A Parasite of the Morbid Mind
Toxoplasma gondii and morbid curiosity
Over two thousand years ago, Plato told the story of a man named Leontius and his morbid curiosity.
One day, Leontius was walking near the city walls of Piraeus, when he caught a glimpse of an executioner standing over a pile of dead bodies. Leontius felt an overwhelming desire to look at the bodies while simultaneously loathing the thought of them. His morbid desires eventually won out and he rushed up to the bodies, shouting, “There! You wretches! Gaze your fill at the beautiful spectacle!”
Leontius felt that one part of his mind had rebelled against him, almost forcing him to confront the mangled bodies. He didn’t delight in the sight or plan his trip to the wall so that he might happen upon the executioner’s work. He didn’t run to see them with joyous excitement. Instead, Leontius actively fought against the urge to look and, ultimately, lost. His curiosity was too strong, overpowering the other aspects of his mind that might have kept him from looking.
The power morbid curiosity has over our attention can sometimes make it seem like we aren’t in the driver’s seat.
Well, what if we aren’t?
T. gondii is a parasite that is found all across the world and can infect virtually any warm-blooded animal. However, the only known definitive hosts (the animal in which the parasite sexually reproduces) are felids — cats. Because T. gondii can only sexually reproduce in the intestines of a cat, it spends its entire lifecycle trying to get into the intestines of a cat. And one of the best ways to get into the intestines of a cat is to hitch a ride with a mouse.
Rodents that are infected with T. gondii begin to act strangely. They becomes less vigilant about predators, explore more, and, most critically, they become attracted to cat urine. The parasite alters the rodent’s behavior in ways that make it more likely to be eaten by a cat, thus helping it complete its lifecycle.
You might say T. gondii makes the rodents a bit more morbidly curious.
When a cat eats a T. gondii infected rodent, the parasite finds itself in the intestines of the cat, where it can sexually reproduce and complete its lifecycle.
Rats aren’t the only animal whose behavior changes when they are infected with T. gondii. I came across a paper recently that describes a peculiar phenomenon among chimpanzees that are infected with the parasite: they lose their natural aversion to felid urine. The effect was particularly pronounced when it came to the urine of leopards, their only natural predator.
One line from the paper really caught my attention:
“However, hominids have long coexisted with large carnivores and were preyed upon at rates comparable to those for extant primates [10]. When feline predation was still an evolutionary driver in our lineage, possible manipulative abilities may have evolved because early hominids were suitable intermediate hosts.”
Could T. Gondii have made humans more morbidly curious?
There are plenty of studies showing strong behavioral, physiological, and pathological associations with T. gondii infection, and over 40 million people in the US alone are infected. Based on this, it’s not unreasonable to assume that T. gondii has played a role in our behavior throughout our evolutionary history.
It’s possible that T. gondii has similar felid-related effects in humans as it does in rodents and our primate relatives. Unfortunately, there aren’t really any studies I could find that looked into this intriguing prospect. For being such a widespread organism, there is surprisingly little research on its behavioral effects in humans.
T. gondii does seem to reduce fear of predators in some animals, but findings are mixed on whether the effects are species-specific or a reduction in cost-benefit calculations generally. In either case, it seems probable that this would influence morbid curiosity.
I’ve written before about the unusual nature of cat domestication, and I’ve suggested that morbid curiosity may have played a role in it given our species’ history of being hunted by big cats. An interesting twist to this story might be the role of T. gondii. Perhaps this felid friendly parasite hijacked our minds to be a bit more curious about our oldest natural enemies — curious enough to let the smaller version hang around our encampments and, when they feel like it, inside our homes.