A Parasite That Allegedly Causes Chimpanzees to Become Sexually Attracted to Leopards

If the behaviour of our closest relatives can change so dramatically, what are the implications of the parasite's hold over humans?

Animals do the most amazing things. Read about them in this series by Janaki Lenin.

T. gondii tissue cyst in a mouse brain; individual bradyzoites can be seen within. Credit: Wikimedia Commons

T. gondii tissue cyst in a mouse brain; individual bradyzoites can be seen within. Credit: Wikimedia Commons

Chimpanzees infected with the parasite Toxoplasmosis gondii become attracted to the urine of leopards, says a new study. If the behaviour of our closest relatives can change so dramatically, what are the implications of the parasite’s hold over humans?

Toxoplasmosis gondii is a protozoan parasite related to the malaria-causing Plasmodium. Toxoplasma infects a whole range of warm-blooded birds and animals, including humans. But it can reproduce sexually in the gastrointestinal tract of cats alone. To get into a cat, the sex-starved protozoan conspires to get its secondary host eaten. It toys with its hosts’ brains, manipulating their behaviour so they put their lives at risk.

Toxoplasma-infected rats not only lose their fear of cats, but become sexually attracted to them. They recklessly frolic in areas that reek of cat urine, enticing a hungry cat to pounce on them. When rats get eaten by cats, the parasite enters the gut and has sex.

Infected cats shed thick-walled oocysts containing zygotes, or embryos, in their stools. A 20-gram faeces can contain between 2 and 20 million oocysts that last in the soil for more than a year. If a foraging rodent or bird picks up these microscopic protozoans, they become infected.

Humans get infected by eating uncooked meat or not washing their hands after gardening. Up to 60% of the human population is infected. A 2014 study says toxo infections detected in pregnant Indian women ranged from about 9% to 37%. Since testing for the parasite is not mandatory, we don’t know how widespread it is.

Just as it does with rodents, the parasite changes people’s personalities. It delays their reaction times and reduces their ability to concentrate. Scientists found infected men were suspicious, jealous, dogmatic, and unlikely to heed the rules of society. Infected women were warm-hearted, extroverted and easy-going. Toxoplasma forms small cysts in the brain that are associated with a range of human mental health conditions, including schizophrenia, bipolar disorder, and depression.

Why does toxo manipulate our behaviour? According to one theory, these changes are inadvertent side-effects of infection. Toxo could form cysts in the brain to escape the onslaught of the host’s immune system. These cysts could coincidentally alter the behaviour of the host. Another hypothesis says the parasite continues to deliberately manipulate our behaviour without realising we are not on any cat’s menu.

To understand whether toxo’s manipulation was coincidental or deliberate, researchers from the Centre d’Écologie Fonctionnelle et Évolutive, Montpellier, France, tested chimpanzees.

The subjects were 33 chimpanzees living in five captive groups in Gabon. The researchers used an antibody test meant for humans to determine if the animals were infected by Toxoplasmosis. Nine were infected while rest were not. They gave the animals human urine, leopard urine or the urine of tigers or lions, one at a time in a random order. Each group had 20 minutes to investigate before the urine was washed away.

The researchers analysed how frequently chimps approached the urine, whether they smelled it by sticking their noses into it, if they sniffed their hands after wetting them in the urine, or if they tasted it with their tongues. Every 30 seconds, they also noted which individuals were within a one metre radius of the urine.

Infected chimps were three times more likely to investigate and approach leopard urine than uninfected ones. Just as the parasite meddled with rats’ sense of smell so they would get eaten by cats, it seemed to make leopard urine smell attractive to its host chimpanzees. Leopards scent-mark their territories with urine. Wild chimps could lurk around these chemical signposts, becoming veritable leopard baits. Leopards would slay these reckless chimpanzees and become the breeding grounds for the parasite.

Interestingly, infected and uninfected chimpanzees were uninterested in the smell of lion and tiger urine. The authors say this is because these cats are not chimpanzee predators. Lions live in open savannahs while chimpanzees live in rainforests. Tigers live in Asia while chimpanzees are African. The researchers say the parasite specifically manipulates its host to fall prey to their natural cat predators.

