The Toxoplasma parasite lives in cats; but can infect humans. If we get infected this parasite can actually change our behaviour. It can increase aggression, slow down reaction time, and even make us attracted to cat urine. Since we get it from cats and it makes us like cats more, it's sometimes nicknamed " crazy cat-lady syndrome ".
For the most part these issues don't cause much of a problem (although the decreased reaction time has been linked to more car accidents). Which is good because almost half of the human population could be infected by the Toxoplasma parasite. It turns out being attracted to little house cats doesn't interfere with our daily lives too much.
But what happens if house cats weren't all we had to worry about? Turns out that's a problem chimps face. They can also get infected by the parasite. But unlike humans, they have to watch out for cats. Their only natural predator are leopards; so the attraction to cat urine can actually put them at a very high risk of being eaten.
Parasite problems
The Toxoplasma parasite can only reproduce in cats. Which cause a bit of a problem. Sure you can make a lot of parasites in one cat, but how do you get the extra ones into a different cat?
The parasite developed a rather dastardly way of spreading. It can exit the cat via their droppings and urine; allowing it to infect any other mammal. But rather than just making a new home for itself, the parasite wants to get back into a cat.
So it starts changing the behaviour of the host to make them vulnerable to getting eaten by cats. In rodents, it almost eliminates the fear associated with cats making them easy prey. And as cherry on the cake, they even give the rodents that attraction to cat urine. Not only do they stop avoiding cats, they start to actively seek them out.
At least humans don't suffer that badly. But it turns out our ancestors might have.
Suicidal chimps
Humans experience some of the symptoms seen in cat "prey". But we aren't cat prey. So it was thought that these mind-control-like symptoms were unintentional. A side-effect of the fact we share some of the same biology as the prey.
But what if these changes weren't side-effects but after-effects? Our ancestors might have been cat prey. Not to house cats, but we did live alongside lions, leopards and all sorts of other felinous felines. Perhaps in the distant past the Toxoplasma parasite infected our ancestors, changed their behaviour and drove them into the arms (or claws) of these predators.
Chimps still have to worry about big cats. Leopards are there only natural predator. So the parasite could still cause problems for them, assuming it ever evolved to turn primates into prey. So a group of researchers infected some captive chimps with the parasite. Sure enough, the parasite changed the chimps behaviour, making them attracted to leopard urine.
This - the paper rather callously states - could
increase the probability of chimpanzee predation by leopards for the parasite's own benefit.
(As an interesting aside, not every big cat carries the parasite. Tigers are a notable exception. Infected chimps and humans still find tiger urine repulsive).
The researchers next aim to see if the parasite actually infects chimps in the wild. If that's the case it reveals our ancestors had to deal with psychological warfare as well as the fact big cats had better claws than us. And that chimps still do have to deal with this. It almost makes you feel sorry for them.
tl;dr
The Toxoplasma parasite changes chimp behaviour, making them more attracted to their predators. This could have been a problem for our ancestors.
References
Flegr, J., 2013. How and why Toxoplasma makes us crazy. Trends in parasitology, 29(4), pp.156-163.
Poirotte, C., Kappeler, P.M., Ngoubangoye, B., Bourgeois, S., Moussodji, M. and Charpentier, M.J., 2016. Morbid attraction to leopard urine in Toxoplasma-infected chimpanzees. Current Biology, 26(3), pp.R98-R99.