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Sex‐related differences in the diet of the mink Mustela vison

Sex‐related differences in the diet of the mink Mustela vison Birks, J. D. S. and Dunstone. N. 1985. Sex-related differences in the diet of the mink Mustela vison. - Holarct. Ecol. 8: 245-252. The diets of free-ranging male and female mink were sampled by analysing faeces collected from radio-tagged individuals. Significant sex differences were apparent in the predation upon three of the five principal prey groups. The larger males preyed much more heavily upon lagomorphs, the largest prey taken, while females preyed more upon fish and crustaceans than did males. These differences were consistent in each season except the autumn (September to November), when males preyed more heavily upon fish and erustacea than did females. Due to their large size, adult lagomorphs are felt to be relatively unavailable to female mink- Male mink are apparently large enough to specialize on lagomorphs, and male mink niche breadth was consistently lower than that of females. Dietary overlap approximated to 40% in all seasons except summer (68%). when female predation upon lagomorphs reached a peak. This reduction in intraspecific feeding competition was felt to be a valuable side-effect of body-size dimorphism. J. D. S. Birks and N. Dunstone. Depl of Zoology. Univ. of Durham, Science Laboratories, South Road, Durham DHl 3LE, England. I. Introduction The smaller members of the family Mustelidae exhibit marked sexual dimorphism of body size, and males are always the larger sex. The evolutionary processes underlying this dimorphism have been considered recently by Erlinge (1979) and Moors (1980). These authors hypothesise that large tnales are favoured by sexual seleclion, whilst energetic constraints favour females which minimize their food requirements by being small, so that a greater proportion of acquired energy may be invested in their offspring during the breeding season. The relationship between body size and prey size in mustelid hunting sets has been explored by Rosenzweig (1966) and Simms (1979). Size differences between sympatric species arc felt to permit resource partitioning because predators of different sizes were found to concentrate upon different sizes of prey. This reasoning formed the basis of earlier hypotheses aimed at explaining mustelid sexual dimorphism: body size dimorphism was thought to have evolved as a means of reducing intra- specific competition by permitting the sexes to exploit different sizes of prey (e.g. Yurgenson 1947, Brown and Lasiewski 1972, Shubin and Shubin 1975). Whatever the nature of its evolutionary origins, sexual dimorphism of body size is likely to have identifiable ecological consequences where either of the following conditions are met: (1) The larger sex utilizes prey which are relatively inaccessible to the smaller sex. (2) The smaller sex utilizes prey which are relatively inaccessible to the larger sex. Moors (1980) has reviewed the sparse information upon sex-related differences in the foods of a number of mustelid species. He observed a consistent trend for females to consume smaller prey than males do. Few detailed studies of this phenomenon have been published, however (e.g. Erlinge 1979). In this paper we present information on the diets of free-ranging male and female mink Mustela vison inhabiting a stretch of coastline in south-west Scotland. Marked sex-related differences in feeding ecology are identified, and explanations to account for these are proposed. Accepted 17 October 1984 © HOLARCnC ECOLOGY HOLARCnC ECOLOGY 8:4 (1985) 2. Methods This paper is based upon the analysis of scats collected from radio-tagged mink of known sex. During radiotracking operations, dens recently vacated by tagged mink were visited and fresh factal material was removed from them. Material from dens known to have been occupied recently by individuals of both sexes was rejected. Scats were collected from one female during May . June and July 1980. and from five males and three females between January 1981 and August 1983. Faeces were treated individually, by soaking in 1% Teepol overnight prior to sieving. Undigested remains were then dried at 50°C for 24 hours and then weighed. Mammalian and avian remains were identified on the basis of hair and feather characteristics (Day 1966). Teeth and jawbones often aided the identification of small mammal prey. Fish remains were identified with the aid of reference collections and keys, particularly that of H. Watson (unpubl.). The nomenclature of fish usedinthispaper follows that of Wheeler (1978). Crustacean remains in scats were easily identified by the presence of carapace fragments. It was often possible to distinguish between different orders on the basis of the shape and texture of carapace fragments, and the shape of appendages. common in the coastal scrub and conifer plantations, and rock doves Columba tivia occupy crevices in the cliffs all year round. Rockpools In the intertidal zone support a range of crustacean and fish prey, notably the shore crab Carcinus maenas. butterfish Pholis gunnellus, shanny Lipophrys pholis and sea scorpion Taurulus bubalis. 5. Terminology The following definitions are adopted in the analysis and presentation of dietary data in this paper: Prey item: The finest possible level of identification of prey detected during analysis. Fish and mammals were commonly identifiable to species, whilst bird and crustacean remains (crabs excepted) generally allowed identification to order only. Prey group: Prey items may be lumped into the following five groups: fish, crustaceans, lagomorphs. small mammals, birds. Prey type: In some analyses, prey are lumped into two categories - aquatic and terrestrial, Aquatic prey include those taxa which are only obtainable from the sea and the littoral zone (i.e. fish and crustaceans); terrestrial prey comprise all remaining taxa, despite the fact that some (such as aquatic birds) may be taken on the sea or in the littoral zone. Prey size: Prey types are classified as small or large on the basis of adult bodyweight. Mammals up to and including rats are classified as small, as are birds of the orders Passeriformes and Columbiformes. Some errors are to be expected in this classification, since juvenile "large" prey should rightly be included in the "small" category if it were possible to identify tbem as such. Furthermore, "large" prey types such as Charadriiformes contain predominantly large species as well as a few which would be classified as "small" if their remains were distinguishable in scats. Three methods of data analysis and presentation have been previously used in the study of carnivore diets, and authors exhibit preferences for each on the grounds of either accuracy, convenience, or ease of statistical analysis (Putnam 1984). Generally the remains of small prey occur in greater proportion relative to the prey's weight, and in lesser proportion relative to the prey's number than do the remains