Gray wolves infected with this parasite are more likely to become pack leaders, scientists say World

Yellowstone Cougar Project

(YELLOWSTONE, WYOMING) — Researchers studying gray wolf populations in Yellowstone National Park have discovered an intriguing reason why some wolves are more likely to become pack leaders.

Gray wolves exposed to Toxoplasma gondii — the parasite that causes the disease toxoplasmosis — are more than 46 times more likely to become pack leaders than uninfected wolves, according to a study published Thursday in Communications Biology.

Researchers analyzed behavioral and distribution data from 1995 to 2020, as well as blood samples from 229 anesthetized wolves to investigate the link between risky behavior and infection with Toxoplasma gondii. They identified associations between parasite infection and high-risk behaviors in both men and women.

Wolves that tested positive for T. gondii were 11 times more likely to break away from their pack and more than 46 times more likely to become pack leaders than uninfected wolves, according to the results. Males were 50% more likely to leave their pack within six months when infected with the parasite, but this increased to 21 months when unaffected. Females showed a 25% chance of abandoning their pack within 30 months when infected, extending to 48 months when uninfected.

Infection with T. gondii often has no adverse effect on the fitness of healthy individuals, but can be fatal to young or immunocompromised wolves, the researchers said. They don’t yet know how this parasite affects things like survival rates, says Connor Meyer, a wildlife biologist with a PhD. Student at the University of Montana and one of the authors of the study.

The results are the first to show that parasite infection influences the species’ decision-making and behavior, the researchers said.

Previous research has identified associations between T. gondii infection and increased courage in hyenas and increased testosterone production in rats. The authors speculate that similar mechanisms might drive the risky behavior seen in wolves that tested positive for the parasite.

The wolves that occupied areas that overlapped with higher population densities of cougars were more likely to be infected with T. gondii than those that did not live near cougars, suggesting that wolves were infected through direct contact with cougars and their environment can be infected with the parasite, the researchers found. Cougars in Yellowstone National Park are known to be hosts of the parasite.

The results “tell the story of this entire ecosystem and how species interact with each other,” said Kira Cassidy, one of the authors and research associate at Yellowstone National Park and Yellowstone Forever, a nonprofit associated with the national park.

The researchers hypothesized that the infection would have a broader impact on the wolf population, as infected pack leaders could lead their packs into higher-risk areas that overlap with cougars, potentially increasing the risk of further infection for uninfected wolves.

“So that’s probably the link to the actual mechanism behind the parasite and the infection,” Meyer said.

The study, which is only the second of its kind examining how toxoplasmosis infection can affect a predator species, is “strong evidence of what long-term research can answer,” Meyer noted.

Cassidy added, “It can be really difficult in many places to look at a research question with an ecosystem approach, but Yellowstone is one of those places where we see all the species that were here hundreds of years ago.”

Gray wolves were largely wiped out in the western United States in the 1940s, but populations have begun to recover in recent decades. Some say the increase is harmful to humans as the wolves can travel great distances and therefore spread disease. The wolves can also be a major factor in the decline of large game herds and the killing of livestock.

Earlier this month, a federal judge in Montana temporarily restricted wolf hunting and capturing near Yellowstone and Glacier National Parks.

However, wolves are usually wary of humans. In Yellowstone, they are “the shyest and most wary” of all large mammals, Cassidy said.

“If you see one, you’re incredibly lucky,” she said. “Overall, I would say they pose essentially no danger to humans.”

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