-aerobic TCRIT should be reduced during exposure to hypoxic environments (Fig. 1B, C). Any reduction in environmental oxygen availability below normoxia will reduce aerobic scope. However, CTMAX will only be reduced if environmental hypoxia is sufficient to cause resting-aerobic TCRIT to drop below subcellular TCRIT (Fig. 1C). The environmental oxygen tension where resting-aerobic TCRIT equals subcellular TCRIT is PCTMAX as defined by Ern et al. (2016). By integrating TCRIT values and aerobic scope, we can derive whole-organism TPCs (Fig. 1, bottom row). Because the active-aerobic TCRIT is the lowest temperature where maximal metabolic rate can be achieved, it closely corresponds to the optimal temperature for aerobic performance (whole-organism TOPT) where aerobic scope is maximized. At temperatures below active-aerobic TCRIT, we predict that performance increases with temperature proportional to aerobic scope. At temperatures above active-aerobic TCRIT, performance will drop until CTMAX is reached, but we predict that the shape of this drop will depend on which TCRIT underlies CTMAX. If resting-aerobic TCRIT underlies CTMAX, the TPC should be more symmetrical with performance and aerobic scope decreasing at a rate mirroring the increase (Fig. 1C). However, if subcellular TCRIT underlies CTMAX, the curve will be asymmetric with loss of performance occurring more rapidly the closer active-aerobic TCRIT is to subcellular TCRIT (Fig. 1A, B). Assumptions, predictions, and evidence for HMTL in reptiles and amphibians The HMTL framework produces numerous testable predictions, some of which can be addressed with available data, while others offer exciting avenues for future research. First, any environmental or organismal characteristics that reduce oxygen availability or capacity for oxygen utilization (e.g., aquatic respiration, reliance on cutaneous gas exchange, or life-stages such as eggs with reduced diffusion potential) should increase the probability of resting-aerobic TCRIT underlying CTMAX. This can explain oxygen capacity setting thermal limits in embryos (Smith et al. 2015) but not adults (Overgaard et al. 2012; Fobian et al. 2014; DuBois et al. 2017). Moreover, the transition between aerobic and subcellular TCRIT predicted by HMTL explains extreme experimental hypoxia reducing thermal tolerance in adult reptiles and amphibians, while moderate hypoxia does not (Fig. 1C and Fig. 2B,D). Similarly, HMTL predicts that CTMAX is lower when governed by oxygen limitation than when governed by subcellular mechanisms. Supporting this prediction, CTMAX drops in tadpoles as oxygen demand increases with growth and when respiratory structures are compromised in late-stage metamorphs, but frequently elevates again in terrestrial adults presumably as a result of increased oxygen capacity (Cupp 1980; Sherman 1980; Floyd 1983). Another important prediction of the HMTL hypothesis is that performance can be limited by oxygen exchange capacity at high temperatures even when CTMAX and TLETHAL are proximally set by subcellular mechanisms. Some interpretations of the OCLTT hypothesis similarly highlight limits on performance at pejus rather than critical temperatures (Pörtner and Knust 2007; Pörtner 2014; Verberk et al. 2016; Pörtner et al. 2017), but other workers suggest OCLTT should explain critical limits to be useful (i.e., Fobian et al. 2014; Smith et al. 2015; Ern et al. 2016; Verberk et al. 2016; DuBois et al. 2017). Under the HMTL framework, activity state is assumed to have no appreciable effect on oxygen capacity, although this may not hold in some highly aerobic taxa (e.g., Wang and Hicks 2004). The extent to which activity-induced increases in oxygen capacity affect thermal performance curves and limits is an important direction for future work. Generally however, individuals in metabolically-demanding states are expected to reach capacity limits at temperatures below those at which such limits are reached by resting individuals. Anaerobic respiration might compensate for short-term mismatches between energy demand and oxygen capacity (reviewed in Gleeson 1991; Fig. 1A, red shaded region), but aerobic constraints are predicted to reduce performance during long-term activity (e.g., digestion, reproduction, recovery; Jackson 2007; Pörtner et al. 2017). In diverse species, active metabolic rate asymptotes despite increases in cardiovascular output, implying oxygen flux becomes limited at temperatures above active-aerobic TCRIT (e.g., Bartholomew and Tucker 1963; Bennett and Licht 1972; Wilson 1974; Overgaard et al. 2012; Fobian et al. 2014). Furthermore, pulmonary diffusion capacity is limited by reduced plasma gas solubility and hemoglobin binding affinity at high temperatures (Wood and Moberly 1970; Kinney et al. 1977; Pough 1980; Jackson 2007; da Silva et al. 2013). Diffusion limitation of cutaneous gas exchange is also well established (reviewed in Burggren 1988; Wang 2011) and will be important in species that spend considerable time submerged (e.g. Ultsch 1973) or are lungless (e.g., Plethodontid salamanders; Whitford and Hutchison 1965; Spotilla 1972). Finally, increases in the products of anaerobic respiration, such as lactic acid, can lead to blood acidification and thereby further declines in blood oxygen affinity (Bennett 1973). The interaction of numerous factors at high temperatures reduces oxygen capacity, which in turn limits aerobic performance at high temperatures in reptiles and amphibians, regardless of the mechanism governing CTMAX (further reviewed in Jackson 2007). The HMTL framework predicts that whole-organism TOPT is the temperature providing maximal aerobic scope, which is governed by active-aerobic TCRIT. Thus, alleviating the limits that underlie active-aerobic TCRIT should allow increased maximal performance (i.e., aerobic scope) and TOPT (Fig. 1). Unfortunately, few data are available comparing whole-organism TOPT and aerobic scope in reptiles or amphibians. In the snake P. regius, whole-organism TOPT and the temperature where aerobic scope is maximized appear to correspond (Fobian et al. 2014). In the toad R. marina, observations are more complex and suggest that the maintenance of aerobic scope to high temperatures could be independent of whole-organism TOPT, at least in some cases. Hopping performance is maximized at ∼30°C (Kearney et al. 2008) whereas aerobic scope can plateau at 30°C, but can also increase to at least 40°C depending on acclimation treatment (Overgaard et al. 2012). Additional data are needed to determine if acclimation elevates TOPT for whole-organism performance similar to maximal aerobic scope in R. marina, as would be predicted by the HMTL framework. Observations for PBT provide additional indirect evidence that whole-organism TOPT corresponds to the temperature that maximizes aerobic scope in reptiles, but again data for amphibians are less clear. Generally, terrestrial ectotherms thermoregulate to within a narrow thermal range during activity if costs to thermoregulation are not prohibitively high (Huey 1982; Bauwens et al. 1995; Angilletta 2009; Kingsolver and Buckley 2015; Sears et al. 2016). Natural selection is predicted to shape thermal preference such that PBT corresponds to, or is slightly below, whole-organism TOPT (Huey 1982; Bauwens et al. 1995; Angilletta et al. 2002; but see Huey and Bennett 1987). As predicted, PBT and temperature of maximum aerobic scope are highly concordant in lizards exposed to normoxic environments (Wilson 1974). On the other hand, in the boreal toad (Anaxyrus boreas), aerobic scope is maximal at 30°C whereas TPREF is 24°C (Carey 1978, 1979). Interestingly, A. boreas and other anurans (L. pipiens, Carey 1979; R. marina, Overgaard et al. 2012) exhibit an increase in lactic acid production with temperatures above PBT in both resting and active animals. Increased lactic acid production indicates that high temperatures induce anaerobic respiration, even as aerobic scope is maintained, and therefore incur an oxygen debt for recovery. Further data are needed to assess whether the potential mismatch between maximal aerobic scope and TOPT in anurans can be explained by animals balancing the increased costs of repaying oxygen debt resulting from increased anaerobic respiration at high temperatures with the benefits of concurrent increases in aerobic scope. Assuming maximal aerobic scope, TOPT, and PBT are linked as predicted by HMTL, they are not uniformly affected by experimental hypoxia as might initially be predicted. Only extreme hypoxia affects PBT (Fig. 2) whereas the temperature that maximizes aerobic scope and presumably whole-organism TOPT is predicted to drop continuously with hypoxia (Fig. 1). This discrepancy might indicate that adult reptiles and amphibians cannot sense and respond to hypoxia-induced changes in aerobic scope in real time, which might be expected given that these animals evolved in terrestrial environments where oxygen availability is relatively stable within a lifetime. Thus, we propose that individuals choose the same body temperature regardless of the oxygen environment so long as basic metabolic demands are met, and thus predict that the “breakpoint” in Fig. 2A occurs when hypoxia causes resting-aerobic TCRIT to fall below evolved PBT. Data comparing resting aerobic TCRIT and PBT when oxygen environment or demand is manipulated are needed to test this prediction. Given the predicted relationships between aerobic scope, TOPT, and PBT, we also expect species adapted to low-oxygen environments to have relatively lower TOPT and PBT, or greater oxygen-handling capacity. However, covariation between temperature and oxygen with elevation make testing this prediction in terrestrial environments difficult. In addition to further exploring the potential importance of resting- and active-aerobic TCRIT, additional research is needed to identify the subcellular components that underlie subcellular TCRIT. A subset of evolutionarily conserved components might constrain thermal tolerance across a diversity of taxa, or the components that are most important could be taxonomically specific. Currently, the data needed to differentiate these possibilities are not available. We think that measures of ATPase and HSP provide useful candidates for further exploration, but caution that focusing on a single or few potential indicators in isolation will likely provide a contorted view of subcellular limitation. Advances in “-omics” technologies, particularly differential expression RNAseq, metabolomics, and proteomics could provide much useful information about subcellular physiological function, and provide additional candidate molecules for detailed analysis (e.g., Verberk et al. 2013; Williams et al. 2014; Campbell-Staton et al. 2017; Telemeco et al. 2017). We recommend that experiments manipulating the thermal and oxygen environment of organisms