of large prey. In this paper we present the data initially in each of the three ways, adopting the following definitions: Percentage occurrence: The relative frequency of each prey category (i.e. item, group, size or type - see above) expressed as a percentage of all prey occurrences in that sample. Percentage of scats: The percentage of scats from a particular sample in which each prey category was present. Percentage bulk: The relative estimated bulk of undigested remains of each prey category, expressed as a percentage of the total bulk of tbe sample. For each HOLARCnC ECOLOGY 8:4 (1985) 3. Study area Scats were collected from the Ross Peninsula, an area of rocky coastline in south-west Scotland some 7 km south of the town of Kirkcudbright. This study area is described in detail in Dunstone and Birks (in prep.); a brief description is presented below. Rough pasture is the abundant habitat type on the 110 ha peninsula, with arable fields and three small conifer plantations making up the remainder. Apart from one 7(K) m stretch of 30 m high cliffs, the coastline is gently sloping and supports a strip of coastal scrub vegetation between the agricultural land and the rocky littoral zone. The average tidal range in 1981 was 7.5 m. Rockpools are abundant at low tide around much of the peninsula. 4. I^ey availability Small mammals, rabbits Oryctolagus cuniculus and brown hares Lepus capensis are particularly numerous on this study area, though all show some seasonal fluctuations in abundance (Dunstone and Birks in prep.). During the summer months, the cliffs support nesting colonies of a range of seabird species, of which the herring gull Lariis argentatus is the commonest and most available to mink. Many seabirds are also available to mink as carrion washed ashore during winter storms. Small numbers of wading birds, such as curlews Numenius arquata and oystercatchers Haematopus ostralegus feed and roost on the peninsula. Small passerines are scut, the relative volume of each prey item was estimated to the nearest 10%. and subsequently used to calculate the estimated dry weight of items in each scat. These were summed for each item and for all items (total bulk) for the derivation of percentage bulk. Tub. 2. Bodyweight information from adult male and female mink live-trapped on Ross Peninsula between September 1981 and September 1983 (data from pregnant females are excluded). N Males Females 15 II Mean of maximum weights (g) 5.3± 175.3 688.2± 64.7 Range 930-1530 560- 770 6. Results 6.1. Seal samples A total of 1024 scats, containing 1563 occurrences of prey items, were collected from mink of known sex (1kb. I). 6.2. Sexual djmorphi.sm ratio A sample of 26 adult mink (15cfcf and 1199) were live-lrapped and weighed on the study area between September I9SI ;ind September 1983 (Tab. 2). The mean of the maximum weights of all individuals gave a male: female hodyweight ratio of 1.74. At its most extreme (i.e. heaviest male: lightest female) this ratio had a value of 2.73; the lowest value at the individual level was 1.21. 6.3. The diets of male and female mink Tab. 3. shows a complete breakdown of the diets of both sexes. The relative contributions of the five prey groups to the diet of each sex are illustrated in Fig. 1. Differenees in the consumption of prey groups by males and females were highly significant in three cases. 6.3.1. Fish Fish prey contributed significantly more to the diet of females than to that of males (x^ = 45.99, P < 0.001). The species represented were mostly typical members ot" the rockpool fauna. Nearly one third of the fish remains in each sample were unidentifiable, due mainly to the fragmentation or absence of characteristic vertebrae in scats. Tab I. The size of seasonal samples used in the analysis of male and female mink diets. Season Scats Males Prev occurrences Scats Females Prey occurrenees 6.3.2. Crustacea As in the case of fish prey, female mink preyed more heavily upon Crustacea than did males (x" = 19.79; P < 0.001). Decapoda were by far the most important order in each case, and the majority of these were identifiable as crabs from the size and texture of carapace fragments. Rockpool sampling revealed that the most abundant crab species in the littoral zone was the shore crab Carcinus maenas. Small quantities of isopod remains, probably Ligia oceanica, were identified in seats from mink of both sexes. It is noteworthy that both fish and erustaeean prey items are relatively small and tend, therefore, to be of exaggerated importance in the diet when expressed as percentage occurrence rather than percentage bulk (Tab. 3). 6.3.3. Lagomorphs The most striking dietary difference between male and female mink is observed in their predation of lagomorphs (x' - 195.17. P < O.(H)l). 65% of male scats contained lagomorph remains, and these comprised 57% of the total bulk of undigested prey remains. Comparable figures for females are 25% and 20%. Lagomorphs are 1 ) Male I I Female "Winter" {Dec.-Feb.) "Spring" (March-May) "Summer" (June-Aug.) "Autumn" (Sept.-Nov.) Total 324 156 481 90 1051 Fig. I. The relative contributions of five prey groups, expressed as percentage bulk of undigested remains, to the eomplete diets of male and female mink. Abbrev.: CRUS. Crustacea; LAG, Lagomorphs; SMALL MAMM. small mammals. HOLARCTIC ECOLOGY 8:4 (1985) Tab. 3. The complete annual diet of male and female mink on the Ross Peninsula expressed as: A: The relative frequency of each prey item as a percentage of all prey occurrences in that sample (termed "percentage occurrence"). B: The percentage of scats from that sample in which eaeh prey item was recorded ("percentage of scats"). C: The relative estimated bulk of undigested remains of each prey item, as a percentage of the total bulk of the sample ("percentage bulk"). Prey item Males A Eel Anguilla anguilla Salmonidae Blenny Lipophrys phoHs Butterfish Pholts gunnellus 5-bearded rockting Ciliata mustela Long-spined sea scorpion Taurutus bubaiis Gobiidae Flatfish 15-spined stickleback Spinachia spinachia Unidentified fish Total fish Crab Other Decapoda Isopoda Unidentified Crustacea Total Crustacea Lagomorph Brown rat Rattus norvegicus Field vole Microrus agrestis Bank vo\e Clethrionomys glareolus Woodmouse Apodemus sylvaticus Shrew Unidentified mammal Total mammal Charadriiformes Passeriformes Anseriformes Columbiformes Galliformes Unidentified bird Totalbird Total number of items Total number of scats Total dryweight (g) 0.6 0.2 5.1 1.0 3.3 1.8 1.6 0.4 6.4 20.4 15.6 1.6 1,2 0.4 18.8 48.4 0.8 3.7 0.4 0.4 0.2 53.9 3.3 1.6 0.6 0.6 1.0 7.1 512 0.8 0.3 6.9 1.3 4.5 2.4 2.1 0.5 8.7 23.2 21.1 2.1 1.6 0.5 25.1 65.4 1.1 5.0 0.5 0.5 0.3 71.2 4.5 2.1 0.8 0.8 1.3 9.5 379 0.2 0.1 3.8 0.6 2.2 1.0 1.3 0.2 4.1 13.6 11.5 0.4 0.4 0.1 12.4 57.2 1.2 3.5 0.2 0.7 0.1 62.7 5.6 2.5 1.7 0.4 1.0 11.2 256.9 3.4 0.1 11.9 1.7 5.9 3.9 1.6 0.3 0.2 8.5 37.5 24.3 0.9 2.4 1.6 29.2 15.2 1.6 3.1 0.9 0.6 0.2 0.3 21.9 4.6 4.1 0.8 O.I 0.1 1.4 11.1 1051 5.6 O.I 19.4 2.8 9.8 6.3 2.6 0.5 03 13.8 46.7 39.7 1,5 3.9 2,6 45.7 24.8 2.6 5.1 1.5 0.9 0,4 0.5 34.6 7.4 6.7 1.4 0.1 0.1 2.3 18.0 645 1.9 0.6 10.2 0.9 4.9 2.9 1.3 0.2 01 9.8 32.4 19.1 0.4 1.4 0.6 21.4 20.2 2.8 3.0 0.9 0.5 0.3 O.I 27,7 7.9 5.6 2.9 0.1 O.OI 2.