|Imagine a disease causing organism - like a parasite - that needs to pass from one host to another to survive and reproduce? How does it make sure it gets where it wants to go? Well one way is to manipulate the behavior of its host. Certain parasitic liver flukes (flatworms) for example - which reproduce in sheep - must pass from sheep, to snails, to ants, and back to sheep to complete their entire complicated life cycle. These clever flukes have found a way to induce infected ants to forego ret |
Imagine a disease causing organism - like a parasite - that needs to pass from one host to another to survive and reproduce? How does it make sure it gets where it wants to go? Well one way is to manipulate the behavior of its host. Certain parasitic liver flukes (flatworms) for example - which reproduce in sheep - must pass from sheep, to snails, to ants, and back to sheep to complete their entire complicated life cycle. These clever flukes have found a way to induce infected ants to forego retiring to their comfy nests come evening and instead climb up stalks of grass - where they're easily eaten by grazing sheep. Then snails eat the sheep poop, ants munch on the snail slime, and the cycle goes on.
Or what about the hairworm - which reproduces in fresh water - but has to pass from a mosquito to a cricket and then back to water to make more of its kind. These cunning operators force infested land-dwelling crickets to (uncharacteristically) jump into a lake or pond. The worms then escape, reproduce, latch onto mosquito larvae, and get back to land in airborne mosquito adults - which make a nice meal for hungry crickets...and so forth.
In the early chapters of her book, Kathleen McAuliffe presents many examples of infectious organisms manipulating their hosts in this fashion - from impelling fish to wiggle their bellies to attract peckish birds; to forcing spiders to spin 'nursery webs' for wasp larvae; to making crabs sprout brood pouches for baby barnacles; and so on. The manipulator's methods may involve forming cysts in the brains/nervous systems of their victims, producing chemicals, activating/inactivating hormones, making proteins, altering DNA, etc. Whatever works for them.
You might think....well....those are 'lowly' invertebrates. Advanced animals, like mammals wouldn't succumb to this kind of tampering. But you'd be wrong. For instance, Toxoplasma gondii ('toxo') is a protozoan parasite that lives and reproduces in cats, forming cysts that are shed in cat feces. When a rat consumes the cat turds the parasite induces the rodent to engage in risky behavior - like purposely cavorting in the path of a hungry feline - so the toxo can get into another cat and continue to propagate its kind.
As it happens toxo can also infect humans, who contract them from litter boxes, unwashed produce, or contaminated water. The protozoans then make themselves at home in the victim's brain where (as in rats) they induce 'risky' behavior. In people this might be dangerous driving, antagonizing enemies, reacting slowly to hazardous situations, perhaps even attempting suicide. Moreover, scientists have found that the parasites may hasten the development of schizophrenia in suspceptible persons.
Parasites aren't the only organisms that alter human behavior. Pathogens (disease causing microbes) - which have probably been around since life evolved - also manipulate their hosts. Anecdotal reports, for example, suggest that terminal aids patients develop fierce cravings for sex - presumably to help the HIV virus find new hosts. And people recently infected with a flu virus may get the urge to go out and socialize - inevitably spreading germs - before aching muscles and a runny nose sends them to bed.
Even organisms that are essential parts of the human body, like gut microbes that help us digest food, can adversely influence our behavior. For instance, patients with gastrointestinal disorders - possibly caused by too many 'bad bacteria' - are more likely to suffer from bipolar disorder, anxiety, and depression. Researchers have shown that probiotic remedies (similar to the ingredients in some kinds of yogurt) can boost the gut's 'good bacteria' population and help alleviate these symptoms.
Of course host organisms aren't going to let parasites and pathogens have it all their own way. They're going to fight back! Thus, animals have evolved a variety of self-protective behaviors. For instance, many species - including primates - perform grooming behavior that removes parasites from the skin; herds isolate or shun sick individuals; animals eat or use medicinal plants; organisms avoid vomit and poop (don't shit where they eat); and most creatures strive to find healthy partners for sex. This explains the appeal of attractive partners with an appealing aroma, who are less likely to have health issues that affect their appearance and smell.
With regard to human avoidance of parasites and pathogens, McAuliffe describes our "behavioral immune system." To put it simply, this is a repertoire of behaviors that helps us avoid 'disgusting' things that (we instinctively feel) may make us ill. This growing field of study is called disgustology and its proponents are dubbed disgustologists. (ha ha ha). Scientific studies (and everyday observations) demonstrate that people are often repulsed by: cockroaches; rats; spiders; worms; people with bad hygiene; individuals with skin rashes; things that smell bad; revolting foods (which vary with culture); and - oddly enough - clusters of little holes...which apparently remind us of insect eggs.
In fact people's avoidance of pathogens and parasites may have led to the development of culture, religion, racial prejudice, dislike of foreigners, liberal or conservative leanings, and so on. These latter speculations are interesting and provide a unique perspective on human history.
I enjoyed the book, which I found very enlightening. My major criticism would be that the topics range all over the place, with some explanations being better than others. Still, I'd highly recommend the book. If nothing else, it will give you a little insight into what people feel and do....and provide some excellent conversation starters for social gatherings.
Thanks to Netgalley, the author, and the publisher for a copy of this book.