endeavor to collect subcellular data as well as whole-organism performance data. Where possible, an integrative approach combining measurements of subcellular components and whole-organism performance will best illuminate the mechanisms that underlie tolerance and their interactions. Finally, the HMTL framework makes predictions for how populations could be affected by global change. Numerous species are expanding or shifting their range to higher elevations in response to climate change-related temperature increases (e.g., Sinervo et al. 2010; Pincheira-Donoso et al. 2013; Pauchard et al. 2016). However, HMTL predicts that reduced oxygen partial pressures at high elevation will lower both TOPT and maximal performance. Thus, species must seek cooler environments as they move to higher elevation to maintain optimal performance, and performance potential will go down regardless of thermal environment selected. An evolutionary change appears necessary for animals to seek out reduced body temperatures because moderate hypoxia does not affect PBT (Fig. 2). The HMTL framework also predicts that species with greater oxygen capacity will be more buffered from lost performance when exposed to increased environmental temperatures. Thus, oxygen capacity might be a prime target of natural selection as climates warm, even if it does not underlie CTMAX or TLETHAL. Finally, given the great diversity in modes of gas exchange, metabolic demands, and shifts across life-history stages in reptiles and amphibians, we emphasize the need to explore these hypotheses in a greater number and variety of taxa. Conclusions Available data suggest that both subcellular- and organ-system-level mechanisms shape thermal performance and tolerance in amphibians and reptiles. The HMTL framework that we propose describes how both mechanisms co-affect animals, with their relative importance driven by their respective TCRIT. We think that the HMTL hypothesis improves upon current frameworks by explicitly removing the false dichotomy between subcellular mechanisms and oxygen limitation, identifying useful parameters for further research (subcellular TCRIT, resting-aerobic TCRIT, and active-aerobic TCRIT), and describing how aerobic and subcellular limitations interact to affect TPCs. Moreover, HMTL appears to explain a wide range of initially perplexing observations in reptiles and amphibians such as reduced aerobic scope at high temperatures without aerobic failure at critical temperatures, extreme hypoxia affecting thermal tolerance and behavior in adults with no effect of moderate hypoxia, moderate hypoxia reducing thermal tolerance in embryos, and HSP and ATPase activity suggesting loss of subcellular function near critical temperatures. Still, the HMTL framework makes numerous predictions for which additional data are needed, including animals only reducing their body temperature when resting-aerobic TCRIT drops below PBT, CTMAX being lower when resting-aerobic TCRIT is responsible than when subcellular TCRIT is responsible, maximum aerobic capacity underlying whole-organism thermal optima, and reduced performance when species invade higher elevations without increased oxygen capacity. Moreover, virtually no data are available to address the potential importance of aerobic- or subcellular-TCRIT as evolutionary constraints shaping the adaptive landscape. Further data better representing the diversity of reptile and amphibian taxa are needed both to understand the potential relevance of the HMTL mechanism in extant reptiles and amphibians, and how such a mechanism could have shaped the evolution of these animals. We think that many of the ideas that make up our HMTL framework are already widely accepted within the scientific community, and hope that explicitly describing them within a single framework with clear, testable predictions will facilitate further research. Although inspired by reptiles and amphibians, this integrated framework could have broad applicability across ectothermic animals. We look forward to continued investigation further integrating, refining, and testing these ideas across mechanisms and taxa. Acknowledgments We are grateful to M. 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This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Integrative and Comparative Biology Oxford University Press

High Temperature, Oxygen, and Performance: Insights from Reptiles and Amphibians

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Oxford University Press
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© The Author(s) 2018. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology. All rights reserved. For permissions please email: journals.permissions@oup.com.
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1540-7063
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10.1093/icb/icy005
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Abstract

Synopsis Much recent theoretical and empirical work has sought to describe the physiological mechanisms underlying thermal tolerance in animals. Leading hypotheses can be broadly divided into two categories that primarily differ in organizational scale: 1) high temperature directly reduces the function of subcellular machinery, such as enzymes and cell membranes, or 2) high temperature disrupts system-level interactions, such as mismatches in the supply and demand of oxygen, prior to having any direct negative effect on the subcellular machinery. Nonetheless, a general framework describing the contexts under which either subcellular component or organ system failure limits organisms at high temperatures remains elusive. With this commentary, we leverage decades of research on the physiology of ectothermic tetrapods (amphibians and non-avian reptiles) to address these hypotheses. Available data suggest both mechanisms are important. Thus, we expand previous work and propose the Hierarchical Mechanisms of Thermal Limitation (HMTL) hypothesis, which explains how subcellular and organ system failures interact to limit performance and set tolerance limits at high temperatures. We further integrate this framework with the thermal performance curve paradigm commonly used to predict the effects of thermal environments on performance and fitness. The HMTL framework appears to successfully explain diverse observations in reptiles and amphibians and makes numerous predictions that remain untested. We hope that this framework spurs further research in diverse taxa and facilitates mechanistic forecasts of biological responses to climate change. Introduction For most animals, the proximate mechanisms that underlie reduced performance and eventual death at high temperatures, and how such mechanisms might change with ontogeny and context, are uncertain (reviewed in Angilletta 2009; Clark et al. 2013; Pörtner et al. 2017). Such a mechanistic understanding would greatly enhance our ability to predict the effects of realistic high-temperature exposures on individuals or populations, which are difficult to forecast with traditional methods because responses can be highly variable (e.g., Gunderson and Stillman 2015; Seebacher et al. 2015; Kingsolver and Woods 2016; Sheldon and Dillon 2016; Williams et al. 2016). Even so, much progress has been made toward developing mechanistic models that use knowledge of individual physiology to predict the effects of thermal environments on populations, largely inspired by growing concern over the impacts of global climate change (Buckley 2008; Kearney and Porter 2009; Huey et al. 2012; Kearney 2012; Levy et al. 2015; Malishev et al. 2018). Commonly, such models employ empirically-derived thermal performance curves (TPC, Huey and Stevenson 1979, all terms and abbreviations are defined in Box 1) to predict the effects of thermal environments on organisms and populations (e.g. Colwell et al. 2008; Deutsch et al. 2008; Vasseur et al. 2014; Buckley and Huey 2016; Dillon et al. 2016). TPCs describe how performance varies with temperature— typically performance increases with temperature above a critical minimum (CTMIN) until an optimum is reached (TOPT), then rapidly drops to zero at the critical maximum (CTMAX, see Fig. 1 for examples). Pejus (getting worse) temperatures above and below TOPT are described by breakpoints in physiological function (e.g., ventilation rate, heart rate, PO2) indicative of rapid declines in whole-organism performance (TPEJUS, Frederich and Pörtner 2000; Pörtner 2002; Pörtner et al. 2017). Typically, TPCs are estimated under controlled laboratory conditions for a single trait and time, but TPC shape can vary with season, ontogeny, trait, and prior experience in ways that are difficult to predict (Rezende et al. 2014; Telemeco 2014; Kingsolver and Woods 2016; Williams et al. 2016). Thus, using empirically-derived TPCs to predict the effects of natural environments on performance is problematic because it requires extrapolating from the traits or environments originally used to estimate TPCs, thereby ignoring probable context-dependency. A mechanistic understanding of the processes that underlie thermal performance is needed to predict the effects of variable or novel environments on TPC shape, which will greatly improve models relying on TPCs to predict population responses. In particular, knowledge of the mechanisms that result in loss of function at high temperatures is needed to predict the rate at which performance will drop in response to thermal challenge, the capacity for animals to recover from sub-lethal thermal exposure, and the capacity for thermal tolerance to change via plasticity or evolution (Helmuth et al. 2005; Buckley and Huey 2016; Williams et al. 2016). Box 1 List of terms and abbreviations ATP  Adenosine triphosphate  active-aerobic TCRIT  Critical temperature where aerobic respiration is maximized for active individuals (i.e., V˙O2MAX is maximized, ˚C)  resting-aerobic TCRIT  Critical temperature where aerobic respiration would be maximized for resting individuals, assuming they survive to such high temperatures (i.e., V˙O2REST is maximized, ˚C)  subcellular TCRIT  Critical temperature for subcellular function (˚C)  CTMAX  Critical thermal maximum (˚C)  HMTL  Hierarchical Mechanisms of Thermal Limitation hypothesis  HSP  Heat shock protein  MPMO  Multiple Performances – Multiple Optima hypothesis (sensu Clark et al. 2013)  OCLTT  Oxygen- and Capacity-Limited Thermal Tolerance hypothesis (sensu Pörtner 2002 and Pörtner et al. 2017)  TGAPE  Gaping temperature: Temperature at which animals gape to promote evaporative cooling (˚C)  TLETHAL  Lethal temperature: Temperature at which an organism dies under acute exposure (˚C)  TOPT  Optimal temperature for performance (˚C)  TPANT  Panting temperature: Temperature at which animals pant to promote evaporative cooling (˚C)  TPEJUS  Pejus (i.e. getting worse) temperature (˚C, sensu