(1 18.6 497,2 Females the largest prey taken by mink on this study area, and both rabbits and hares have been recorded amongst prey remains at mink dens (Birks and Dunstone, 1984). Hares, however, were probably taken only occasionally due to their greater size and strength and their non-fossorial habits. 6.3.4. Small mammals Small mammals comprised less than 8% (bulk) of the diet of both sexes, and no sex-related difference was apparent in their consumption by mink (x^ = 0.76, P > 0.2). The most important species to both sexes, in terms of both bulk and frequency, was the field vole Microtus agrestis followed by the brown rat Rattus norvegicus. 248 6.3.5. Birds Female mink preyed upon birds rather more heavily than did males (x^ = 6.1, P<0.02). Predictably, on this coastal site, Charadriiformes contributed most to the diets of both males and females. Several of the larger species, such as the herring gull, are most likely to have been taken as juveniles or as strandline earrion, and some prey remains reeovered from mink dens in 1982 were clearly those of oiled auks (Birks and Dunstone 1984). Despite marked differences in their consumption of particular prey groups, male and female mink exhibited very similar patterns of relative consumption of prey items within groups (Tab. 4). Both sexes, for example, showed an identical order of relative importance for the four bird Orders in common in their diets. HOLARCTIC ECOLOGY 8:4 (1985) Tab. 4. Spearman's rank coefficients, showing similarities in the order of relative importance to male and female mink ot the constituent prey items of three prey groups tested (percentage bulk values were used). Prey group N Fish 8 Mammals 5 Birds 4 Spearman's rank coefficient r,=().815 r,= n.9 r .= 1.0 Probability <0.05 <0.001 <0.001 lOOn I 80I small ] large 60- 40- 6.4. Prey type and size When data shown in Tab. 3 are categorized into terrestrial and aquatic prey types (Fig. 2). and "large" and "stnall" prey sizes (Fig. 3). female mink emerge as predators of prey items which are predominantly small and of aquatic origin whilst males consume predominantly large terrestrial prey. Furthermore, there is evidence to suggest that where certain "large" items occur in the diet of female mink, a substantial proportion may be taken as juveniles. For example, of 9 rabbit carcasses recovered from the dens used by a breeding female during summer 1983. only one had a hindfoot length exceeding 57 mm. Hindfoot lengths for adult rabbits lie in the range 75 to 95 mm (Corbet and Southern 1977). Male mink are known to take adult rabbits regularly, as indicated by recoveries of carcasses from dens. A.5 Seasonal variations 20- O-' male female Fig. 3. The relative contributions of small and large prey to the complete diets of male and female mink, expressed as A: percentage occurrence, B: percentage bulk. Female predation upon lagomorphs peaked at 31% bulk during the summer (when juveniles are abundant), and declined subsequently as in the case of males. 60-1 60, Crustac ea Figs 4, 5 and 6 illustrate seasonal variations in the consumption of prey groups, types and sizes by male and female mink. Males may be regarded as lagomorph specialists during the spring and summer, when this prey item contributed more than 80% (bulk) to their diet. S S A W 40- 1 00-, B Smalt Mammal LagomoroU LJlerrestrial 80aquatic cD 60- W female male Fig. 2, The relative contributions of terrestrial and aquatic prey to the complete diets of male and female mink, expressed as A: percentage occurrence, B: percentage bulk. HOLARCTIC ECOLOGY 8:4 (1985) Fig. 4. Variations in the consumption of five prey groups by male (solid line) and female (broken line) mink, expressed as percent bulk of undigested remains, over the four seasons: spring (S), summer (S), autumn (A) and winter (W). 80-1 of e male 60- CD 40- 20- male Fig. 5. Seasonal variations in the consumption of aquatic prey, expressed as percent bulk, by male (solid line) and female (broken line) mink. Terrestrial prey make up the remainder of the diets in this analysis. though birds were preyed upon most heavily (ca. 55% bulk) by female mink in the spring. Bird predation by male mink was insignificant in all seasons except the winter (33% bulk). Charadriiformes contributed more to the diet of each sex in every season than did any other bird order. Female mink preyed most heavily upon fish during the winter months (70% bulk). Blennies were the fish prey consumed most by both sexes in three out of the four seasons. An examination of Fig. 5 indicates that aquatic prey were at least twice as important to female mink during summer and winter, relative to their consumption in spring and autumn. Aquatic prey did not exceed 10% of the diet of males during spring and summer, but were more important subsequently. Fig. 6 suggests that there is a tendency for both sexes to consume smaller prey as the seasons progress from spring to winter. 6.6. Niche breadth and niche overlap Values for niche breadth (NB) were derived from Simpson's index: NB - 1/^ i^ One obvious divergence from the overall pattern of sex-related differences shown in Fig. 1 concerns the autumn consumption of fish and erustacea. During this season male mink preyed more heavily (x' — 12.76, P < O.(Kll) upon aquatic prey than did females (Fig. 5). The marked decrease in the consumption of aquatic prey by females in autumn was paralleled by an increase in their predation upon small mammals and birds, 1 00-1 o f ema le 80- Where ^.i denotes the relative proportions of each prey item in the diet (percentage bulk values were used). The nice overlap (a) between male and female mink was calculated by means of Pianka's formula: a = Where pi denotes the relative proportions of each prey item in the diet, and j and k signify male and female mink. Overall niche breadths for male and female mink were 2.88 and 8.81 respectively. Overall niche overiap was 0.78. Of greater significance, however, are the seasonal variations in both these parameters (Fig. 7). Female niche breadth was consistently greater than that of males., and both sexes exhibited a trend of increasing niche breadth from spring to winter. Females, however, showed a marked decrease in this parameter during the summer. In three out of four seasons, niche overlap approximataed to 0.4; in the summer, however, it increased to 0.68. 