Many of our primate ancestors probably ended up in the bellies of big cats. How else to explain bite marks on the bones of ancient hominins, the apparent gnawing of leopards or other African felines?
Big cats still pose a threat to primates. In one study of chimpanzees in Ivory Coast, for example, scientists estimated that each chimp ran a 30 percent risk of being attacked by a leopard every year.
A new study suggests that the big cats may be getting some tiny help on the hunt. A parasite infecting the brains of some primates, including perhaps our forebears, may make them less wary.
What does the parasite get out of it? A ride into its feline host.
The parasite is Toxoplasma gondii, a remarkably successful single-celled organism. An estimated 11 percent of Americans have dormant Toxoplasma cysts in their brains; in some countries, the rate is as high as 90 percent. Infection with the parasite poses a serious threat to fetuses and to people with compromised immune systems. But the vast majority of those infected appear to show no serious symptoms. Their healthy immune systems keep the parasite in check.
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Mammals and birds can also be infected. But cats in particular play a crucial part in the life cycle of the parasite: When a cat eats an infected animal, Toxoplasma gondii ends up in its gut. It reproduces there, generating offspring called oocysts that are shed in the cat’s feces. The oocysts can last for months in the environment, where they can be taken up by new hosts.
In the 1990s, scientists discovered that mice and rats infected with Toxoplasma gondii lose their natural fear of cat odors — and in some cases even appear to become attracted to them. It was possible, researchers speculated, that the parasite had evolved an ability to influence the behavior of its rodent hosts, to raise the chances they might be eaten by cats.
Subsequent studies have shown that the parasite can change the wiring of fear-related regions of the rat brain. Robert M. Sapolsky, a biologist at Stanford University, said that these findings led many researchers to see Toxoplasma gondii as a parasite exquisitely adapted to rodents. According to this view, he said, “Toxo being able to infect a zillion nonrodent species is just some sort of irrelevant evolutionary dead end.”
Even so, Toxoplasma gondii can cause intriguing changes in our brains as well. In a 2015 study, for example, researchers found that women infected with the parasite are more aggressive than those without it; infected men behave more impulsively than parasite-free men.
Clémence Poirotte, an evolutionary biologist at the Center for Functional and Evolutionary Ecology in Montpellier, France, wondered if our understanding of Toxoplasma might be limited by the paltry number of species in which its manipulations had been studied. She and her colleagues decided to focus on chimpanzees, running an experiment on 33 apes at a primate research center in Gabon, nine of which had Toxoplasma infections.
Instead of testing the reactions of chimpanzees to the odor of house cats, Ms. Poirotte and her colleagues turned to leopards, their natural predators. A veterinarian at a Gabon zoo supplied them with leopard urine, and they poured drops of it on the fence enclosing the space in which the chimpanzees lived.
Stepping back from the fence, the scientists observed the apes to see how they responded. They also ran the same experiment with urine from three species that are not chimpanzees’ natural predators: humans, lions and tigers.
Sometimes, the chimpanzees would approach the fence and investigate the smell; other times, they would ignore it. Ms. Poirotte and her colleagues found that chimpanzees not infected with Toxoplasma investigated the smell of leopard urine less than the smell of humans.
That’s the sort of behavior you would expect if the smell of leopard urine alarmed the chimpanzees — a healthy instinct that could keep them out of leopard territory and reduce their chances of getting killed.
The Toxoplasma-infected chimpanzees, on the other hand, checked out the leopard urine more often than that of humans, not less. They appeared to have developed the same recklessness observed in Toxoplasma-infected rodents.
“It’s so interesting to see that Toxo seems to have evolved the same manipulation ability in an ape with respect to its natural feline predator,” said Dr. Sapolsky, who was not involved in the new study.
Other experts were also intrigued by the report. But Michael B. Eisen, a biologist at the University of California, Berkeley, said he didn’t think it was powerful enough to rule out other explanations for how the chimpanzees behaved.
There might be innate differences in how the chimpanzees respond to odors, for example, that have nothing to do with being infected with Toxoplasma. “I’d have to file this, at best, in the ‘interesting but nowhere near convincing’ file,” Dr. Eisen said.
Ms. Poirotte acknowledged that it might be possible to tease apart these different possibilities by testing the chimpanzees before and after being infected with Toxoplasma. That would be a very challenging experiment to set up, however.
But the current study provided another piece of evidence that the parasite really was manipulating the chimpanzees. “It works only with leopard urine, and not with other felines which aren’t their natural predator,” Ms. Poirotte said.
That’s the kind of precision you’d expect from a parasite that has evolved a strategy for getting into one particular animal. “It’s so specific that it suggests it’s Toxoplasma causing the behavior modification,” Ms. Poirotte said.
She added that it would be useful now to study Toxoplasma’s effects on other primate species. It may even turn out that our primate ancestors were once the primary targets of the parasite.
When domesticated cats emerged several thousand years ago, the parasite might have expanded into a new host population that favored rodents rather than primates.
“It certainly suggests that Toxo’s behavioral effects in humans may be less of an irrelevant dead end than was always assumed,” Dr. Sapolsky said.
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