Frederich and Pörtner 2000)  TPC  Thermal performance curve (sensu Huey and Stevenson 1979)  PBT  Preferred body temperature (˚C)  PCTMAX  Oxygen partial pressure below which CTMAX is reduced (kPa) (sensu Ern et al. 2016)  PO2  Partial pressure of oxygen (kPa)  PCO2  Partial pressure of carbon dioxide (kPa)  V˙O2  Oxygen consumption rate (generally mL O2 min−1)  ATP  Adenosine triphosphate  active-aerobic TCRIT  Critical temperature where aerobic respiration is maximized for active individuals (i.e., V˙O2MAX is maximized, ˚C)  resting-aerobic TCRIT  Critical temperature where aerobic respiration would be maximized for resting individuals, assuming they survive to such high temperatures (i.e., V˙O2REST is maximized, ˚C)  subcellular TCRIT  Critical temperature for subcellular function (˚C)  CTMAX  Critical thermal maximum (˚C)  HMTL  Hierarchical Mechanisms of Thermal Limitation hypothesis  HSP  Heat shock protein  MPMO  Multiple Performances – Multiple Optima hypothesis (sensu Clark et al. 2013)  OCLTT  Oxygen- and Capacity-Limited Thermal Tolerance hypothesis (sensu Pörtner 2002 and Pörtner et al. 2017)  TGAPE  Gaping temperature: Temperature at which animals gape to promote evaporative cooling (˚C)  TLETHAL  Lethal temperature: Temperature at which an organism dies under acute exposure (˚C)  TOPT  Optimal temperature for performance (˚C)  TPANT  Panting temperature: Temperature at which animals pant to promote evaporative cooling (˚C)  TPEJUS  Pejus (i.e. getting worse) temperature (˚C, sensu Frederich and Pörtner 2000)  TPC  Thermal performance curve (sensu Huey and Stevenson 1979)  PBT  Preferred body temperature (˚C)  PCTMAX  Oxygen partial pressure below which CTMAX is reduced (kPa) (sensu Ern et al. 2016)  PO2  Partial pressure of oxygen (kPa)  PCO2  Partial pressure of carbon dioxide (kPa)  V˙O2  Oxygen consumption rate (generally mL O2 min−1)  Box 1 List of terms and abbreviations ATP  Adenosine triphosphate  active-aerobic TCRIT  Critical temperature where aerobic respiration is maximized for active individuals (i.e., V˙O2MAX is maximized, ˚C)  resting-aerobic TCRIT  Critical temperature where aerobic respiration would be maximized for resting individuals, assuming they survive to such high temperatures (i.e., V˙O2REST is maximized, ˚C)  subcellular TCRIT  Critical temperature for subcellular function (˚C)  CTMAX  Critical thermal maximum (˚C)  HMTL  Hierarchical Mechanisms of Thermal Limitation hypothesis  HSP  Heat shock protein  MPMO  Multiple Performances – Multiple Optima hypothesis (sensu Clark et al. 2013)  OCLTT  Oxygen- and Capacity-Limited Thermal Tolerance hypothesis (sensu Pörtner 2002 and Pörtner et al. 2017)  TGAPE  Gaping temperature: Temperature at which animals gape to promote evaporative cooling (˚C)  TLETHAL  Lethal temperature: Temperature at which an organism dies under acute exposure (˚C)  TOPT  Optimal temperature for performance (˚C)  TPANT  Panting temperature: Temperature at which animals pant to promote evaporative cooling (˚C)  TPEJUS  Pejus (i.e. getting worse) temperature (˚C, sensu Frederich and Pörtner 2000)  TPC  Thermal performance curve (sensu Huey and Stevenson 1979)  PBT  Preferred body temperature (˚C)  PCTMAX  Oxygen partial pressure below which CTMAX is reduced (kPa) (sensu Ern et al. 2016)  PO2  Partial pressure of oxygen (kPa)  PCO2  Partial pressure of carbon dioxide (kPa)  V˙O2  Oxygen consumption rate (generally mL O2 min−1)  ATP  Adenosine triphosphate  active-aerobic TCRIT  Critical temperature where aerobic respiration is maximized for active individuals (i.e., V˙O2MAX is maximized, ˚C)  resting-aerobic TCRIT  Critical temperature where aerobic respiration would be maximized for resting individuals, assuming they survive to such high temperatures (i.e., V˙O2REST is maximized, ˚C)  subcellular TCRIT  Critical temperature for subcellular function (˚C)  CTMAX  Critical thermal maximum (˚C)  HMTL  Hierarchical Mechanisms of Thermal Limitation hypothesis  HSP  Heat shock protein  MPMO  Multiple Performances – Multiple Optima hypothesis (sensu Clark et al. 2013)  OCLTT  Oxygen- and Capacity-Limited Thermal Tolerance hypothesis (sensu Pörtner 2002 and Pörtner et al. 2017)  TGAPE  Gaping temperature: Temperature at which animals gape to promote evaporative cooling (˚C)  TLETHAL  Lethal temperature: Temperature at which an organism dies under acute exposure (˚C)  TOPT  Optimal temperature for performance (˚C)  TPANT  Panting temperature: Temperature at which animals pant to promote evaporative cooling (˚C)  TPEJUS  Pejus (i.e. getting worse) temperature (˚C, sensu Frederich and Pörtner 2000)  TPC  Thermal performance curve (sensu Huey and Stevenson 1979)  PBT  Preferred body temperature (˚C)  PCTMAX  Oxygen partial pressure below which CTMAX is reduced (kPa) (sensu Ern et al. 2016)  PO2  Partial pressure of oxygen (kPa)  PCO2  Partial pressure of carbon dioxide (kPa)  V˙O2  Oxygen consumption rate (generally mL O2 min−1)  Fig. 1 View largeDownload slide Schematic of the HMTL framework illustrating proposed relationships between body temperature, oxygen environment, metabolism, thermal limits, and performance. The top row of plots displays resting and active metabolic rates as a function of temperature within thermal and physiological limits, and the bottom row displays predicted thermal performance curves. Panel (A) displays these relationships in normoxic environments. Data are means ± s.e.m. for the snake, P. regius, from Fobian et al. (2014). These data were used to fit curves for resting and active metabolic rate, which were then used to estimate TOPT (maximal aerobic scope), resting-aerobic TCRIT, active-aerobic TCRIT, and thermal performance curve shape. The critical thermal maximum is 42˚C (Fobian et al. 2014). Panel (B) Moderate hypoxia (10–20 kPa) is predicted to have no effect on CTMAX or resting metabolism because resting aerobic TCRIT is above subcellular TCRIT. Even so, active-aerobic TCRIT will be reduced thereby lowering aerobic scope, maximal performance, and TOPT. Panel (C) Exposure to extreme hypoxia (< 10 kPa) will reduce CTMAX because resting-aerobic TCRIT drops below subcellular TCRIT. Active-aerobic TCRIT will be strongly affected by such hypoxic environments as well, with large reductions in aerobic scope, maximal performance, and TOPT. In Panels B) and C), gray lines illustrate values predicted for normoxia and are included to aid interpretation of changes predicted to result from reduced environmental oxygen availability. Similarly, arrows in Panels B) and C) indicate predicted displacement of maximum oxygen capacity, resting-aerobic TCRIT, TOPT, and CTMAX values given the oxygen environment. Subcellular TCRIT is not predicted to be affected by oxygen environment and therefore is never displaced. Fig. 1 View largeDownload slide Schematic of the HMTL framework illustrating proposed relationships between body temperature, oxygen environment, metabolism, thermal limits, and performance. The top row of plots displays resting and active metabolic rates as a function of temperature within thermal and physiological limits, and the bottom row displays predicted thermal performance curves. Panel (A) displays these relationships in normoxic environments. Data are means ± s.e.m. for the snake, P. regius, from Fobian et al. (2014). These data were used to fit curves for resting and active metabolic rate, which were then used to estimate TOPT (maximal aerobic scope), resting-aerobic TCRIT, active-aerobic TCRIT, and thermal performance curve shape. The critical thermal maximum is 42˚C (Fobian et al. 2014). Panel (B) Moderate hypoxia (10–20 kPa) is predicted to have no effect on CTMAX or resting metabolism because resting aerobic TCRIT is above subcellular TCRIT. Even so, active-aerobic TCRIT will be reduced thereby lowering aerobic scope, maximal performance, and TOPT. Panel (C) Exposure to extreme hypoxia (< 10 kPa) will reduce CTMAX because resting-aerobic TCRIT drops below subcellular TCRIT. Active-aerobic TCRIT will be strongly affected by such hypoxic environments as well, with large reductions in aerobic scope, maximal performance, and TOPT. In Panels B) and C), gray lines illustrate values predicted for normoxia and are included to aid interpretation of changes predicted to result from reduced environmental oxygen availability. Similarly, arrows in Panels B) and C) indicate predicted displacement of maximum oxygen capacity, resting-aerobic TCRIT, TOPT, and CTMAX values given the oxygen environment. Subcellular TCRIT is not predicted to be affected by oxygen environment and therefore is never displaced. Potential mechanisms explaining why animals lose function at high temperatures can be divided into two major categories differing in the level of organization first affected. At lower levels of organization, subcellular components could be critically impaired when animals are exposed to temperatures above the optimum. Subcellular impairment results from either denaturation of key molecules, such as proteins and cell membranes, or reduced efficiency of these molecules to perform their biological functions (reviewed in Hochachka and Somero 2002; Angilletta 2009; Schulte 2015). Impairment of subcellular components would result in the breakdown of higher levels of organization and lead to rapid performance loss. Alternatively, higher-levels of organization, such as organ systems, could be impaired at temperatures below those that directly affect the performance of their subcellular components if high temperatures disrupt subcellular interactions or pathways necessary for organ system function. A recent mechanistic model explaining organ-system impairment at high temperatures is the oxygen and capacity limited thermal tolerance (OCLTT) hypothesis, which proposes that oxygen demand for aerobic metabolism at high temperatures outpaces the ability of the cardiovascular and respiratory systems to provide sufficient amounts of oxygen to tissues (Pörtner 2002; Pörtner and Knust 2007; Verberk et al. 2016; Pörtner et al. 2017). Under this hypothesis, oxygen diffusion and transport capacity limit metabolic rates such that resting rates converge with potential maximum rates at high temperatures thereby reducing aerobic scope to zero (i.e., zero aerobic power budget sensu Pörtner et al. 2017). This mismatch in supply and demand will first reduce aerobic performance at TPEJUS but eventually result in basal oxygen demands not being met and thus collapse of organismal systems at CTMAX. Proponents of OCLTT argue that this mechanism integrates across levels of organization from systemic to molecular mechanisms and can explain diverse evolutionary and ecological phenomena (Pörtner 2002; Pörtner et al. 2017). However, the temporal window, activity range, magnitude, and biological relevancy of reduced aerobic scope at high temperature are debated (Clark et al. 2013; Gräns et al. 