7. Discussion Analysis of the content of just over one hundred stomachs and intestines from winter-trapped mink in Michigan led Sealander (1943) to comment that "there was an apparent discrimination by sex as to size of prey items taken". The results of the present study clearly reinforce this statement: female mink were found to prey most heavily upon small, littoral species, while the diet of males was dominated by lagomorphs - the largest HOLARCnC ECOLOGY 84 o.. CO 40- "•male 20- A n.s Fig. 6. Seasonal variations in the consumption of small prey, expressed as percent bulk, by male (solid line) and female (broken line) mink. Large prey make up the remainder of the diets in this analysis. (a) 10,0-1 f ema le, 1.0-1 (b) o- 8 0CO CO 0.8- OQ 6.0- male 0.6- o o 4.0- ii 2.0- 0.4- 0.2- Fig. 7. Seasonal variations in (a) the niche breadth (NB) of male and female mink, and (b) the niche overlap (a) between the two sexes. See text for Simpson's and Pianka's formulae used in the calculation of NB and a. prey item recorded in this study. A sex-related difference in tagomorph predation was also noticed by Birks (198!) in Devon, where radio-tagged male mink were found to hunt rabbits successfully in their burrows to a much greater degree than did female mink. Rabbits must be regarded as a highly profitable resoucc for small carnivores: their fossorial habits make them vulnerable to underground predation, a situation in which speed of pursuit is not vital; an underground kill may be consumed in the relative comfort and security of the burrow; furthermore, at a weight of approximately 2 kg, a fully grown rabbit may sustain an adull male mink for two to three days (Linn and Birks 1981). In the light of these benefits, the low overall contribution of lagomorphs to the diet of female mink suggest that this valuable resource is relatively unavailable to them. Il is likely that the two- or even three-fold difference in bodyweighl between female mink and adult rabbits precludes successful predation on a regular basis. The observation that lagomorph predation by females is at a peak in the summer, when juvenile rabbits are most abundant (pers. obs.), supports the evidence from prey remains that smaller individuals arc selected. A similar pattern has been observed in Swedish stoats Mmtela erminca, which prey mainly on juvenile rabbits (Erlinge pers. comm.). It has been demonstrated that whereas relatively large predators may consume foods unavailable to smaller predators in areas of sympatry, the reverse is much less true (Wilson 1975). This imbalance is apparently reflected in the diets of male and female mink, particularly if seasonal variations are taken into acHOLARCTIC ECOLOGY 8:4 (1^85) count. Whereas the largest prey (lagomorphs) were exploited very heavily by males during some seasons, and relatively lightly by females in all seasons, males also exploited small, littoral prey quite heavily during the autumn. This peak in littoral predation by males occurred at a time when lagomorphs were relatively abundant (Dunstone and Birks, in prep.), so it cannot be regarded as a simple response to a shortage of preferred prey. It may, however, have been a response to a seasonal increase in the availability of large Carcinus specimens. Naylor (1962) noted that the number of large, paired Carcinus found on the littoral zone at low tide in South Wales reached a peak in August and September. A similar pattern was also noted on Ross peninsula (pers. obs.). The vulnerability of erabs to predation during August and September is suggested by their occurrence in the diet of foxes Vulpes vulpcs on Ross peninsula during these two months only. The extent to which the observed dietary differences are a consequence of either sex-related differences in prey availability and preference or some form of intraspecific competitive interaction remains undetermined. Male and female mink occupy overlapping home ranges on this study area (Dunstone and Birks 1983), and female mink are clearly subordinate to males outside the breeding season (Birks 1981). Erlinge (1977) has demonstrated that the normal strategy of a female stoat outside the breeding season is to avoid males or to become inactive when a male is present in the vicinity. A similar relationship might explain, for example, the reduction in the littoral component of the female mink diet during the autumn, when males exploited the spatially and temporally restricted littoral foraging areas most heavily. Radio-tracking evidence indicates that mink do exhibit clear sex-related differences in habitat use. Further data are needed, however, in order to elucidate the role of intersexual Interactions in causing these differences and the associated dietary differences presented in this paper. Unlike Eriinge's (1979) stoat study, in which no consistent difference in niche breadth of males and females was apparent, the present study indicates that male mink niche breadth is less than that of females in each of the four seasons of the year. Dunstone and Birks (in prep.) suggest that the niche breadth of mink on this study area is inversely related to the proportion of lagomorph remains in the diet. The seasonal patterns of niche breadth and lagomorph predation observed in both sexes in the present study tend to support this conclusion. It is significant, therefore, that whilst the larger sex may enjoy a wider range of available prey than the smaller sex in a sexually dimorphic species (Schoener 1967, Wilson 1975). this advantage has permitted more dietary specialization, through greater predation upon lagomorphs, in male mink relative to females. Niche overlap values derived from dietary information gathered over more than one season, or over the whole year, should be interpreted with caution. In the present study, seasonal variations in the diet of males and females resulted in much lower niche overlap In each season when compared with that derived from the overall diet. The summer peak in niche overlap observed in this study occurs when female niche breadth is low. This coincides with the breeding season, when females are restricted to the vicinity of breeding dens for several weeks (N. Dunstone, J. Birks and M. Ireland, unpubl.) and a narrower range of prey is probably encountered. Under certain circumstances, small mustelid populations may be limited by a shortage of food (e.g. Erlinge 1983). Where a mink population is affected in such a way, as Gerell (1971) has suggested happened in southern Sweden following the elimination of an important summer prey species (crayfish Astacus sp.), the degree of niche overlap between the sexes may be instrumental in determining its fate. The low (ca. 40%) niche overlap outside the breeding season observed in the present study would clearly minimize intraspecific competition to a considerable extent under those circumstances, and as such it may be regarded as an adaptive side-effeet of body-size dimorphism. Indeed Moors (1980) has suggested that this beneficial side-effect might augment the main selective pressures which favour body size dimorphism in mustelids. are also duo to A. Rosscr. D. Moltu. T. Birks and M. Ireland for assistance with fieldwork, and to Paul Loftus, Chris Moore and Tom Jackson for technical assistance at Durham. 8. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Ecography Wiley

Sex‐related differences in the diet of the mink Mustela vison

Ecography , Volume 8 (4) – Dec 1, 1985

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Wiley
