The parasite gives wolves what it takes to be pack leaders

A lone wolf pup in Yellowstone National Park.

Some wolves in Wyoming’s Yellowstone National Park share territory with cougars—and can become infected with their parasites.Credit: mtnmichelle/Getty

According to an analysis of more than 200 North American wolves, wolves infected with the common parasite are more likely than uninfected animals to lead a pack.1. Infected animals are also more likely to leave their home packs and strike out on their own.

parasite, Toxoplasma gondii, makes its hosts brave—a mechanism that increases its survival. To reproduce sexually, T. gondii must enter a cat’s body, usually when its host is eaten by one. This becomes much more likely if the parasite alters the host’s behavior so that it is foolish. Research results are mixed, but in rodents, infection generally correlates with reduced fear of cats and increased exploratory behavior. Physical and behavioral changes have also been found in humans: the production of testosterone and dopamine increases and there is a greater risk.

Warm-blooded mammals can catch the parasite by ingesting an infected animal or by ingesting the molds T. gondii excreted in the feces of infected cats. After a period of acute infection, semi-dormant cysts form in the muscle and brain tissue and persist for the rest of the host’s life. Up to one-third of people may be chronically infected.

A unique data set

T. gondii it is known to infect wildlife, but few studies have examined its behavioral effects. In one episode, infected hyenas in Kenya were more likely to be eaten by lions2. Connor Meyer and Kira Cassidy, wildlife ecologists at the University of Montana in Missoula, came up with a rare opportunity to link infection to wild wolf behavior: data on gray wolves (Canis lupus) collected intensively in Yellowstone National Park, Wyoming for nearly 27 years. Some wolves in Yellowstone live near cougars and sometimes steal their prey (Puma concolor), which are known to transmit the parasite. Wolves can become infected by ingesting cats – or their feces.

The team looked at 256 blood samples from 229 wolves that had been closely followed throughout their lives, and recorded their life history and social status. Meyer and Cassidy found that infected wolves were 11 times more likely than uninfected ones to leave their birth family to start a new pack, and 46 times more likely to become pack leaders—often the only wolves in the pack to reproduce.

“We got this result and we were just staring at each other with our mouths hanging open,” says Meyer. “It’s a lot bigger than we thought. The work is published today in Communication biology.

Dan Macnulty, a wolf biologist at Utah State University in Logan, says the study “provides compelling evidence of the profound impact pathogens can have on the ecology and behavior of wild animal populations.” He adds that this demonstrates the immense value of long-term studies of wolves and other wildlife in Yellowstone National Park.

Ecosystem effects

In the future, the team hopes to look at whether infection could cause wolves to reproduce successfully – and what the ripple effects of low or high infection rates might be across ecosystems. Wolf population with a high incidence rate T. gondii the infection can spread faster across the landscape as individual wolves decide to disperse. Aggressive and risk-taking pack leaders could influence the behavior of entire packs—perhaps even increasing their chances of encountering cougars and exposing more members to infection.

For Meyer, the moral of the story is that parasites can be major players in ecosystems. “Parasites can play a much bigger role than anyone generally gives them credit for,” he says.

However, wolves are known to kill cougars, so even bold, risk-taking wolves infected with the parasite are unlikely to end up as a cougars’ lunch, Meyer says. He speculates that in the past, infected wolves may have been more likely to prey on American lions (Panthera atrox), massive feline predators weighing around 200 kilograms that roamed North America until they became extinct more than 11,000 years ago.

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