Does a Brain Parasite Make Chimps Morbidly Attracted to Leopards?

New insight into a parasite that makes some animals suicidal.

When a mouse smells the heady aroma of cat urine, it will typically run in the sensible direction: away. But if that mouse is infected with a parasite called Toxoplasma gondii (or Toxo for short), it will instead make a fatal beeline towards the source of the smell, and the jaws of the feline that created it.

Its suicidal behaviour is Toxo’s doing. This parasite infects a wide variety of mammals but it can only complete its life cycle in the guts of a cat. If it ends up in a mouse or rat, it’s stuck—but not for long. It permanently hacks the rodent’s brain chemistry, so that it reacts to the smell of cat with something more like sexual arousal than fear. It becomes a vehicle for sneaking Toxo into its ideal host.

But what happens when Toxo gets inside a bigger-brained animal like a chimpanzee? And what happens when that infected chimp smells the scent of a big cat, like a leopard?

To find out Clémence Poirotte from the Center for Functional and Evolutionary Ecology in Montpelier travelled to a primate research centre in Gabon and worked with 33 captive chimps, 9 of which were infected with Toxo. Her team dabbed spots of leopard urine around the chimps’ fences, and found that the infected animals approached and investigated the urine more frequently than the uninfected animals.

That, the team say, is a sign of “morbid attraction”. The curious chimps may be behaving like the suicidal rats, compelled by Toxo to put themselves in harm’s way and so smuggle the parasite into a cat—albeit a very large one. As they write, “Since leopards urine-mark their territory, a decreased avoidance of leopard urine in the wild may therefore increase the probability of prey–predator encounter.”

The team admits that their results are correlative, and “a pre-existing recklessness” might explain why some chimps are both infected by Toxo and more drawn to leopard urine. Alternatively, the infected chimps might just be acting weirdly as a side effect of infection, because Toxo has caused inflammation in their brains. (This is likely why infected sea otters are more vulnerable to shark attacks, and not some kind of manipulation as has sometimes been suggested.)

But these hypotheses don’t explain why the chimps react specifically to the smell of leopards—a predator that kills substantial numbers of chimps every year. The team found that infected and uninfected chimps didn’t behave differently when they smelled the urine of humans (which aren’t good final hosts for Toxo), tigers (which don’t live in Africa), or lions (which don’t hunt chimps)—just leopards.

But that actually makes me less convinced that the team have found a genuine case of manipulation. When Toxo gets into rodents, it changes their response to the urine of all cats, whether domestic tabbies, tigers, or bobcats. And yet here, it’s apparently changing a chimp’s response to Eau de Leopard, but not the scents of lions or tigers. If that’s right, it must be capable of host-specific manipulations, changing the behaviour of different mammals according to their most relevant predators.

That would be amazing if true, but it also seems far-fetched—or, more generously, something that needs to be supported by stronger evidence than this small study of just 9 infected chimps. With such a small sample size, it’s entirely possible that the results are a statistical fluke.

By comparison, it’s worth noting just how much work has gone into establishing Toxo’s manipulations of rats. Scientists first discovered its ability in the 1990s, and have been replicated those findings again and again. They’ve studied the behaviour of the infected animals, peered into their brains, looked at their immune systems, and more. They’ve done experiments to show that the rodents’ suicidal tendencies are a direct result of the parasite and not some inadvertent side effect.

Of course, as Poirotte’s team says, such evidence would be difficult—if not impossible—to get for chimps. You can’t very well expose infected and uninfected individuals to leopards and see if the former are more likely to die. Still, that leaves their study in a difficult place. It’s certainly too early to make any claims about how this relates to Toxo’s effect on people.

One in three people around the world apparently carry the parasite. Some scientists have suggested that Toxo can affect our personality, and perhaps even increasing our risk of schizophrenia. Poirotte suggests that these effects may be leftovers from a prehistoric time when humans were hunted by big cats, and Toxo could have benefited from hacking our brains.

But as I’ve written before, the study of Toxo’s effects on humans is rife with tenuous evidence. It’s not implausible that a parasite could be manipulating our behaviour but no one has demonstrated that with any certainty yet—and the chimp experiment doesn’t change that status quo. Parasite whisperer Carl Zimmer was the first to write about the study, and he quoted Michael Eisen as saying “I’d have to file this, at best, in the ‘interesting but nowhere near convincing’ file.” I think he’s right.

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