2014; Jutfelt et al. 2014; Pörtner 2014; Verberk et al. 2016; Jutfelt et al. 2018). Identifying the level of organization first impaired by high temperature is necessary to understand how temperature affects whole-organism function. Recent work exploring the mechanisms underlying thermal tolerance in ectotherms focuses on aquatic taxa (e.g., Frederich and Pörtner 2000; Lucassen et al. 2006; Verberk et al. 2013; Gräns et al. 2014; Ern et al. 2016; Pörtner and Gutt 2016; Verberk et al. 2018) and terrestrial arthropods, particularly insects (reviewed in McCue and De Los Santos 2013; Verberk et al. 2016). These observations suggest that subcellular-level mechanisms are more important determinants of thermal tolerance than oxygen capacity limitations in terrestrial taxa, perhaps because oxygen is more abundant in air than in water. However, recent work includes few studies of terrestrial vertebrate ectotherms: non-avian reptiles (hereafter, reptiles) and amphibians. Reptiles and amphibians display varied ecology, life-history, and thermal tolerance (Vitt and Caldwell 2009; Sunday et al. 2011) and are important for a broad understanding of thermal tolerance. Additionally, these animals frequently contend with challenging temperatures, with some species inhabiting the hottest terrestrial environments (Cowles and Bogert 1944; Vitt and Caldwell 2009; Sunday et al. 2014). Moreover, both the thermal and oxygen environment can vary throughout ontogeny (e.g., aquatic to terrestrial transition in amphibians), and modes of respiration can vary (e.g., cutaneous respiration in many amphibians and some reptiles, such as turtles; Hutchison et al. 1968; Glass et al. 1981; Wang 2011). Because of these traits, reptiles and amphibians have been important models for thermal physiology and ecology for the last century (e.g., Cowles and Bogert 1944; Snyder and Weathers 1975; Huey and Stevenson 1979; Huey 1982; Huey and Berrigan 2001; Vitt and Caldwell 2009; Kearney 2012). This historic work can be leveraged to address modern ideas, such as the OCLTT hypothesis. Furthermore, these taxa are at risk of high-temperature induced extinction resulting from global climate change (Thomas et al. 2004; Huey et al. 2010; Rohr and Raffel 2010; Sinervo et al. 2010; Kearney 2013). Here, we apply the rich history of physiological studies in reptiles and amphibians to modern hypotheses for mechanisms underlying thermal performance and tolerance. This will complete taxonomic coverage of recent reviews, and, more importantly, further understanding of how organismal performance and survival are restricted by high temperatures. In this commentary, we begin by synthesizing evidence for subcellular- and organ-system-level failures underlying high temperature tolerance and performance in reptiles and amphibians. Because neither framework satisfactorily explains observed patterns individually, we next build upon work in other taxa to present an integrative framework combining both subcellular and organ-system sensitivity to high temperatures and the thermal performance curve paradigm. Using data for reptiles and amphibians, we illustrate how this combined framework appears to predict a broad array of physiological and behavioral observations. We conclude by identifying future research directions to test this framework and discuss how it might be applied to other systems to predict consequences of future high-temperature exposure for organisms and populations. Evidence for mechanisms underlying thermal tolerance and performance in reptiles and amphibians Subcellular mechanisms High temperatures compromise the structure and function of subcellular components, such as enzymes and cell membranes (Fields 2001; Hochachka and Somero 2002) but such damage will only underlie organismal thermal tolerance if it occurs at temperatures below those affecting function at higher levels of organization, such as organ systems (Pörtner 2002). The complexities of enzymatic interactions make extending observations for individual reactions to the whole cell or organism potentially problematic (Schulte 2015). Nonetheless, the observation that subcellular components are frequently stable to temperatures above the critical and lethal limits of animals (hereafter, CTMAX and TLETHAL), commonly maintaining function to temperatures >50˚C and capable of evolving stability to 120˚C (Fields 2001), was an important motivation for the development of hypotheses such as OCLTT (Pörtner 2001; 2002; Pörtner et al. 2017). For example, CTMAX in reptiles and amphibians generally ranges from mid-30s to low 40s ˚C (Brattstrom 1965; Sunday et al. 2011, 2014), although a few warm-adapted reptiles tolerate acute exposure to 47.5˚C such as the desert iguana (Dipsosaurus dorsalis) (Cowles and Bogert 1944; Brattstrom 1965). Even so, most reptile and amphibian proteins do not lose function until they experience temperatures well above CTMAX. For example, ribonucleases from three frog species were stable up to 85˚C and maintained high activity at temperatures >50˚C (Irie et al. 1998), lactate dehydrogenase in Agama stellio lizards maintained both stability and function up to 70˚C (Al-Jassabi 2002), and alkaline phosphatase maintained high function up to 50˚C in four lizard species (Licht 1964). Similarly, acute exposure to CTMAX did not cause tissue damage or reduce function of serum glutamic-oxaloacetic or glutamic-pyruvic transaminases in Great Plains toads (Anaxyrus cognatus, Paulson and Hutchinson 1987) and 2.5-h exposure to temperatures just below CTMAX had no effect on mitochondrial respiration or free-radical production in alligator lizards (Elgaria coerulea and E. multicarinata; Telemeco et al. 2017), implying no subcellular damage. That said, only one key component needs to lose function for the entire organism to become compromised. For example, the activity of myosin ATPase, a key enzyme for organismal muscle function, closely resembled whole-organism thermal performance curves and denatured at relatively low temperatures in eight lizard species corresponding closely to their respective CTMAX (20% denatured between 37˚C and 45.2˚C; Licht 1964). Subcellular components such as ATPase could underlie thermal tolerance even though most components maintain function to higher temperature. Given the complexities of subcellular interactions and the paucity of data for the thermal performance of subcellular components in reptiles and amphibians, heat shock protein (HSP) production might better indicate whether subcellular-level components are challenged at sub-critical high temperatures. HSPs commonly act as molecular chaperones, maintaining protein structure and preventing aggregations of denatured proteins, and inducible variants are produced in response to cellular stress or damage (Fernando and Heikkila 2000; Kregel 2002; Daugaard et al. 2007). In diverse reptiles and amphibians, HSPs (particularly HSPA family members) are produced in response to sub-critical high temperatures, and production of HSPs can allow acclimation for increased thermal tolerance (Ulmasov et al. 1992; Fernando and Heikkila 2000; Zatespina et al. 2000; McMillan et al. 2011; Gao et al. 2014; Simoniello et al. 2016; Tedeschi et al. 2016). Moreover, patterns of HSP expression are correlated with thermal tolerance in lizards: warm-adapted species display higher constitutive HSP concentrations, and both initiate and maintain synthesis of HSPs to higher temperatures than more cold-adapted species (Ulmasov et al. 1992; Zatespina et al. 2000). This pattern of increased HSP production correlating with increases in thermal tolerance in reptiles and amphibians, along with observations for myosin ATPase in lizards, provides compelling evidence for subcellular-function loss playing a role in setting thermal limits, despite other subcellular components displaying little loss of function at relevant temperatures. Organ-system mechanisms Even though subcellular-level traits will be compromised at sufficiently high temperatures, higher-order systems might break down at lower temperatures and thus be proximally responsible for setting thermal tolerances (Pörtner 2001; 2002; Storch et al. 2014, but see Clark et al. 2013). The OCLTT hypothesis proposes that the highest organizational level in animals is the integrated cardiovascular and respiratory system because all tissues will be limited by their ability to acquire oxygen for respiration, and that this system is compromised by high temperatures prior to other systems (Pörtner 2001; 2002; Storch et al. 2014). However, similar to other terrestrial species (Klok et al. 2004; McCue and De Los Santos 2013; Verberk et al. 2016) and many fish (Clark et al. 2013; Gräns et al. 2014; Norin et al. 2014; Wang et al. 2014; Ern et al. 2016), evidence for the OCLTT mechanism underlying thermal tolerance in reptiles and amphibians is limited. Under the OCLTT hypothesis, maximal and resting rates of oxygen consumption are expected to converge as animals reach their physiological limits at high temperatures, thereby reducing aerobic scope and potentially inducing a short-term reliance on anaerobic respiration (Frederich and Pörtner 2000; Pörtner and Knust 2007; Elaison et al. 2011; Verberk et al. 2013, 2016; Table 1). Some evidence points to such a mechanism playing an important role in early animal evolution, notably in the transition to air breathing (Berner et al. 2007; Giomi et al. 2014; Teague et al. 2017). However, the few studies exposing animals to high temperatures and measuring indicators of aerobic and anaerobic respiration fail to find evidence for oxygen limitation in adult reptiles and amphibians (Carey 1979; Overgaard et al. 2012; Fobian et al. 2014; Gangloff et al. 2016; Telemeco et al. 2017). For example, oxygen consumption ( V˙O2) by pythons (Python regius) did not plateau at temperatures approaching CTMAX either when at rest or during periods of high metabolic demand (Fobian et al. 2014, Fig. 1), and resting oxygen consumption in garter snakes (Thamnophis elegans) increased with temperature with no apparent limit when animals experienced near-lethal temperatures (Gangloff et al. 2016). Moreover, neither garter snakes (T. elegans) nor alligator lizards (E. coerulea and E. multicarinata) transitioned to anaerobic respiration when exposed to near-critical temperatures (Gangloff et al. 2016; Telemeco et al. 2017), despite snakes and lizards rapidly transitioning when oxygen availability is limited during exercise (reviewed in Gleeson 1991). Observations in amphibians are similar to those for reptiles. For example, oxygen consumption, arterial oxygen saturation, and the proportion of saturated hemoglobin did not plateau at high temperatures in active or resting cane toads (Rhinella marina), thus providing evidence for these toads’ ability to maintain a positive aerobic power budget at near-critical temperatures (Seebacher