Chimpanzee under the influence

What would drive a chimpanzee into the clutches of its only predator, the leopard?
Answer: Toxoplasma gondii. Researchers have shown that when these primates are infected with the toxoplasmosis parasite, they are attracted by the urine of their natural predator. A manipulative phenomenon that could well be expressed in humans too...

Chimpanzee at the Limbe Wildlife Center in Cameroon. Sabrina Locatelli © IRD

We're manipulating you! "We? It's Toxoplasma gondii, the parasite responsible for toxoplasmosis. This protozoan can only reproduce in the body of a feline, but in the course of its wanderings, it temporarily takes up residence in numerous "intermediate" hosts, such as mice, chimpanzees and humans. To return to the feline and perpetuate its lineage, there is only one solution: to see this intermediate host end up in the stomach of a felid. To achieve this, Toxoplasma gondii seems to be able to "manipulate" its host. Studies have shown that mice infected with the parasite are attracted to the urine of cats, increasing the likelihood of them being eaten.

Parasitic cul-de-sac

What about humans, 1/3 of whose population is infected with the parasite? The days when humans were the natural prey of other animals are long gone, so the likelihood of this host ending up in the stomach of some felid is very slim indeed. A real dead end for the parasite... ". Yet numerous studies suggest that contaminated humans also show behavioral changes: increased reaction times or reduced concentration", explains Marie Charpentier, researcher at theInstitut des sciences de l'évolution de Montpellier (Isem).

Are these secondary effects of toxoplasmosis, or atavistic attempts by the parasite to escape this human impasse? To better understand these changes in behavior, Marie Charpentier and Clémence Poirotte took an interest in man's closest cousin, the chimpanzee, whose natural predator has always been a big cat, the leopard.

Easy prey

"We observed the behavior of 33 chimpanzees, 9 of which were infected with toxoplasma," explains Marie Charpentier. The researchers spread leopard urine in the primates' enclosures and observed their reactions. The verdict: "The healthy animals avoided the leopard urine much more than their infected counterparts, who approached it more and seemed intrigued by the smell, deliberately moving towards the urine. At the controls, Toxoplasma gondiifait makes infected chimpanzees easier prey for the feline.

The fact that monkeys close to the human species are indeed manipulated by the parasite suggests that the same mechanism could be at work in humans. "The behavioural changes observed in infected individuals could be the remnants of an ancient parasite manipulation, from the time when our ancestors were preyed upon by big cats.