Parasites change the behavior of animals. They change the behavior of the animals that they infect, and they change the behavior of animals that attempt to avoid infection. What does this mean for concepts like free will, for the treasured notion that our decisions depend on some form of rational thinking? I began a 1984 Scientific American article about parasites and behavior with a nod in the direction of free will, but then I proceeded to stick to the science, as sparse as it was. Since then, the science itself has burgeoned. It now reaches far beyond its roots in parasitology and animal behavior and intrigues psychologists, social scientists, and even jury-selection consultants. [Disclosure: Because of my early and continued involvement in this field, I appear in this book; that said, the coverage is thorough, and many of my colleagues in the field also make appearances.] Kathleen McAuliffe has embraced this riotous expanse of hard data, exciting conjecture and endless questions, and has written a book that is well worth reading. It is as good a story as we would expect from a writer whose work was included in The Best American Science Writing 2010 and who has amassed significant honors in the field of science writing. Indeed, one of my favorite things about the book is the way that McAuliffe wrestles this unruly field into a surprisingly clear and beckoning narrative arc. As I neared the end of the book, I was surprised at how far (and painlessly) I had traveled conceptually. The book begins with an exploration of parasite manipulation of animal behavior. It touches such things as horsehair worms (nematomorphs) that compel their cricket hosts to plunge into water, tapeworms of flamingos that turn their crustacean intermediate hosts a bright pink, trematodes that cause fish intermediate hosts to flash their silvery undersides at final host predators, and vectors of blood-borne diseases in which blood feeding is altered—to list a few. View largeDownload slide View largeDownload slide This provides a natural gateway into a consideration of parasites and human behavior. McAuliffe explores the possible effects of parasites on sex drive and propensity for accidents, as well as their subtle and worrisome effects on learning and memory in children. The intestinal microbiome has been in the news of late, and McAuliffe reveals why. After all, we live with more than 100 trillion organisms (or as McAuliffe puts it, “a rainforest growing within each of us”); should we not expect some interaction? These companions secrete neuroactive metabolites and activate the immune system in ways that might well influence everything from weight gain to personality, especially given the fact that the abundant neurons surrounding the gut are in close contact with that other brain—the one found in our cranium. So what might this tell us about behavioral modification and probiotics, or for that matter, antibiotics? The possibilities are vast. McAuliffe points out that because most of our microbiome has evolved with animals for hundreds of millions of years, the effect of these organisms on our behavior is part of the fabric of our own evolution. She cautions, “Make no mistake: Their influence on our behavior is dramatic. In fact, I’m not sure we will ever truly be able to separate their motives from our own.” And then, of course, there are the behaviors that prevent or mitigate encounters with parasites. These range from the obvious, such as grooming and fly swatting, to the subtle, such as mate choice and the use of medicinal plants. Indeed, they range further, into that territory called “disgust,” or as McAuliffe titles it, “The Forgotten Emotion.” None other than Darwin noted that the facial expressions associated with disgust were similar around the world and were almost identical to those conveying contempt. Today's evolutionary psychologists link disgust to a variety of personality disorders, as well as the avoidance of contamination. This places disgust squarely in the domain of human behavior, and psychologists doubt that many other animals experience this emotion; it may be associated with our big brains and our enhanced imaginations. As a behavioral biologist, I add that the history of science is littered with cognitive attributes once thought exclusive to humans, ranging from tool use to mental time travel. Stay tuned. Protecting the body and protecting the soul are concepts that share a certain similarity, and at its outermost reach, McAuliffe notes that disgust and its evil twin, prejudice, may have influenced the development of some of humankind's basest inclinations and most soaring ideals. One of those baser inclinations is xenophobia. Evolutionary psychologists now see this as possibly an evolved response that holds novel germs at bay. This was precisely the hypothesis put forward and tested with baboons by William Freeland in 1979 (doi:10.2307/1936609); the results were affirmative. Beyond xenophobia, there is the heartbreaking inability of many folks to interact normally with people who have somehow triggered a disgusted response because of conditions that cannot be contagious, ranging from cancer and mutilation to incontinence. As a nasty corollary, the victim also may feel shame. Much like a hyperactive immune system, a hyperactive disgust response is no one's friend. Indeed, political inclinations and religious outlook can be related to how easily a person is revolted by standard images that elicit disgust. On a more optimistic note, shared disgust can create social cohesion and may well be the impetus toward what we now view as good manners. Could it be a driving force behind what we call culture? To what extent does culture then mediate what we find disgusting—or uplifting? Starting with tales of trematodes and flamingos, in the space of 268 pages, McAuliffe leads us to question whether our most deeply held convictions and gut-level reactions are somehow related to parasites. She does this without sensationalism; she is careful to remind us that correlation should never be confused with causation. She puts nightmare scenarios into common-sense contexts: For instance, even if Toxoplasma does increase our risk tolerance in ways that could contribute to car accidents, its effect is miniscule compared with that of texting and other forms of distracted driving. And if parasites do influence our behavior to the point that it is actually observable, then we are still in no danger of becoming automatons, marching in lockstep—because no one has the same symbiotic community. Without so much as a wobble in her storytelling, McAuliffe also manages to teach a good bit of biology—deftly leading us through the sometimes-serpentine world of parasite life cycles and mosquito gonotrophic cycles and explaining the pathology of rabies in clear detail. She hits that target nestled between arcane hand waving and dumbing down; from “epigenetic transformation” to “enteric nervous system,” her definitions are clear and concise and never detract from the larger story. And then there are other definitions: You’ll have to read the book to find out about “crapsules.” They are for real. McAuliffe's curiosity drives this book. I found that palpable; it is part of what makes the book such a fun read. She is eager to lead us to the edge of what is known. Might parasite-induced behavioral alterations profoundly affect food webs? If Toxoplasma affects drivers’ propensity for car accidents, should that influence insurance rates? If (and that is a very big “if”) parasites can take swipes at free will, what implication does that have for our social contract? The book is rich with interviews of people who create this scientific literature. To see its promise and its limits through their eyes is to encounter parasites, behavior, and how well we know ourselves through multiple lenses. It turns out that I was not the only one who had to struggle to convince the scientific establishment of the importance of parasites to ecology and behavior. Despite its homage to the scientific method and the testing of countless hypotheses, science cannot escape its human core. It is a social construct, and that means that it can be pretty hidebound, at least until it meets up with creative, determined scientists. Finally, McAuliffe's story glitters with random nuggets that are themselves worth the price of the book. For instance, many animals, from lemurs to insects, are particular about where they release feces, thus avoiding contamination. Bees are no exception, and they frequently leave the hive as a group, releasing a “yucky yellow mist.” McAuliffe reveals that Alexander Haig Jr., secretary of state during his 1985 visit to Laos, mistook this for chemical warfare. She adds spice to her masterful synopsis of rabies pathology with a paragraph detailing the parallels between the course of that disease and what we know about the origin of vampire myths. Because clay is valued around the globe for its medicinal, stomach-settling properties, McAuliffe went to the trouble of tracking down Grandma's Georgia White Dirt—available on the Internet—and tasting it. She is not a fan but admits that she was not feeling all that ill when she tried it. McAuliffe attributes her skill as a writer to time spent in Irish pubs during her student days. As she puts it, “Your status in those lean times hinged almost entirely on how good a yarn you could spin.” McAuliffe has done her early training proud. Your Brain on Parasites is definitely a good yarn. © The Author(s) 2017. Published by Oxford University Press on behalf of the American Institute of Biological Sciences. All rights reserved. For permissions, please e-mail: firstname.lastname@example.org
BioScience – Oxford University Press
Published: Jan 1, 2018
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