and Franklin 2011; Overgaard et al. 2012; Winwood-Smith et al. 2015). In both the boreal toad (Anaxyrus boreas) and leopard frog (Lithobates pipiens), aerobic scope increased with temperature up to 30°C (Carey 1979). Whole-organism lactate concentration also increased with temperature, but there is no indication that either species becomes oxygen limited up to at least 30°C (Carey 1979). These results are in line with previous work showing that amphibians can maintain substantial aerobic scope at temperatures above active and preferred temperatures, although not necessarily at temperatures approaching TLETHAL (Whitford 1973). While such studies provide strong evidence for adult reptiles and amphibians maintaining aerobic scope at high temperatures, we currently lack data on tissue and cellular oxygen supply, such as venous PO2, which would more directly test the mechanisms described by the OCLTT hypothesis (Pörtner et al. 2017, 2018). Table 1 Experimental designs for testing aspects of the HMTL hypothesis in reptiles and amphibians with example studies Experiment type: Factor manipulated  Manipulation  Dependent variable  What it demonstrates  Examples  Temperature  Temperature treatments or ramp in lab  Oxygen capacity parameters ( V˙O2, Active V˙O2, alveolar/arterial PO2, heart rate), lactate production  Maintenance of aerobic scope at high temperatures; No evidence of transition to anaerobic respiration  Carey (1979), Seebacher and Franklin (2011), Overgaard et al. (2012), Fobian et al. (2014), Gangloff et al. (2016)  Temperature treatments in lab  Skeletal muscle metabolome, mitochondrial function  No transition from aerobic to anaerobic metabolism or subcellular damage at high temperatures  Telemeco et al. (2017)  Ex vivo temperature treatments  Enzyme activity  Temperature where subcellular components lose function  Licht (1964), Paulson and Hutchinson (1987), Irie et al. (1998), Al-Jassabi (2002)  Temperature treatments in lab  HSP induction  Temperature that induces a subcellular-protection response  Ulmasov et al. (1992), Fernando and Heikkila (2000), Zatespina et al. (2000), McMillan et al. (2011), Gao et al. (2014), Simoniello et al. (2016), Tedeschi et al. (2016)  Ambient oxygen  Oxygen treatments in lab  Oxygen capacity parameters ( V˙O2, heart rate, alveolar/arterial PO2, ventilation rate)  Aerobic capacity is maintained under conditions of mild hypoxia, but is limited under extreme hypoxia  Boyer (1963, 1966), Withers and Hillman (1983), Pörtner et al. (1991), Branco et al. (1993), Wang et al. (1994)  Oxygen treatments in lab  PBT  PBT is unaffected by mild hypoxia, but is reduced under extreme hypoxia  Hicks and Wood (1985), Branco et al. (1993), Cadena and Tattersall (2009)  Both ambient oxygen levels and temperature  Temperature and oxygen treatments in lab  Embryo development and survival  Hyperoxia increases survivorship while hypoxia reduces survivorship at high temperatures; Hypoxia reduces development, growth, and hatchling performance at high temperatures  Flewelling and Parker (2015), Liang et al. (2015), Smith et al. (2015)  Temperature gradient and/or ramp and oxygen treatments in lab  CTMAX, TGAPE, TPANT  Behavioral responses to high temperatures depend on ambient O2, but only under extreme hypoxia  Dupre et al. (1986), Tattersall and Gerlach (2005), Shea et al. (2016)  Transplant across altitudinal gradients within species’ ranges; Manipulation of ambient O2 in field; Temperature ramp  CTMAX, TGAPE, TPANT  Behavioral responses to high temperatures depend on ambient O2, but only under extreme hypoxia  DuBois et al. (2017)  Oxygen capacity  Hematocrit reduction  PBT  Reduced oxygen capacity affects temperature perception and animals choose lower temperatures  Wood (1990), Hicks and Wood (1985)  Blood volume reduction  Oxygen capacity parameters (heart rate, alveolar/arterial PO2, ventilation rate)  Heart rate increases while ventilation rate is unchanged by reduced oxygen carrying capacity  Wang et al. (1994)  Observational  –  Quantification of CTMAX across life stages  Oxygen capacity limits thermal tolerance at some developmental stages in larval anurans  Cupp (1980), Sherman (1980), Floyd (1983)  –  Comparison of PBT, resting V˙O2, Active V˙O2,  PBT matches temperature of maximal aerobic scope in lizards  Wilson (1974)  Experiment type: Factor manipulated  Manipulation  Dependent variable  What it demonstrates  Examples  Temperature  Temperature treatments or ramp in lab  Oxygen capacity parameters ( V˙O2, Active V˙O2, alveolar/arterial PO2, heart rate), lactate production  Maintenance of aerobic scope at high temperatures; No evidence of transition to anaerobic respiration  Carey (1979), Seebacher and Franklin (2011), Overgaard et al. (2012), Fobian et al. (2014), Gangloff et al. (2016)  Temperature treatments in lab  Skeletal muscle metabolome, mitochondrial function  No transition from aerobic to anaerobic metabolism or subcellular damage at high temperatures  Telemeco et al. (2017)  Ex vivo temperature treatments  Enzyme activity  Temperature where subcellular components lose function  Licht (1964), Paulson and Hutchinson (1987), Irie et al. (1998), Al-Jassabi (2002)  Temperature treatments in lab  HSP induction  Temperature that induces a subcellular-protection response  Ulmasov et al. (1992), Fernando and Heikkila (2000), Zatespina et al. (2000), McMillan et al. (2011), Gao et al. (2014), Simoniello et al. (2016), Tedeschi et al. (2016)  Ambient oxygen  Oxygen treatments in lab  Oxygen capacity parameters ( V˙O2, heart rate, alveolar/arterial PO2, ventilation rate)  Aerobic capacity is maintained under conditions of mild hypoxia, but is limited under extreme hypoxia  Boyer (1963, 1966), Withers and Hillman (1983), Pörtner et al. (1991), Branco et al. (1993), Wang et al. (1994)  Oxygen treatments in lab  PBT  PBT is unaffected by mild hypoxia, but is reduced under extreme hypoxia  Hicks and Wood (1985), Branco et al. (1993), Cadena and Tattersall (2009)  Both ambient oxygen levels and temperature  Temperature and oxygen treatments in lab  Embryo development and survival  Hyperoxia increases survivorship while hypoxia reduces survivorship at high temperatures; Hypoxia reduces development, growth, and hatchling performance at high temperatures  Flewelling and Parker (2015), Liang et al. (2015), Smith et al. (2015)  Temperature gradient and/or ramp and oxygen treatments in lab  CTMAX, TGAPE, TPANT  Behavioral responses to high temperatures depend on ambient O2, but only under extreme hypoxia  Dupre et al. (1986), Tattersall and Gerlach (2005), Shea et al. (2016)  Transplant across altitudinal gradients within species’ ranges; Manipulation of ambient O2 in field; Temperature ramp  CTMAX, TGAPE, TPANT  Behavioral responses to high temperatures depend on ambient O2, but only under extreme hypoxia  DuBois et al. (2017)  Oxygen capacity  Hematocrit reduction  PBT  Reduced oxygen capacity affects temperature perception and animals choose lower temperatures  Wood (1990), Hicks and Wood (1985)  Blood volume reduction  Oxygen capacity parameters (heart rate, alveolar/arterial PO2, ventilation rate)  Heart rate increases while ventilation rate is unchanged by reduced oxygen carrying capacity  Wang et al. (1994)  Observational  –  Quantification of CTMAX across life stages  Oxygen capacity limits thermal tolerance at some developmental stages in larval anurans  Cupp (1980), Sherman (1980), Floyd (1983)  –  Comparison of PBT, resting V˙O2, Active V˙O2,  PBT matches temperature of maximal aerobic scope in lizards  Wilson (1974)  Table 1 Experimental designs for testing aspects of the HMTL hypothesis in reptiles and amphibians with example studies Experiment type: Factor manipulated  Manipulation  Dependent variable  What it demonstrates  Examples  Temperature  Temperature treatments or ramp in lab  Oxygen capacity parameters ( V˙O2, Active V˙O2, alveolar/arterial PO2, heart rate), lactate production  Maintenance of aerobic scope at high temperatures; No evidence of transition to anaerobic respiration  Carey (1979), Seebacher and Franklin (2011), Overgaard et al. (2012), Fobian et al. (2014), Gangloff et al. (2016)  Temperature treatments in lab  Skeletal muscle metabolome, mitochondrial function  No transition from aerobic to anaerobic metabolism or subcellular damage at high temperatures  Telemeco et al. (2017)  Ex vivo temperature treatments  Enzyme activity  Temperature where subcellular components lose function  Licht (1964), Paulson and Hutchinson (1987), Irie et al. (1998), Al-Jassabi (2002)  Temperature treatments in lab  HSP induction  Temperature that induces a subcellular-protection response  Ulmasov et al. (1992), Fernando and Heikkila (2000), Zatespina et al. (2000), McMillan et al. (2011), Gao et al. (2014), Simoniello et al. (2016), Tedeschi et al. (2016)  Ambient oxygen  Oxygen treatments in lab  Oxygen capacity parameters ( V˙O2, heart rate, alveolar/arterial PO2, ventilation rate)  Aerobic capacity is maintained under conditions of mild hypoxia, but is limited under extreme hypoxia  Boyer (1963, 1966), Withers and Hillman (1983), Pörtner et al. (1991), Branco et al. (1993), Wang et al. (1994)  Oxygen treatments in lab  PBT  PBT is unaffected by mild hypoxia, but is reduced under extreme hypoxia  Hicks and Wood (1985), Branco et al. (1993), Cadena and Tattersall (2009)  Both ambient oxygen levels and temperature  Temperature and oxygen treatments in lab  Embryo development and survival  Hyperoxia increases survivorship while hypoxia reduces survivorship at high temperatures; Hypoxia reduces development, growth, and hatchling performance at high temperatures  Flewelling and Parker (2015), Liang et al. (2015), Smith et al. (2015)  Temperature gradient and/or ramp and oxygen treatments in lab  CTMAX, TGAPE, TPANT  Behavioral responses to high temperatures depend on ambient O2, but only under extreme hypoxia  Dupre et al. (1986), Tattersall and Gerlach (2005), Shea et al. (2016)  Transplant across altitudinal gradients within species’ ranges; Manipulation of ambient O2 in field; Temperature ramp  CTMAX, TGAPE, TPANT  Behavioral responses to high temperatures depend on ambient O2, but only under extreme hypoxia  DuBois et al. (2017)  Oxygen capacity  Hematocrit reduction  PBT  Reduced oxygen capacity affects temperature perception and animals choose lower temperatures  Wood (1990), Hicks and Wood (1985)  Blood volume reduction  Oxygen capacity parameters (heart rate, alveolar/arterial PO2, ventilation rate)  Heart rate increases while ventilation rate is unchanged by reduced oxygen carrying capacity  Wang et al. (1994)  Observational  –  Quantification of CTMAX across life stages  Oxygen capacity limits thermal tolerance at some developmental stages in larval anurans  Cupp (1980), Sherman (1980), Floyd (1983)  –  Comparison of PBT, resting V˙O2, Active V˙O2,  PBT matches temperature of maximal