Since the reactions of infected chimpanzees are the same as infected rats, the authors speculate the parasite causes the change in the primates’ behaviour. Do toxo-infected chimps fall prey to leopards more often than unparasitised ones? Unlike rodents, it’s impossible to prove, say the authors.

If the brains of chimpanzees can become addled by parasites, what happens to humans? Are millions of sex-starved protozoans stuck within human bodies desperate to make the leap into cats? Stone Age people regularly fell prey to sabre-toothed cats, tigers, lions, and leopards. Over the centuries, we’ve taken ourselves out of cat menu.

Almost every research paper says modern humans are a dead-end for the parasite. About 85 people are killed or injured by tigers alone in India. And in the Garhwal and Kumaon, 180 people were killed by leopards in the eight years since 2000. Is it likely that communities working the soil are more prone to toxo-infection? Would they behave as carelessly around tigers and leopards as infected rats and chimpanzees?

The study raises three issues: Toxoplasma affects a diverse range of warm-blooded animals and birds. How does it know which animal it has affected and what that animal’s natural predators are? Since lions, tigers, and leopards are cats, what would the parasite gain by being so finicky in its choice of cat species?

“An alternative explanation is that x ml of predator A urine is not perceptively equal to x ml of predator B urine,” Ajai Vyas, with the school of biological sciences at Nanyang Technological University, Singapore, told The Wire. “One of the stimuli is a weaker scent. It could be because of anything, may be one animal produces more urine per body mass per hour. So the researchers might be measuring different doses of predator odours when they think they are measuring two odours. I say this because none of us have ever been able to come up with a mechanism that can reliably separate two phylogenetically similar predators in the brain of a prey. Not in neurobiological studies of fear outside the context of the parasite; and definitely not in toxoplasma context.

“All mechanisms proposed for post-toxoplasma behavioural change will return an expectation of general reduction in defensive behaviours. So I will take a more parsimonious view that difference in leopard and other feline predators are probably artefacts of the experimental approach.”

Secondly, the authors cite a 1991 paper that estimates an individual chimpanzee’s risk of being eaten by a leopard at 30% per year in Ivory Coast. A more recent study, published in 2002, conducted in the same forest shows leopards don’t prey on chimpanzees. Only one in 200 leopard faeces had chimpanzee remains. Neither is the case of lion predation settled. In 1989, two lions preyed on four chimpanzees in Tanzania’s Mahale Mountains National Park. Other chimps reacted to the lions by alarm calling, whimpering, climbing into trees or being silent. This may be a rare event. But in southeastern Senegal, lions and leopards prey on these primates. In sum, the selective attraction of leopard urine alone isn’t convincing.

Thirdly, if toxo is so specific about targeting its host’s predators, it doesn’t explain the findings of another paper published in 2011. Much like this study, Czech researchers tested toxo-infected and uninfected people’s reactions to the smell of domestic cat and tiger urine. Infected men rated domestic cat urine pleasant, but they thought tiger urine smelled bad. The researchers speculated that an amino acid called felinine could make domestic cat urine smell pleasant. The compound is found only in small cats but not in large ones like leopards and tigers. However, infected women didn’t like the smell of cat or tiger urine. If toxo specifically manipulated its hosts to give themselves up to their predators, infected men and women ought to be attracted by tiger urine. There’s no explanation for the differences in men and women’s reactions.

Although as a species, we aren’t on the list of cat prey anymore, we may have played successful hosts to toxoplasma in the past. Vyas says a common ancestor of humans and chimpanzees may have evolved with the parasite, but many of its current strains are of recent origin. Or, it’s possible that our brains, whether rodent or primate, react to the infection in similar ways. “The last option seems the most plausible to me, and that contradicts the idea that chimps will show specific change only to leopard and not other feline predators,” says Vyas.

While the effects of the protozoan on rats seem in no doubt, the consequences for humans raise several questions. The only animals to escape toxo’s fiddling are cats. Toxoplasma makes hunting so much easier for felines. Most cats don’t seem to suffer any ill-effects, while severe infestation may cause problems. After all, it’s not in toxo’s interest to kill cats.

The study was published on February 8, 2016, in the journal Current Biology.

Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.

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