Parasite makes wolves more likely to become pack leaders

Toxoplasma gondii is sometimes called the “mind control” parasite: it can infect the brains of animals and disrupt their behavior in ways that kill the host, but ensure the spread of the parasite. But now researchers have found that infected wolves can actually benefit from these mind-altering tricks. A Toxoplasma Infection, they found, makes wolves bolder and more likely to become pack leaders or disperse to other habitats, giving them more opportunities to breed.

“We really underestimated some of the consequences of this parasite,” says Eben Gering, a biologist at Nova Southeastern University who was not involved in the work. “The results probably represent the tip of the iceberg regarding the importance of the parasite for the dynamics of wild ecosystems.

T. gondii, a single-celled parasite, breeds only in domestic cats and other felines. Infected cats excrete oocysts filled with spores in their feces, which can survive on plants or in soil or water. They can also persist in undercooked meat from livestock or game. When a host, including humans, consumes an oocyst, the spores are released and spread throughout the brain and muscles, forming new cysts. Worldwide, approximately one in four people are infected. Usually the immune system controls the parasite, but it can cause spontaneous abortion and other serious problems during pregnancy.

It has long been known that rodents infected with Toxoplasma lose their fear of predators. Cysts in the brain somehow increase dopamine and testosterone, stimulating boldness and risk-taking and increasing the chances of the host being eaten by cats. “These parasites use generic mind or personality control that helps them complete their life cycle,” says Jaap de Roode, a biologist at Emory University who was not involved in the new study. “And it has all kinds of interesting consequences that we may not have even thought of before.”

The consequences are not limited to rodents. In 2016, Gabonese researchers discovered that Toxoplasma– infected captive chimpanzees lost their aversion to leopard urine. And last year, another team described how Toxoplasma-Infected baby hyenas in Kenya venture closer to lions, making them more likely to be killed.

When researchers learned a few years ago that some wolves in Yellowstone National Park were infected with Toxoplasma, Connor Meyer, Ph.D. student at the University of Montana, teamed up with park biologist Kira Cassidy to see if the parasite also changes wolf behavior.

Meyer and Cassidy looked back on 26 years of research on the park’s gray wolves, including Toxoplasma results of analyzes of blood samples taken from various areas of the park. They also looked at data on cougars, in which Toxoplasma can reproduce. Wolves that were in areas with lots of cougars were more likely to be infected with Toxoplasma, they found. According to the authors, it’s likely that these wolves picked up their infections from cougars, possibly by scavenging or eating the big cats’ feces.

By combining infection data and past field observations, they also found that infected wolves were much more likely to become pack leaders, the team reports today in Communications Biology. Infected wolves were also more likely to leave their packs at a younger age and seek out new territory or other packs, just as infected rodents become more eager to explore. “There might be a few instances where wolves or even their packs are really successful because they push those boundaries and are more accepting of risk,” Cassidy says.

The study is one of the few to examine Toxoplasma in nature. “We know that infection can change animal behavior, but it’s very difficult to document that in wildlife populations,” says Meggan Craft, wildlife disease ecologist at the University of Minnesota. “The cool thing about this study is that it relies on a fabulous long-term study to be able to tease out these subtle impacts of infection and behavior.”

As with rodents, boldness in wolves also carries risks. Wolves that roam widely might be more susceptible to being run over by a car or leaving park boundaries and being shot by hunters. “Dispersal is one of the most dangerous things a wolf can do,” says Meyer. It is also possible for an infected pack leader to transmit the parasite during mating, as can happen in dogs, potentially jeopardizing a pregnancy. Overall, Cassidy suspects the risks of infection likely outweigh the long-term benefits. “Wolves live on a knife edge to survive,” Cassidy says.

Because wolves are one of the park’s keystone species, this parasite “can really have very significant impacts on ecosystems,” de Roode says. “They can control food webs; they can control the flow of energy within ecosystems.

Infected pack leaders could even influence uninfected wolves, the researchers speculate in their paper. Pack members can mimic their leader’s boldness or curiosity about cougar scents, resulting in more wolves being infected. “It’s a brilliant idea, and I think it’s very likely,” says Gering.

Ultimately, wolves appear to be a dead-end host for Toxoplasma, however, as they are unlikely to transmit the parasite to cougars. Still, Meyer wonders if the parasite’s effect on wolves means the animals played a role in the infection cycle at some point in the distant past. During the last Ice Age, he notes, large lions roamed North America that might have preyed on these infected and emboldened beasts.

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