Researchers studying grey wolf populations in Yellowstone National Park have discovered an intriguing reason why some wolves may be more inclined to become pack leaders.
Grey wolves exposed to Toxoplasma gondii -- the parasite that causes the disease toxoplasmosis -- are more than 46 times more likely to become a pack leader than uninfected wolves, according to a study published Thursday in Communications Biology.
The researchers analyzed behavioral and distribution data from 1995 to 2020 as well as blood samples from 229 anaesthetized wolves to study the association between risk-taking behaviors and infection with Toxoplasma gondii. They identified associations between parasite infection and high-risk behaviors in both males and females.
Wolves that tested positive for T. gondii were 11 times more likely to disperse from their pack and more than 46 times more likely to become a pack leader than uninfected wolves, according to the findings. Males were 50% more likely to leave their pack within a six-month period if infected with the parasite but that length of time jumped to 21 months if unaffected. Females displayed a 25% chance of leaving their pack within 30 months if infected, extending to 48 months if uninfected.
Infection with T. gondii often has no negative effects on the fitness of healthy individuals but can be fatal to young or immunosuppressed wolves, according to the researchers. They don't yet know how this parasite influences things like survival rates, according to Connor Meyer, a wildlife biology Ph.D. student at the University of Montana and one of the authors of the study.
The findings are the first to demonstrate parasite infection affecting decision-making and behavior in the species, the researchers said.
Previous research has identified associations between T. gondii infection and increased boldness in hyenas as well as increased testosterone production in rats, the authors speculate that similar mechanisms could drive the risky behaviors observed in wolves that tested positive for the parasite.
The wolves occupying areas that overlapped with a higher population density of cougars were more likely to be infected with T. gondii than those not living near cougars, suggesting that wolves may become infected with the parasite as a result of direct contact with cougars and their environments, the researchers found. Cougars in Yellowstone National Park are known to be hosts of the parasite.
The findings "tell the story of this entire ecosystem and how species interact with each other," said Kira Cassidy, one of the authors and a research associate for Yellowstone National Park and Yellowstone Forever, a nonprofit associated with the national park.
The researchers hypothesized that the infection would have wider implications on the wolf population, as infected pack leaders could lead their packs into more high-risk areas that overlap with cougars, potentially increasing the risk of further infection for uninfected wolves.
"So that's probably the the link there with the actual mechanism behind the parasite and the infection," Meyer said.
The study, only the second of its kind to look at how a toxoplasmosis infection can affect a species of predators, is a "powerful kind of testament to what long-term research is able to answer," Meyer noted.
Added Cassidy: "Taking an ecosystem approach to a research question can be really difficult in a lot of places but Yellowstone is one of these places where we see all of the species that were here hundreds of years ago."
Grey wolves were widely eradicated in the western U.S. in the 1940s but populations have begun to rebound in recent decades. Some say the increase is detrimental to humans due to the wolves' ability to travel vast distances and therefore spread diseases. The wolves can also be a significant factor in the decline of big game herds and the killing of livestock.
Earlier this month, a federal judge in Montana temporarily restricted wolf hunting and trapping near Yellowstone and Glacier National Parks.
Wolves, however, are typically cautious of people. At Yellowstone, they are "the most shy and cautious" of all the large mammals, Cassidy said.
"If you see one, you're incredibly lucky," she said. "I would say overall, they are essentially no danger to people."