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Copyright © 1985 Wiley Subscription Services, Inc., A Wiley Company
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0906-7590
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1600-0587
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10.1111/j.1600-0587.1985.tb01175.x
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Abstract

Birks, J. D. S. and Dunstone. N. 1985. Sex-related differences in the diet of the mink Mustela vison. - Holarct. Ecol. 8: 245-252. The diets of free-ranging male and female mink were sampled by analysing faeces collected from radio-tagged individuals. Significant sex differences were apparent in the predation upon three of the five principal prey groups. The larger males preyed much more heavily upon lagomorphs, the largest prey taken, while females preyed more upon fish and crustaceans than did males. These differences were consistent in each season except the autumn (September to November), when males preyed more heavily upon fish and erustacea than did females. Due to their large size, adult lagomorphs are felt to be relatively unavailable to female mink- Male mink are apparently large enough to specialize on lagomorphs, and male mink niche breadth was consistently lower than that of females. Dietary overlap approximated to 40% in all seasons except summer (68%). when female predation upon lagomorphs reached a peak. This reduction in intraspecific feeding competition was felt to be a valuable side-effect of body-size dimorphism. J. D. S. Birks and N. Dunstone. Depl of Zoology. Univ. of Durham, Science Laboratories, South Road, Durham DHl 3LE, England. I. Introduction The smaller members of the family Mustelidae exhibit marked sexual dimorphism of body size, and males are always the larger sex. The evolutionary processes underlying this dimorphism have been considered recently by Erlinge (1979) and Moors (1980). These authors hypothesise that large tnales are favoured by sexual seleclion, whilst energetic constraints favour females which minimize their food requirements by being small, so that a greater proportion of acquired energy may be invested in their offspring during the breeding season. The relationship between body size and prey size in mustelid hunting sets has been explored by Rosenzweig (1966) and Simms (1979). Size differences between sympatric species arc felt to permit resource partitioning because predators of different sizes were found to concentrate upon different sizes of prey. This reasoning formed the basis of earlier hypotheses aimed at explaining mustelid sexual dimorphism: body size dimorphism was thought to have evolved as a means of reducing intra- specific competition by permitting the sexes to exploit different sizes of prey (e.g. Yurgenson 1947, Brown and Lasiewski 1972, Shubin and Shubin 1975). Whatever the nature of its evolutionary origins, sexual dimorphism of body size is likely to have identifiable ecological consequences where either of the following conditions are met: (1) The larger sex utilizes prey which are relatively inaccessible to the smaller sex. (2) The smaller sex utilizes prey which are relatively inaccessible to the larger sex. Moors (1980) has reviewed the sparse information upon sex-related differences in the foods of a number of mustelid species. He observed a consistent trend for females to consume smaller prey than males do. Few detailed studies of this phenomenon have been published, however (e.g. Erlinge 1979). In this paper we present information on the diets of free-ranging male and female mink Mustela vison inhabiting a stretch of coastline in south-west Scotland. Marked sex-related differences in feeding ecology are identified, and explanations to account for these are proposed. Accepted 17 October 1984 © HOLARCnC ECOLOGY HOLARCnC ECOLOGY 8:4 (1985) 2. Methods This paper is based upon the analysis of scats collected from radio-tagged mink of known sex. During radiotracking operations, dens recently vacated by tagged mink were visited and fresh factal material was removed from them. Material from dens known to have been occupied recently by individuals of both sexes was rejected. Scats were collected from one female during May . June and July 1980. and from five males and three females between January 1981 and August 1983. Faeces were treated individually, by soaking in 1% Teepol overnight prior to sieving. Undigested remains were then dried at 50°C for 24 hours and then weighed. Mammalian and avian remains were identified on the basis of hair and feather characteristics (Day 1966). Teeth and jawbones often aided the identification of small mammal prey. Fish remains were identified with the aid of reference collections and keys, particularly that of H. Watson (unpubl.). The nomenclature of fish usedinthispaper follows that of Wheeler (1978). Crustacean remains in scats were easily identified by the presence of carapace fragments. It was often possible to distinguish between different orders on the basis of the shape and texture of carapace fragments, and the shape of appendages. common in the coastal scrub and conifer plantations, and rock doves Columba tivia occupy crevices in the cliffs all year round. Rockpools In the intertidal zone support a range of crustacean and fish prey, notably the shore crab Carcinus maenas. butterfish Pholis gunnellus, shanny Lipophrys pholis and sea scorpion Taurulus bubalis. 5. Terminology The following definitions are adopted in the analysis and presentation of dietary data in this paper: Prey item: The finest possible level of identification of prey detected during analysis. Fish and mammals were commonly identifiable to species, whilst bird and crustacean remains (crabs excepted) generally allowed identification to order only. Prey group: Prey items may be lumped into the following five groups: fish, crustaceans, lagomorphs. small mammals, birds. Prey type: In some analyses, prey are lumped into two categories - aquatic and terrestrial, Aquatic prey include those taxa which are only obtainable from the sea and the littoral zone (i.e. fish and crustaceans); terrestrial prey comprise all remaining taxa, despite the fact that some (such as aquatic birds) may be taken on the sea or in the littoral zone. Prey size: Prey types are classified as small or large on the basis of adult bodyweight. Mammals up to and including rats are classified as small, as are birds of the orders Passeriformes and Columbiformes. Some errors are to be expected in this classification, since juvenile "large" prey should rightly be included in the "small" category if it were possible to identify tbem as such. Furthermore, "large" prey types such as Charadriiformes contain predominantly large species as well as a few which would be classified as "small" if their remains were distinguishable in scats. Three methods of data analysis and presentation have been previously used in the study of carnivore diets, and authors exhibit preferences for each on the grounds of either accuracy, convenience, or ease of statistical analysis (Putnam 