Parasite may create risk-taking wolves in Yellowstone

The story was picked up by major news outlets. The research originally was published in the journal Communications Biology.

"I've been blown away by it," said Connor Meyer, a wildlife biology doctoral student in UM's Ungulate Ecology Lab, part of the W.A. Franke College of Forestry & Conservation. "I'm surprised and grateful, but it's been a bit of a nerve-wracking experience with all the attention."

Meyer and his team created the story sensation by studying a single-celled creature named Toxoplasma gondii -- often nicknamed the "mind-control parasite." It prefers to live in felines, and infected cats spread spore-packed oocysts in their feces. T. gondii -- which Meyer calls "toxo" for short -- is the reason pregnant people aren't supposed to clean the litterbox. Human immune systems usually keep it in check, but the parasite causes sickness that can be dangerous to fetuses, as well as those who are immunocompromised, such as HIV/AIDS patients.

T. gondii can infect all warm-blooded mammals, and it's estimated a third of all people are carriers. The parasite settles in muscles and brains, and it's known to boost dopamine and testosterone. This affects behavior: Studies have shown that infected rodents lose their fear of feline urine or cats and move around in the open more, making them more likely to be eaten. Infected captive chimpanzees lose their aversion to leopard urine.

It's almost like they are being biologically controlled so the parasite can return to the comfy insides of its preferred feline host. But do other beasts get affected that aren't part of the regular T. gondii life cycle?

Meyer and his fellow lead author, Yellowstone park biologist Kira Cassidy, started a serious study of the prevalence of T. gondii among park wolves in spring 2021. They discovered a toxo-positive wolf becomes more of a risk taker -- 11 times more likely to disperse from its original pack and 46 times more likely to become a pack leader.

Yellowstone wolves are among the most studied animals in the world. Since they were reintroduced in 1995, park managers have taken blood samples every time a wolf is captured and collared. Meyer and his team wound up testing blood from 243 wolves for toxo antibodies with assistance from a Cornell University diagnostics lab. They also used data from long-term and ongoing Yellowstone Wolf Project research. More than 27% of the wolves they looked at -- about 74 individuals all told -- were infected with T. gondii.

The researchers first suspected wolves were getting infected by eating elk, their chief prey. But when they tested more than 100 elk, none were positive for the parasite.

"Eventually we found the most significant predictor of infection with wolves was when their range overlapped areas with high mountain lion density," Meyer said. "So, with no elk testing positive, we hypothesized they were getting infected directly by cougars."

Yellowstone wolves can slay and eat mountain lions, but there only have been 10 or so documented cases of that since 1995. Meyer said it's more likely wolves they get toxo infection by nosing around "scrape sites," where cougars defecate and mark their territory.

"We also have a litter box theory," he said. "Almost anyone who has a dog and cat at home knows that, if the dog gets an opportunity, they are going to raid the litter box. We don't have direct evidence of wolves eating mountain lion scat, but we have lots of photos of wolves at mountain lion scrapes. Wolves eat lots of things, so we don't think it's much of a stretch."

Meyer said they want to emphasize they aren't claiming that toxo causes wolves to become leaders.

"Toxo is not the only factor that predicts whether wolves will lead the pack," he said. "It's one of the many things that affect wolf behavior, just like in humans. With our study, being toxo-positive shortened the time it took for individuals to disperse, but toxo-negative individuals would still disperse and still become pack leaders. So we aren't saying that toxo runs the world -- we are saying it may accelerate some of these behaviors."

He also said wolf packs generally have two leaders, a male and a female, and both are equally likely to test positive for the parasite.

A native of Whidbey Island, Washington, Meyer first became fascinated by the T. gondii life cycle as an undergraduate at the University of Washington. He then was hired by Dr. Matthew Metz -- who earned his Ph.D. from UM last year -- to work for the Yellowstone Wolf Project and soon after also began work with the Yellowstone Cougar Project. Over six years he worked on a variety of research efforts, which brought him into the orbit of Professor Mark Hebblewhite, leader of UM's Ungulate Ecology Lab. Meyer started making inquiries about grad school.

"UM is one of the best -- if not the best -- wildlife biology graduate schools in the nation," Meyer said, "so I definitely had an interest in coming here. Mark said working on the toxo paper could help me get into his program. I started at UM in 2021, working with Mark on an elk migration study in Canada. Doing this paper gave me a little more confidence as I jumped straight into a super-intense Ph.D. program."

Though Meyer believes stories about T. gondii may be getting a bit sensationalized, and that too much may be attributed to its supposed mind-bending powers, he said we need to learn more about the parasite. Studies suggest that humans infected with toxo are more likely to like cats, develop schizophrenia or engage in road rage. He said a recent study on a college campus found students infected with toxo generally were rated more attractive.

Is it messing with our minds?

"More work definitely needs to be done," Meyer said. "Luckily for us with our study, we had all that excellent data, we had all the blood serum and we had the time, interest and encouragement to check it out."