aerobic scope in lizards  Wilson (1974)  Experiment type: Factor manipulated  Manipulation  Dependent variable  What it demonstrates  Examples  Temperature  Temperature treatments or ramp in lab  Oxygen capacity parameters ( V˙O2, Active V˙O2, alveolar/arterial PO2, heart rate), lactate production  Maintenance of aerobic scope at high temperatures; No evidence of transition to anaerobic respiration  Carey (1979), Seebacher and Franklin (2011), Overgaard et al. (2012), Fobian et al. (2014), Gangloff et al. (2016)  Temperature treatments in lab  Skeletal muscle metabolome, mitochondrial function  No transition from aerobic to anaerobic metabolism or subcellular damage at high temperatures  Telemeco et al. (2017)  Ex vivo temperature treatments  Enzyme activity  Temperature where subcellular components lose function  Licht (1964), Paulson and Hutchinson (1987), Irie et al. (1998), Al-Jassabi (2002)  Temperature treatments in lab  HSP induction  Temperature that induces a subcellular-protection response  Ulmasov et al. (1992), Fernando and Heikkila (2000), Zatespina et al. (2000), McMillan et al. (2011), Gao et al. (2014), Simoniello et al. (2016), Tedeschi et al. (2016)  Ambient oxygen  Oxygen treatments in lab  Oxygen capacity parameters ( V˙O2, heart rate, alveolar/arterial PO2, ventilation rate)  Aerobic capacity is maintained under conditions of mild hypoxia, but is limited under extreme hypoxia  Boyer (1963, 1966), Withers and Hillman (1983), Pörtner et al. (1991), Branco et al. (1993), Wang et al. (1994)  Oxygen treatments in lab  PBT  PBT is unaffected by mild hypoxia, but is reduced under extreme hypoxia  Hicks and Wood (1985), Branco et al. (1993), Cadena and Tattersall (2009)  Both ambient oxygen levels and temperature  Temperature and oxygen treatments in lab  Embryo development and survival  Hyperoxia increases survivorship while hypoxia reduces survivorship at high temperatures; Hypoxia reduces development, growth, and hatchling performance at high temperatures  Flewelling and Parker (2015), Liang et al. (2015), Smith et al. (2015)  Temperature gradient and/or ramp and oxygen treatments in lab  CTMAX, TGAPE, TPANT  Behavioral responses to high temperatures depend on ambient O2, but only under extreme hypoxia  Dupre et al. (1986), Tattersall and Gerlach (2005), Shea et al. (2016)  Transplant across altitudinal gradients within species’ ranges; Manipulation of ambient O2 in field; Temperature ramp  CTMAX, TGAPE, TPANT  Behavioral responses to high temperatures depend on ambient O2, but only under extreme hypoxia  DuBois et al. (2017)  Oxygen capacity  Hematocrit reduction  PBT  Reduced oxygen capacity affects temperature perception and animals choose lower temperatures  Wood (1990), Hicks and Wood (1985)  Blood volume reduction  Oxygen capacity parameters (heart rate, alveolar/arterial PO2, ventilation rate)  Heart rate increases while ventilation rate is unchanged by reduced oxygen carrying capacity  Wang et al. (1994)  Observational  –  Quantification of CTMAX across life stages  Oxygen capacity limits thermal tolerance at some developmental stages in larval anurans  Cupp (1980), Sherman (1980), Floyd (1983)  –  Comparison of PBT, resting V˙O2, Active V˙O2,  PBT matches temperature of maximal aerobic scope in lizards  Wilson (1974)  A second approach for testing the influence of oxygen capacity on thermal constraints is to manipulate oxygen availability and examine changes in thermal behavior or tolerance (Table 1). Under the OCLTT hypothesis, hypoxia is predicted to reduce thermal optima and tolerance limits (Smith et al. 2015; Verberk et al. 2016; DuBois et al. 2017), and this prediction is somewhat supported in reptiles and amphibians. For example, CTMAX, preferred body temperature (PBT), panting temperature (TPANT), and gaping temperature (TGAPE) are reduced when diverse species are exposed to very-low oxygen environments (<10 kPa; mostly lizards examined; Hicks and Wood 1985; Dupre et al. 1986; Branco et al. 1993; Cadena and Tattersall 2009; Shea et al. 2016; DuBois et al. 2017; Fig. 2) and when hematocrit is experimentally reduced (only PBT examined; Hicks and Wood 1985; Wood 1990). Hypoxia-induced PBT reduction is a well-described phenomenon in ectotherms, termed “behavioral anapyrexia,” that allows adaptive reduction of metabolic demand when oxygen is limited (Hicks and Wood 1985; Wood and Gonzales 1996; Steiner and Branco 2002; Hicks and Wang 2004), supporting the hypothesis that oxygen limitation influences thermal preference and possibly tolerance. Along with CTMAX, reductions in TGAPE and TPANT, which provide an indication of perceived heat stress (Heatwole et al. 1973; Tattersall et al. 2006; DuBois et al. 2017), imply reduced thermal tolerance under hypoxia. Hypoxia also induces elevated heart rates and reduces both resting and active oxygen consumption ( V˙O2) in diverse species (Fig. 3), suggesting observed shifts in thermal tolerance and behavior are related to physiological limits of oxygen capacity (including diffusion and transport). Interestingly, only extreme hypoxia had the predicted effects on thermal tolerance and behavior, with levels of hypoxia within the range generally found in terrestrial environments having no effect (Figs. 2 and 3). Thus, it is not clear that the OCLTT mechanism will be generally relevant in nature. Fig. 2 View largeDownload slide Thermal preference and tolerance are unaffected by moderate hypoxia but reduced in extremely hypoxic environments (<10 kPa). This “broken-stick” pattern is apparent for (A) PBT (i.e. behavioral anapyrexia), (B) critical thermal maximum, (C) gaping temperature, and (D) panting temperature across species. Data are mostly available for lizards (solid lines) but PBT data are also available for alligator (dashed line). Lines between points are included to aid visualization, but connections between values at normoxia and hypoxia are likely nonlinear. Colors (online only) and initials denote species: A.m., Alligator mississippiensis; B.v., Basiliscus vittatus; C.p., Ctenosaura pectinata; I.i., Iguana iguana; P.v., Pogona vitticeps; S.g., Sceloporus graciosus; S.o., Sceloporus occidentalis; V.e., Varanus exanthematicus; V.v., Varanus varius. Data are means ± s.e.m. derived from the literature. See Supplementary Table S1 for data and citations. Fig. 2 View largeDownload slide Thermal preference and tolerance are unaffected by moderate hypoxia but reduced in extremely hypoxic environments (<10 kPa). This “broken-stick” pattern is apparent for (A) PBT (i.e. behavioral anapyrexia), (B) critical thermal maximum, (C) gaping temperature, and (D) panting temperature across species. Data are mostly available for lizards (solid lines) but PBT data are also available for alligator (dashed line). Lines between points are included to aid visualization, but connections between values at normoxia and hypoxia are likely nonlinear. Colors (online only) and initials denote species: A.m., Alligator mississippiensis; B.v., Basiliscus vittatus; C.p., Ctenosaura pectinata; I.i., Iguana iguana; P.v., Pogona vitticeps; S.g., Sceloporus graciosus; S.o., Sceloporus occidentalis; V.e., Varanus exanthematicus; V.v., Varanus varius. Data are means ± s.e.m. derived from the literature. See Supplementary Table S1 for data and citations. Fig. 3 View largeDownload slide As the environment becomes increasingly hypoxic, (A) arterial PO2 decreases, whereas (B) resting metabolic rate, (C) heart rate, and (D) maximal metabolic rate are relatively unaffected until the environment becomes extremely hypoxic (<10 kPa), similar to thermal preference and tolerance (see Fig. 2 for comparison). Colors (online only) denote taxonomic group whereas initials denote species: A.c., Anaxyrus cognatus (formerly Bufo cognatus); A.m., Alligator mississippiensis; C.s., Chelydra serpentina; D.d, Dipsosaurus dorsalis; L.p., Lithobates pipiens (formerly Rana pipiens); P.m., Pituophis melanoleucus; R.m., Rhinella marina (formerly Bufo marinus); R.s., Rhinella schneideri (formerly Bufo paracnemis). A.c., L.p., R.s., and R.m. are frogs, D.d. is a lizard, P.m. is a snake, C.s. is a turtle, and A.m. is an alligator. Experimental temperature is given to the right of the species’ initials. Data are means ± s.e.m. derived from the literature. See Supplementary Table S1 for data and citations. Fig. 3 View largeDownload slide As the environment becomes increasingly hypoxic, (A) arterial PO2 decreases, whereas (B) resting metabolic rate, (C) heart rate, and (D) maximal metabolic rate are relatively unaffected until the environment becomes extremely hypoxic (<10 kPa), similar to thermal preference and tolerance (see Fig. 2 for comparison). Colors (online only) denote taxonomic group whereas initials denote species: A.c., Anaxyrus cognatus (formerly Bufo cognatus); A.m., Alligator mississippiensis; C.s., Chelydra serpentina; D.d, Dipsosaurus dorsalis; L.p., Lithobates pipiens (formerly Rana pipiens); P.m., Pituophis melanoleucus; R.m., Rhinella marina (formerly Bufo marinus); R.s., Rhinella schneideri (formerly Bufo paracnemis). A.c., L.p., R.s., and R.m. are frogs, D.d. is a lizard, P.m. is a snake, C.s. is a turtle, and A.m. is an alligator. Experimental temperature is given to the right of the species’ initials. Data are means ± s.e.m. derived from the literature. See Supplementary Table S1 for data and citations. In contrast to adult stages, naturalistic hypoxia reduces thermal performance and tolerance in eggs and larvae of reptiles and amphibians. Because amphibian eggs and larvae inhabit aqueous environments, environmental oxygen availability is reduced relative to terrestrial stages, thereby increasing the potential importance of the OCLTT mechanism similar to some fully aquatic ectotherms (e.g., Pörtner and Knust 2007; Verberk et al. 2013; Pörtner and Gutt 2016). Oxygen limitation is further exacerbated when species develop in small water bodies that can rapidly lose dissolved oxygen, such as ephemeral pools (Seymour and Bradford 1995; Sacerdote and King 2009), and because amphibian eggs are commonly produced within large gelatinous masses with low oxygen diffusion potential (Pinder and Friet 1994; Woods 1999; Sacerdote and King 2009). Although data are limited, these constraints appear to affect early stage amphibians congruent with oxygen limiting thermal tolerances. For example, amphibians that nest in warm water produce smaller eggs and egg masses to facilitate oxygen diffusion to embryos (Woods 1999; Sacerdote and King 2009), and aquatic larvae born into warm water (from aquatic or terrestrial eggs) are smaller and have lower metabolic demands (Kuramoto 1975; Rollinson and Rowe 2018). In oviparous reptiles, embryonic gas exchange occurs via passive diffusion across the shell into the chorioallantoic membrane, which is much less efficient than adult respiration (Vitt and Caldwell 2009). As predicted by the OCLTT