1984). Generally the remains of small prey occur in greater proportion relative to the prey's weight, and in lesser proportion relative to the prey's number than do the remains of large prey. In this paper we present the data initially in each of the three ways, adopting the following definitions: Percentage occurrence: The relative frequency of each prey category (i.e. item, group, size or type - see above) expressed as a percentage of all prey occurrences in that sample. Percentage of scats: The percentage of scats from a particular sample in which each prey category was present. Percentage bulk: The relative estimated bulk of undigested remains of each prey category, expressed as a percentage of the total bulk of tbe sample. For each HOLARCnC ECOLOGY 8:4 (1985) 3. Study area Scats were collected from the Ross Peninsula, an area of rocky coastline in south-west Scotland some 7 km south of the town of Kirkcudbright. This study area is described in detail in Dunstone and Birks (in prep.); a brief description is presented below. Rough pasture is the abundant habitat type on the 110 ha peninsula, with arable fields and three small conifer plantations making up the remainder. Apart from one 7(K) m stretch of 30 m high cliffs, the coastline is gently sloping and supports a strip of coastal scrub vegetation between the agricultural land and the rocky littoral zone. The average tidal range in 1981 was 7.5 m. Rockpools are abundant at low tide around much of the peninsula. 4. I^ey availability Small mammals, rabbits Oryctolagus cuniculus and brown hares Lepus capensis are particularly numerous on this study area, though all show some seasonal fluctuations in abundance (Dunstone and Birks in prep.). During the summer months, the cliffs support nesting colonies of a range of seabird species, of which the herring gull Lariis argentatus is the commonest and most available to mink. Many seabirds are also available to mink as carrion washed ashore during winter storms. Small numbers of wading birds, such as curlews Numenius arquata and oystercatchers Haematopus ostralegus feed and roost on the peninsula. Small passerines are scut, the relative volume of each prey item was estimated to the nearest 10%. and subsequently used to calculate the estimated dry weight of items in each scat. These were summed for each item and for all items (total bulk) for the derivation of percentage bulk. Tub. 2. Bodyweight information from adult male and female mink live-trapped on Ross Peninsula between September 1981 and September 1983 (data from pregnant females are excluded). N Males Females 15 II Mean of maximum weights (g) 5.3± 175.3 688.2± 64.7 Range 930-1530 560- 770 6. Results 6.1. Seal samples A total of 1024 scats, containing 1563 occurrences of prey items, were collected from mink of known sex (1kb. I). 6.2. Sexual djmorphi.sm ratio A sample of 26 adult mink (15cfcf and 1199) were live-lrapped and weighed on the study area between September I9SI ;ind September 1983 (Tab. 2). The mean of the maximum weights of all individuals gave a male: female hodyweight ratio of 1.74. At its most extreme (i.e. heaviest male: lightest female) this ratio had a value of 2.73; the lowest value at the individual level was 1.21. 6.3. The diets of male and female mink Tab. 3. shows a complete breakdown of the diets of both sexes. The relative contributions of the five prey groups to the diet of each sex are illustrated in Fig. 1. Differenees in the consumption of prey groups by males and females were highly significant in three cases. 6.3.1. Fish Fish prey contributed significantly more to the diet of females than to that of males (x^ = 45.99, P < 0.001). The species represented were mostly typical members ot" the rockpool fauna. Nearly one third of the fish remains in each sample were unidentifiable, due mainly to the fragmentation or absence of characteristic vertebrae in scats. Tab I. The size of seasonal samples used in the analysis of male and female mink diets. Season Scats Males Prev occurrences Scats Females Prey occurrenees 6.3.2. Crustacea As in the case of fish prey, female mink preyed more heavily upon Crustacea than did males (x" = 19.79; P < 0.001). Decapoda were by far the most important order in each case, and the majority of these were identifiable as crabs from the size and texture of carapace fragments. Rockpool sampling revealed that the most abundant crab species in the littoral zone was the shore crab Carcinus maenas. Small quantities of isopod remains, probably Ligia oceanica, were identified in seats from mink of both sexes. It is noteworthy that both fish and erustaeean prey items are relatively small and tend, therefore, to be of exaggerated importance in the diet when expressed as percentage occurrence rather than percentage bulk (Tab. 3). 6.3.3. Lagomorphs The most striking dietary difference between male and female mink is observed in their predation of lagomorphs (x' - 195.17. P < O.(H)l). 65% of male scats contained lagomorph remains, and these comprised 57% of the total bulk of undigested prey remains. Comparable figures for females are 25% and 20%. Lagomorphs are 1 ) Male I I Female "Winter" {Dec.-Feb.) "Spring" (March-May) "Summer" (June-Aug.) "Autumn" (Sept.-Nov.) Total 324 156 481 90 1051 Fig. I. The relative contributions of five prey groups, expressed as percentage bulk of undigested remains, to the eomplete diets of male and female mink. Abbrev.: CRUS. Crustacea; LAG, Lagomorphs; SMALL MAMM. small mammals. HOLARCTIC ECOLOGY 8:4 (1985) Tab. 3. The complete annual diet of male and female mink on the Ross Peninsula expressed as: A: The relative frequency of each prey item as a percentage of all prey occurrences in that sample (termed "percentage occurrence"). B: The percentage of scats from that sample in which eaeh prey item was recorded ("percentage of scats"). C: The relative estimated bulk of undigested remains of each prey item, as a percentage of the total bulk of the sample ("percentage bulk"). Prey item Males A Eel Anguilla anguilla Salmonidae Blenny Lipophrys phoHs Butterfish Pholts gunnellus 5-bearded rockting Ciliata mustela Long-spined sea scorpion Taurutus bubaiis Gobiidae Flatfish 15-spined stickleback Spinachia spinachia Unidentified fish Total fish Crab Other Decapoda Isopoda Unidentified Crustacea Total Crustacea Lagomorph Brown rat Rattus norvegicus Field vole Microrus agrestis Bank vo\e Clethrionomys glareolus Woodmouse Apodemus sylvaticus Shrew Unidentified mammal Total mammal Charadriiformes Passeriformes Anseriformes Columbiformes Galliformes Unidentified bird Totalbird Total number of items Total number of scats Total dryweight (g) 0.6 0.2 5.1 1.0 3.3 1.8 1.6 0.4 6.4 20.4 15.6 1.6 1,2 0.4 18.8 48.4 0.8 3.7 0.4 0.4 0.2 53.9 3.3 1.6 0.6 0.6 1.0 7.1 512 0.8 0.3 6.9 1.3 4.5 2.4 2.1 0.5 8.7 23.2 21.1 2.1 1.6 0.5 25.1 65.4 1.1 5.0 0.5 0.5 0.3 71.2 4.5 2.1 0.8 0.8 1.3 9.5 379 0.2 0.1 3.8 0.6 2.2 1.0 1.3 0.2 4.1 13.6 11.5 0.4 0.4 0.1 12.4 57.2 1.2 3.5 0.2 0.7 0.1 62.7 5.6 2.5 1.7 0.4 1.0 11.2 256.9 3.4 0.1 11.9 1.7 5.9 3.9 1.6 0.3 0.2 8.5 37.5 24.3 0.9 2.4 1.6 29.2 15.2 1.6 3.1 0.9 0.6 0.2 0.3 21.9 4.6 4.1 0.8 O.I 0.1 1.4 11.1 1051 5.6 O.I 19.4 2.8 9.8 6.3 2.6 0.5 03 13.8 46.7 39.7 1,5 3.9 2,6 45.7 24.8 2.6 5.1 1.5 0.9 0,4 0.5 34.6 7.4 6.7 1.4 0.1 0.1 2.3 18.0 645 1.9 0.6 10.2 0.9 4.9 2.9 1.3 0.2 01 9.8 32.4 19.1 0.4 1.4 0.6 21.4 20.2 2.8 3.0 0.9 0.5 0.3 O.I 27,7 7.9 5.6 2.9 0.1 O.OI 2.