Parasites Make Grey Wolves More Likely to Become Pack Leaders

Research has shown that infected animals can engage in riskier behavior than their uninfected peers

A line of grey wolves walking across the snow in Yellowstone National Park
A pack of grey wolves in Yellowstone National Park.  milehightraveler via Getty Images

Scientists already know that the parasite Toxoplasma gondii can influence animal behavior. For example, rodents infected with the parasite are less fearful around predators, according to National Geographic’s Mary Bates.

Now, in a new study published in Communications Biology, researchers have teased out the parasite’s influence on grey wolves in Yellowstone National Park. Wolves infected by Toxoplasma gondii were more likely to become pack leaders than uninfected wolves.

“We’ve really underestimated some of the consequences this parasite has,” Eben Gering, a biologist at Nova Southeastern University who did not contribute to the study, tells Science’s Erik Stokstad. “The findings probably represent the tip of the iceberg concerning the parasite’s significance to the dynamics of wild ecosystems.”

Three grey wolves in the snow in Yellowstone National Park.
Grey wolves in Yellowstone National Park take in the snowy scenery.  Russell Burden via Getty Images

Toxoplasma gondii is a single-celled parasite that can persist in the bodies of humans and other animals for as long as a lifetime. The parasite is ubiquitous—as many as one in three humans might be chronically infected, per Emma Marris in Nature News. But healthy people’s immune systems usually prevent the parasite from causing any illness, according to the Centers for Disease Control and Prevention.

While T. gondii can be found in a lot of different places, it can only reproduce in domestic or wild cats, per National Geographic. That’s because cat intestines contain extra amounts of an acid the parasite uses to reproduce, writes CNN’s Kate Golembiewski. Other animals can catch the parasite by eating an infected animal or coming into contact with cat feces, per National Geographic.

The parasite seems to induce risky behavior in animals that might help it spread to new hosts. Research has shown that infected rodents lose their fear of cat urine, writes National Geographic. Other studies have found that infected captive chimpanzees lost their fear of leopard urine and infected hyena cubs are more likely to get too close to lions, per Science.

“These parasites are using some generic mind control or personality control that helps them fulfill their lifecycle,” Jaap de Roode, a biologist at Emory University who did not contribute to the study, tells Science.

In the new study, the researchers examined data on wolf behavior and movement, as well as parasite infections, collected over 26 years in Yellowstone National Park. Some wolves had their blood tested, and many were tracked using trail cameras, planes and collars, according to CNN.

​​The researchers looked at the blood tests of 229 wolves. They found that infected wolves were 11 times more likely to leave their birth family to start a new pack and 46 times more likely to become pack leaders than uninfected wolves, according to Nature News.

“We focus so much on vertebrate dynamics—wolves and elk, and how they affect each other—and for a long time, it seems like we have generally ignored the fact that parasites might play a role in those relationships,” Connor Meyer, a wildlife ecologist at the University of Montana and co-author of the study, tells The Daily Beast’s Maddie Bender. “With something like Toxo, it seems like we should be giving parasites a little more credit.”

“We know that infection can change animal behavior, but it’s very hard to document that in wildlife populations,” Meggan Craft, a wildlife disease ecologist at the University of Minnesota who did not contribute to the study, tells Science. “What’s cool about this study is that it leverages a fabulous long-term study to be able to tease apart these subtle impacts of infection and behavior.”

The researchers also might have found how the wolves contracted the parasite—wolves whose territory overlapped with areas with lots of cougars (which are cats) were more likely to be infected.

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