hypothesis, thermal tolerance is reduced when developing embryos experience modest to extreme hypoxia (Flewelling and Parker 2015; Liang et al. 2015; Smith et al. 2015). For example, lower environmental PO2 experienced at high elevations reduces TLETHAL in plateau fence lizard (Sceloporus tristichus) embryos compared to PO2 at sea level (Smith et al. 2015), although such modest hypoxia does not affect the thermal tolerance of adult congeners (S. occidentalis and S. graciosus; Shea et al. 2016; DuBois et al. 2017). Thus, available data suggest that reptiles and amphibians experience ontogenetic shifts in the proximate mechanism underlying thermal tolerance: early stages appear more affected by organ-system-level failures, such as the OCLTT mechanism, whereas later stages are more impacted by subcellular mechanisms. An integrative framework: HMTL Available evidence implies that neither failure of subcellular components nor oxygen and capacity limitation are solely responsible for loss of performance at high temperatures in reptiles and amphibians. Rather, mechanisms across levels of organization appear to play partial, context-dependent roles. We propose a unified framework that combines subcellular mechanisms, the oxygen and capacity limited thermal tolerance (OCLTT) hypothesis, and the thermal performance curve (TPC) paradigm. We call this integrative framework “Hierarchical Mechanisms of Thermal Limitation” (HMTL, Fig. 1). Like the OCLTT hypothesis, we propose that oxygen diffusion and transport capacity largely shape thermal performance at sub-critical temperatures via effects of temperature on aerobic scope. However, the HMTL framework explicitly recognizes the importance of subcellular-level mechanisms and allows them to underlie absolute thermal limits, such as CTMAX and TLETHAL. Under this hypothesis, the relative importance of subcellular- and organ-system-level mechanisms on thermal tolerance is context dependent, but predictable. We propose that the HMTL framework explains diverse empirical results and can bring together decades of physiological studies in reptiles and amphibians. Given that numerous models of thermal performance and tolerance have been proposed, we think that it is useful to state their similarities and differences with HMTL. The HMTL framework builds upon ideas developed for the OCLTT hypothesis proposed by Pörtner and colleagues (recently reviewed in Pörtner et al. 2017) and is similar to that put forth by Ern et al. (2016) for fishes (see Fig. 1 in Ern et al. 2016), but more formally incorporates the TPC paradigm and explicitly describes mechanism-dependent critical temperatures. Importantly, this hypothesis predicts conditions under which subcellular and organ-system failure will set thermal tolerance limits and when transitions are likely to occur. Our framework also differs importantly from the Multiple Performances—Multiple Optima (MPMO) framework put forth by Clark et al. (2013) for fishes. The MPMO suggests a single thermal limit set by an undefined mechanism but multiple, trait-specific optima that can dramatically differ from the temperature that maximizes aerobic scope (summarized in Fig. 7 of Clark et al. 2013). By contrast, we propose multiple potential mechanisms of thermal limitation but a single whole-organism thermal optimum for performance correlated with the temperature that maximizes aerobic scope (although this will be an integration across traits). Figure 1 illustrates the HMTL framework. In the top row of panels, solid blue and red lines represent the effect of temperature on oxygen consumption during rest and activity, respectively. Assuming activity is maximal, aerobic scope is the difference between these lines. To illustrate this concept with empirical data, we derived these lines by fitting a generalized logistic function to data from Fobian et al. (2014) for pythons (P.regius) assuming the shape of the response curve is the same during rest and activity. Based on available data, the general shape of these curves is qualitatively similar across diverse taxa (e.g., Carey 1979; Frederich and Pörtner 2000; Overgaard 2012; Fobian et al. 2014; Ern et al. 2016) and we think that the predictions of the HMTL framework can be generalized beyond the specific data used to generate this illustration. The dotted blue and red lines depict exponential functions fit to these data and illustrate predicted oxygen demand/use if organisms were unconstrained by capacity limitations. The columns illustrate predicted effects of variable oxygen environments, and the bottom row displays TPCs predicted to result given relationships in the top row. A central assumption of the HMTL framework is that critical thermal limits exist for both subcellular mechanisms (subcellular TCRIT) and aerobic respiration (aerobic TCRIT), and that organismal thermal limits are proximally caused by the lower TCRIT (Fig. 1). The environment and oxygen-handling capacity of the organism will co-determine the hierarchy of these critical limits and thus their relative importance. The subcellular TCRIT is the temperature at which key subcellular components begin to lose function, and should be insensitive to activity state or oxygen environment. By contrast, aerobic TCRIT is the temperature at which oxygen capacity is maximized, and is affected by activity state and oxygen environment. For example, aerobic TCRIT during rest will be higher than aerobic TCRIT during activity because elevated O2 demand during activity causes capacity limits to be reached more readily (Fig. 1). Moreover, both active- and resting-aerobic TCRIT should be reduced during exposure to hypoxic environments (Fig. 1B, C). Any reduction in environmental oxygen availability below normoxia will reduce aerobic scope. However, CTMAX will only be reduced if environmental hypoxia is sufficient to cause resting-aerobic TCRIT to drop below subcellular TCRIT (Fig. 1C). The environmental oxygen tension where resting-aerobic TCRIT equals subcellular TCRIT is PCTMAX as defined by Ern et al. (2016). By integrating TCRIT values and aerobic scope, we can derive whole-organism TPCs (Fig. 1, bottom row). Because the active-aerobic TCRIT is the lowest temperature where maximal metabolic rate can be achieved, it closely corresponds to the optimal temperature for aerobic performance (whole-organism TOPT) where aerobic scope is maximized. At temperatures below active-aerobic TCRIT, we predict that performance increases with temperature proportional to aerobic scope. At temperatures above active-aerobic TCRIT, performance will drop until CTMAX is reached, but we predict that the shape of this drop will depend on which TCRIT underlies CTMAX. If resting-aerobic TCRIT underlies CTMAX, the TPC should be more symmetrical with performance and aerobic scope decreasing at a rate mirroring the increase (Fig. 1C). However, if subcellular TCRIT underlies CTMAX, the curve will be asymmetric with loss of performance occurring more rapidly the closer active-aerobic TCRIT is to subcellular TCRIT (Fig. 1A, B). Assumptions, predictions, and evidence for HMTL in reptiles and amphibians The HMTL framework produces numerous testable predictions, some of which can be addressed with available data, while others offer exciting avenues for future research. First, any environmental or organismal characteristics that reduce oxygen availability or capacity for oxygen utilization (e.g., aquatic respiration, reliance on cutaneous gas exchange, or life-stages such as eggs with reduced diffusion potential) should increase the probability of resting-aerobic TCRIT underlying CTMAX. This can explain oxygen capacity setting thermal limits in embryos (Smith et al. 2015) but not adults (Overgaard et al. 2012; Fobian et al. 2014; DuBois et al. 2017). Moreover, the transition between aerobic and subcellular TCRIT predicted by HMTL explains extreme experimental hypoxia reducing thermal tolerance in adult reptiles and amphibians, while moderate hypoxia does not (Fig. 1C and Fig. 2B,D). Similarly, HMTL predicts that CTMAX is lower when governed by oxygen limitation than when governed by subcellular mechanisms. Supporting this prediction, CTMAX drops in tadpoles as oxygen demand increases with growth and when respiratory structures are compromised in late-stage metamorphs, but frequently elevates again in terrestrial adults presumably as a result of increased oxygen capacity (Cupp 1980; Sherman 1980; Floyd 1983). Another important prediction of the HMTL hypothesis is that performance can be limited by oxygen exchange capacity at high temperatures even when CTMAX and TLETHAL are proximally set by subcellular mechanisms. Some interpretations of the OCLTT hypothesis similarly highlight limits on performance at pejus rather than critical temperatures (Pörtner and Knust 2007; Pörtner 2014; Verberk et al. 2016; Pörtner et al. 2017), but other workers suggest OCLTT should explain critical limits to be useful (i.e., Fobian et al. 2014; Smith et al. 2015; Ern et al. 2016; Verberk et al. 2016; DuBois et al. 2017). Under the HMTL framework, activity state is assumed to have no appreciable effect on oxygen capacity, although this may not hold in some highly aerobic taxa (e.g., Wang and Hicks 2004). The extent to which activity-induced increases in oxygen capacity affect thermal performance curves and limits is an important direction for future work. Generally however, individuals in metabolically-demanding states are expected to reach capacity limits at temperatures below those at which such limits are reached by resting individuals. Anaerobic respiration might compensate for short-term mismatches between energy demand and oxygen capacity (reviewed in Gleeson 1991; Fig. 1A, red shaded region), but aerobic constraints are predicted to reduce performance during long-term activity (e.g., digestion, reproduction, recovery; Jackson 2007; Pörtner et al. 2017). In diverse species, active metabolic rate asymptotes despite increases in cardiovascular output, implying oxygen flux becomes limited at temperatures above active-aerobic TCRIT (e.g., Bartholomew and Tucker 1963; Bennett and Licht 1972; Wilson 1974; Overgaard et al. 2012; Fobian et al. 2014). Furthermore, pulmonary diffusion capacity is limited by reduced plasma gas solubility and hemoglobin binding affinity at high temperatures (Wood and Moberly 1970; Kinney et al. 1977; Pough 1980; Jackson 2007; da Silva et al. 2013). Diffusion limitation of cutaneous gas exchange is also well established (reviewed in Burggren 1988; Wang 2011) and will be important in species that spend considerable time submerged (e.g. Ultsch 1973) or are lungless (e.g., Plethodontid salamanders; Whitford and Hutchison 1965; Spotilla 1972). Finally, increases in the products of anaerobic respiration, such as lactic acid, can lead to blood acidification and thereby further declines in blood oxygen affinity (Bennett 1973). The interaction of numerous factors at high temperatures reduces oxygen capacity, which in turn limits aerobic performance at high temperatures in reptiles and amphibians, regardless of the mechanism governing CTMAX (further reviewed in Jackson 2007). The HMTL framework predicts that whole-organism TOPT is the temperature providing maximal aerobic scope, which is governed by active-aerobic TCRIT. Thus, alleviating the limits that underlie active-aerobic TCRIT should allow increased maximal performance (i.e., aerobic scope) and TOPT (Fig. 1). Unfortunately, few data are available comparing whole-organism TOPT and aerobic scope in reptiles or amphibians. In the snake P. regius, whole-organism TOPT and the temperature where aerobic scope is maximized appear to correspond (Fobian et al. 2014). In the toad R. marina, observations are more complex and suggest that the maintenance of aerobic scope to high temperatures could be independent of whole-organism TOPT, at least in some cases. Hopping performance is maximized at ∼30°C (Kearney et al. 2008) whereas aerobic scope can plateau at 30°C, but can also increase to at least 40°C depending on acclimation treatment (Overgaard et al. 2012). Additional data are needed to determine if acclimation elevates TOPT for whole-organism performance similar to maximal aerobic scope in R. marina, as would be predicted by the HMTL framework. Observations for PBT provide additional indirect evidence that whole-organism TOPT corresponds to the temperature that maximizes aerobic scope in reptiles, but again data for amphibians are less clear. Generally, terrestrial ectotherms thermoregulate to within a narrow thermal range during activity if costs to thermoregulation are not prohibitively high (Huey 1982; Bauwens et al. 1995; Angilletta 2009; Kingsolver and Buckley 2015; Sears et al. 2016). Natural selection is predicted to shape thermal preference such that PBT corresponds to, or is slightly below, whole-organism TOPT (Huey 1982; Bauwens et al. 1995; Angilletta et al. 2002; but see Huey and Bennett 1987). As predicted, PBT and temperature of maximum aerobic scope are highly concordant in lizards exposed to normoxic environments (Wilson 1974). On the other hand, in the boreal toad (Anaxyrus boreas), aerobic scope is maximal at 30°C whereas TPREF is 24°C (Carey 1978, 1979). Interestingly, A. boreas and other anurans (L. pipiens, Carey 1979; R. marina, Overgaard et al. 2012) exhibit an increase in lactic acid production with temperatures above PBT in both resting and active animals. Increased lactic acid production indicates that high temperatures induce anaerobic respiration, even as aerobic scope is maintained, and therefore incur an oxygen debt for recovery. Further data are needed to assess whether the potential mismatch between maximal aerobic scope and TOPT in anurans can be explained by animals balancing the increased costs of repaying oxygen debt resulting from increased anaerobic respiration at high temperatures with the benefits of concurrent increases in aerobic scope. Assuming maximal aerobic scope, TOPT, and PBT are linked as predicted by HMTL, they are not uniformly affected by experimental hypoxia as might initially be predicted. Only extreme hypoxia affects PBT (Fig. 2) whereas the temperature that maximizes aerobic scope and presumably whole-organism TOPT is predicted to drop continuously with hypoxia (Fig. 1). This discrepancy might indicate that adult reptiles and amphibians cannot sense and respond to hypoxia-induced changes in aerobic scope in real time, which might be expected given that these animals evolved in terrestrial environments where oxygen availability is relatively stable within a lifetime. Thus, we propose that individuals choose the same body temperature regardless of the oxygen environment so long as basic metabolic demands are met, and thus predict that the “breakpoint” in Fig. 2A occurs when hypoxia causes resting-aerobic TCRIT to fall below evolved PBT. Data comparing resting aerobic TCRIT and PBT when oxygen environment or demand is manipulated are needed to test this prediction. Given the predicted relationships between aerobic scope, TOPT, and PBT, we also expect species adapted to low-oxygen environments to have relatively lower TOPT and PBT, or greater oxygen-handling capacity. However, covariation between temperature and oxygen with elevation make testing this prediction in terrestrial environments difficult. In addition to further exploring the potential importance of resting- and active-aerobic TCRIT, additional research is needed to identify the subcellular components that underlie subcellular TCRIT. A subset of evolutionarily conserved components might constrain thermal tolerance across a diversity of taxa, or the components that are most important could be taxonomically specific. Currently, the data needed to differentiate these possibilities are not available. We think that measures of ATPase and HSP provide useful candidates for further exploration, but caution that focusing on a single or few potential indicators in isolation will likely provide a contorted view of subcellular limitation. Advances in “-omics” technologies, particularly differential expression RNAseq, metabolomics, and proteomics could provide much useful information about subcellular physiological function, and provide additional candidate molecules for detailed analysis (e.g., Verberk et al. 2013; Williams et al. 2014; Campbell-Staton et al. 2017; Telemeco et al. 2017). We recommend that experiments manipulating the thermal and oxygen environment of organisms endeavor to collect subcellular data as well as whole-organism performance data. Where possible, an integrative approach combining measurements of subcellular components and whole-organism performance will best illuminate the mechanisms that underlie tolerance and their interactions. Finally, the HMTL framework makes predictions for how populations could be affected by global change. Numerous species are expanding or shifting their range to higher elevations in response to climate change-related temperature increases (e.g., Sinervo et al. 2010; Pincheira-Donoso et al. 2013; Pauchard et al. 2016). However, HMTL predicts that reduced oxygen partial pressures at high elevation will lower both TOPT and maximal performance. Thus, species must seek cooler environments as they move to higher elevation to maintain optimal performance, and performance potential will go down regardless of thermal environment selected. An evolutionary change appears necessary for animals to seek out reduced body temperatures because moderate hypoxia does not affect PBT (Fig. 2). The HMTL framework also predicts that species with greater oxygen capacity will be more buffered from lost performance when exposed to increased environmental temperatures. Thus, oxygen capacity might be a prime target of natural selection as climates warm, even if it does not underlie CTMAX or TLETHAL. Finally, given the great diversity in modes of gas exchange, metabolic demands, and shifts across life-history stages in reptiles and amphibians, we emphasize the need to explore these hypotheses in a greater number and variety of taxa. Conclusions Available data suggest that both subcellular- and organ-system-level mechanisms shape thermal performance and tolerance in amphibians and reptiles. The HMTL framework that we propose describes how both mechanisms co-affect animals, with their relative importance driven by their respective TCRIT. We think that the HMTL hypothesis improves upon current frameworks by explicitly removing the false dichotomy between subcellular mechanisms and oxygen limitation, identifying useful parameters for further research (subcellular TCRIT, resting-aerobic TCRIT, and active-aerobic TCRIT), and describing how aerobic and subcellular limitations interact to affect TPCs. Moreover, HMTL appears to explain a wide range of initially perplexing observations in reptiles and amphibians such as reduced aerobic scope at high temperatures without aerobic failure at critical temperatures, extreme hypoxia affecting thermal tolerance and behavior in adults with no effect of moderate hypoxia, moderate hypoxia reducing thermal tolerance in embryos, and HSP and ATPase activity suggesting loss of subcellular function near critical temperatures. Still, the HMTL framework makes numerous predictions for which additional data are needed, including animals only reducing their body temperature when resting-aerobic TCRIT drops below PBT, CTMAX being lower when resting-aerobic TCRIT is responsible than when subcellular TCRIT is responsible, maximum aerobic capacity underlying whole-organism thermal optima, and reduced performance when species invade higher elevations without increased oxygen capacity. Moreover, virtually no data are available to address the potential importance of aerobic- or subcellular-TCRIT as evolutionary constraints shaping the adaptive landscape. Further data better representing the diversity of reptile and amphibian taxa are needed both to understand the potential relevance of the HMTL mechanism in extant reptiles and amphibians, and how such a mechanism could have shaped the evolution of these animals. We think that many of the ideas that make up our HMTL framework are already widely accepted within the scientific community, and hope that explicitly describing them within a single framework with clear, testable predictions will facilitate further research. Although inspired by reptiles and amphibians, this integrated framework could have broad applicability across ectothermic animals. We look forward to continued investigation further integrating, refining, and testing these ideas across mechanisms and taxa. Acknowledgments We are grateful to M. Angilletta, B. Bodensteiner, and J. VandenBrooks for comments on earlier drafts of this manuscript, and for the support of A. Bronikowski, T. Schwartz, B. Slater, M. Telemeco, and A. Toth. Funding This work was supported by Auburn University and California State University, Fresno (to R.S.T.), E.J.G. was supported by the U.S. National Science Foundation (IOS-1558071 to A. Bronikowski), the “Laboratoire d’Excellence (LABEX)” TULIP (ANR-10-LABX-41), and the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 752299. Supplementary data Supplementary data are available at ICB online. References Al-Jassabi S. 2002. Purification and kinetic properties of skeletal muscle lactate dehydrogenase from the lizard Agama stellio stellio. Biochemistry (Moscow)  67: 786– 9. Google Scholar CrossRef Search ADS   Angilletta MJ. 2009. Thermal adaptation: a theoretical and empirical synthesis . New York (NY): Oxford University Press. 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Integrative and Comparative BiologyOxford University Press

Published: May 2, 2018

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