Saying “The Devil made me do it” is a well-known and lighthearted – but ultimately meaningless – way of exploiting theology to avoid accountability for our actions. But saying “The protozoans made me do it,” even if it doesn’t roll off the tongue quite so easily, actually has some scientific backing. There is a growing body of evidence that parasites play a large role in influencing animal behaviors… and one of the best known, Toxoplasma gondii, lives in the bellies of North America’s second-most beloved pet: the common housecat.
Many animal lovers already know about the effect of toxoplasmosis on rodents. In a nutshell, cats are the only host T. gondii can reproduce inside of… and wouldn’t you know it: toxoplasmosis-infected rats don’t show the same aversion to cat urine, making these rodents (and the parasites they are carrying) more likely to find their way into a cat’s stomach. This is a convenient coincidence (except for the rat)!
But such convenient coincidences may be just the tip of the iceberg. Newer research in the wild shows T. gondii-infected chimps and hyena cubs attracted to the urine of large, predatory cats. And infected humans display differences in behavior, too. While the odds of us being eaten by a cat are quite small, people suffering from toxoplasmosis engage in notably more risk-taking behaviors, and demonstrate less fear of failure when compared to the general population. Past experiences, our environment, genes, and social expectations are well-known factors that shape behavior, but as we’re learning with parasites like T. gondii, there may be numerous lesser-known – but significant – factors at play, as well!