(1 18.6 497,2 Females the largest prey taken by mink on this study area, and both rabbits and hares have been recorded amongst prey remains at mink dens (Birks and Dunstone, 1984). Hares, however, were probably taken only occasionally due to their greater size and strength and their non-fossorial habits. 6.3.4. Small mammals Small mammals comprised less than 8% (bulk) of the diet of both sexes, and no sex-related difference was apparent in their consumption by mink (x^ = 0.76, P > 0.2). The most important species to both sexes, in terms of both bulk and frequency, was the field vole Microtus agrestis followed by the brown rat Rattus norvegicus. 248 6.3.5. Birds Female mink preyed upon birds rather more heavily than did males (x^ = 6.1, P<0.02). Predictably, on this coastal site, Charadriiformes contributed most to the diets of both males and females. Several of the larger species, such as the herring gull, are most likely to have been taken as juveniles or as strandline earrion, and some prey remains reeovered from mink dens in 1982 were clearly those of oiled auks (Birks and Dunstone 1984). Despite marked differences in their consumption of particular prey groups, male and female mink exhibited very similar patterns of relative consumption of prey items within groups (Tab. 4). Both sexes, for example, showed an identical order of relative importance for the four bird Orders in common in their diets. HOLARCTIC ECOLOGY 8:4 (1985) Tab. 4. Spearman's rank coefficients, showing similarities in the order of relative importance to male and female mink ot the constituent prey items of three prey groups tested (percentage bulk values were used). Prey group N Fish 8 Mammals 5 Birds 4 Spearman's rank coefficient r,=().815 r,= n.9 r .= 1.0 Probability <0.05 <0.001 <0.001 lOOn I 80I small ] large 60- 40- 6.4. Prey type and size When data shown in Tab. 3 are categorized into terrestrial and aquatic prey types (Fig. 2). and "large" and "stnall" prey sizes (Fig. 3). female mink emerge as predators of prey items which are predominantly small and of aquatic origin whilst males consume predominantly large terrestrial prey. Furthermore, there is evidence to suggest that where certain "large" items occur in the diet of female mink, a substantial proportion may be taken as juveniles. For example, of 9 rabbit carcasses recovered from the dens used by a breeding female during summer 1983. only one had a hindfoot length exceeding 57 mm. Hindfoot lengths for adult rabbits lie in the range 75 to 95 mm (Corbet and Southern 1977). Male mink are known to take adult rabbits regularly, as indicated by recoveries of carcasses from dens. A.5 Seasonal variations 20- O-' male female Fig. 3. The relative contributions of small and large prey to the complete diets of male and female mink, expressed as A: percentage occurrence, B: percentage bulk. Female predation upon lagomorphs peaked at 31% bulk during the summer (when juveniles are abundant), and declined subsequently as in the case of males. 60-1 60, Crustac ea Figs 4, 5 and 6 illustrate seasonal variations in the consumption of prey groups, types and sizes by male and female mink. Males may be regarded as lagomorph specialists during the spring and summer, when this prey item contributed more than 80% (bulk) to their diet. S S A W 40- 1 00-, B Smalt Mammal LagomoroU LJlerrestrial 80aquatic cD 60- W female male Fig. 2, The relative contributions of terrestrial and aquatic prey to the complete diets of male and female mink, expressed as A: percentage occurrence, B: percentage bulk. HOLARCTIC ECOLOGY 8:4 (1985) Fig. 4. Variations in the consumption of five prey groups by male (solid line) and female (broken line) mink, expressed as percent bulk of undigested remains, over the four seasons: spring (S), summer (S), autumn (A) and winter (W). 80-1 of e male 60- CD 40- 20- male Fig. 5. Seasonal variations in the consumption of aquatic prey, expressed as percent bulk, by male (solid line) and female (broken line) mink. Terrestrial prey make up the remainder of the diets in this analysis. though birds were preyed upon most heavily (ca. 55% bulk) by female mink in the spring. Bird predation by male mink was insignificant in all seasons except the winter (33% bulk). Charadriiformes contributed more to the diet of each sex in every season than did any other bird order. Female mink preyed most heavily upon fish during the winter months (70% bulk). Blennies were the fish prey consumed most by both sexes in three out of the four seasons. An examination of Fig. 5 indicates that aquatic prey were at least twice as important to female mink during summer and winter, relative to their consumption in spring and autumn. Aquatic prey did not exceed 10% of the diet of males during spring and summer, but were more important subsequently. Fig. 6 suggests that there is a tendency for both sexes to consume smaller prey as the seasons progress from spring to winter. 6.6. Niche breadth and niche overlap Values for niche breadth (NB) were derived from Simpson's index: NB - 1/^ i^ One obvious divergence from the overall pattern of sex-related differences shown in Fig. 1 concerns the autumn consumption of fish and erustacea. During this season male mink preyed more heavily (x' — 12.76, P < O.(Kll) upon aquatic prey than did females (Fig. 5). The marked decrease in the consumption of aquatic prey by females in autumn was paralleled by an increase in their predation upon small mammals and birds, 1 00-1 o f ema le 80- Where ^.i denotes the relative proportions of each prey item in the diet (percentage bulk values were used). The nice overlap (a) between male and female mink was calculated by means of Pianka's formula: a = Where pi denotes the relative proportions of each prey item in the diet, and j and k signify male and female mink. Overall niche breadths for male and female mink were 2.88 and 8.81 respectively. Overall niche overiap was 0.78. Of greater significance, however, are the seasonal variations in both these parameters (Fig. 7). Female niche breadth was consistently greater than that of males., and both sexes exhibited a trend of increasing niche breadth from spring to winter. Females, however, showed a marked decrease in this parameter during the summer. In three out of four seasons, niche overlap approximataed to 0.4; in the summer, however, it increased to 0.68. 7. Discussion Analysis of the content of just over one hundred stomachs and intestines from winter-trapped mink in Michigan led Sealander (1943) to comment that "there was an apparent discrimination by sex as to size of prey items taken". The results of the present study clearly reinforce this statement: female mink were found to prey most heavily upon small, littoral species, while the diet of males was dominated by lagomorphs - the largest HOLARCnC ECOLOGY 84 o.. CO 40- "•male 20- A n.s Fig. 6. Seasonal variations in the consumption of small prey, expressed as percent bulk, by male (solid line) and female (broken line) mink. Large prey make up the remainder of the diets in this analysis. (a) 10,0-1 f ema le, 1.0-1 (b) o- 8 0CO CO 0.8- OQ 6.0- male 0.6- o o 4.0- ii 2.0- 0.4- 0.2- Fig. 7. Seasonal variations in (a) the niche breadth (NB) of male and female mink, and (b) the niche overlap (a) between the two sexes. See text for Simpson's and Pianka's formulae used in the calculation of NB and a. prey item recorded in this study. A sex-related difference in tagomorph predation was also noticed by Birks (198!) in Devon, where radio-tagged male mink were found to hunt rabbits successfully in their burrows to a much greater degree than did female mink. Rabbits must be regarded as a highly profitable resoucc for small carnivores: their fossorial habits make them vulnerable to underground predation, a situation in which speed of pursuit is not vital; an underground kill may be consumed in the relative comfort and security of the burrow; furthermore, at a weight of approximately 2 kg, a fully grown rabbit may sustain an adull male mink for two to three days (Linn and Birks 1981). In the light of these benefits, the low overall contribution of lagomorphs to the diet of female mink suggest that this valuable resource is relatively unavailable to them. Il is likely that the two- or even three-fold difference in bodyweighl between female mink and adult rabbits precludes successful predation on a regular basis. The observation that lagomorph predation by females is at a peak in the summer, when juvenile rabbits are most abundant (pers. obs.), supports the evidence from prey remains that smaller individuals arc selected. A similar pattern has been observed in Swedish stoats Mmtela erminca, which prey mainly on juvenile rabbits (Erlinge pers. comm.). It has been demonstrated that whereas relatively large predators may consume foods unavailable to smaller predators in areas of sympatry, the reverse is much less true (Wilson 1975). This imbalance is apparently reflected in the diets of male and female mink, particularly if seasonal variations are taken into acHOLARCTIC ECOLOGY 8:4 (1^85) count. Whereas the largest prey (lagomorphs) were exploited very heavily by males during some seasons, and relatively lightly by females in all seasons, males also exploited small, littoral prey quite heavily during the autumn. This peak in littoral predation by males occurred at a time when lagomorphs were relatively abundant (Dunstone and Birks, in prep.), so it cannot be regarded as a simple response to a shortage of preferred prey. It may, however, have been a response to a seasonal increase in the availability of large Carcinus specimens. Naylor (1962) noted that the number of large, paired Carcinus found on the littoral zone at low tide in South Wales reached a peak in August and September. A similar pattern was also noted on Ross peninsula (pers. obs.). The vulnerability of erabs to predation during August and September is suggested by their occurrence in the diet of foxes Vulpes vulpcs on Ross peninsula during these two months only. The extent to which the observed dietary differences are a consequence of either sex-related differences in prey availability and preference or some form of intraspecific competitive interaction remains undetermined. Male and female mink occupy overlapping home ranges on this study area (Dunstone and Birks 1983), and female mink are clearly subordinate to males outside the breeding season (Birks 1981). Erlinge (1977) has demonstrated that the normal strategy of a female stoat outside the breeding season is to avoid males or to become inactive when a male is present in the vicinity. A similar relationship might explain, for example, the reduction in the littoral component of the female mink diet during the autumn, when males exploited the spatially and temporally restricted littoral foraging areas most heavily. Radio-tracking evidence indicates that mink do exhibit clear sex-related differences in habitat use. Further data are needed, however, in order to elucidate the role of intersexual Interactions in causing these differences and the associated dietary differences presented in this paper. Unlike Eriinge's (1979) stoat study, in which no consistent difference in niche breadth of males and females was apparent, the present study indicates that male mink niche breadth is less than that of females in each of the four seasons of the year. Dunstone and Birks (in prep.) suggest that the niche breadth of mink on this study area is inversely related to the proportion of lagomorph remains in the diet. The seasonal patterns of niche breadth and lagomorph predation observed in both sexes in the present study tend to support this conclusion. It is significant, therefore, that whilst the larger sex may enjoy a wider range of available prey than the smaller sex in a sexually dimorphic species (Schoener 1967, Wilson 1975). this advantage has permitted more dietary specialization, through greater predation upon lagomorphs, in male mink relative to females. Niche overlap values derived from dietary information gathered over more than one season, or over the whole year, should be interpreted with caution. In the present study, seasonal variations in the diet of males and females resulted in much lower niche overlap In each season when compared with that derived from the overall diet. The summer peak in niche overlap observed in this study occurs when female niche breadth is low. This coincides with the breeding season, when females are restricted to the vicinity of breeding dens for several weeks (N. Dunstone, J. Birks and M. Ireland, unpubl.) and a narrower range of prey is probably encountered. Under certain circumstances, small mustelid populations may be limited by a shortage of food (e.g. Erlinge 1983). Where a mink population is affected in such a way, as Gerell (1971) has suggested happened in southern Sweden following the elimination of an important summer prey species (crayfish Astacus sp.), the degree of niche overlap between the sexes may be instrumental in determining its fate. The low (ca. 40%) niche overlap outside the breeding season observed in the present study would clearly minimize intraspecific competition to a considerable extent under those circumstances, and as such it may be regarded as an adaptive side-effeet of body-size dimorphism. Indeed Moors (1980) has suggested that this beneficial side-effect might augment the main selective pressures which favour body size dimorphism in mustelids. are also duo to A. Rosscr. D. Moltu. T. Birks and M. Ireland for assistance with fieldwork, and to Paul Loftus, Chris Moore and Tom Jackson for technical assistance at Durham. 8.

Journal

EcographyWiley

Published: Dec 1, 1985

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