Parasite gives wolves what it takes to be pack leaders

A lone wolf pup in Yellowstone National Park.

Some wolves in Yellowstone National Park, Wyoming, 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 a common parasite are more likely to lead a pack than uninfected animals1. Infected animals are also more likely to leave their home packs and strike out on their own.

the parasite, Toxoplasma gondii, makes its hosts brave – a mechanism that increases its survival. To reproduce sexually T. gondii must reach a cat’s body, usually when their host is eaten by one. This becomes much more likely when the parasite changes the host’s behavior and makes it foolhardy. Research is mixed, but in rodents, infection generally correlates with reduced fear of cats and increased exploratory behavior. Physical and behavioral changes have also been noted in people: testosterone and dopamine production is increased and more risks are taken.

Warm-blooded mammals can acquire the parasite by eating or taking forms of an infected animal T. gondii excreted in the feces of infected cats. After a period of acute infection, semi-dormant cysts form in muscle and brain tissue and persist for the remainder of the host’s life. Up to a third of people could be chronically infected.

Unique record

T. gondii It is known to infect wildlife, but few studies have examined its behavioral infections. In one paper, infected hyenas became more likely to be eaten by lions in Kenya2. Connor Meyer and Kira Cassidy, wildlife ecologists at the University of Montana at Missoula, reflected on a rare opportunity to link infection to wild wolf behavior: gray wolf data (wolf) intensively collected in Yellowstone National Park, Wyoming for almost 27 years. Some wolves in Yellowstone live near cougars and sometimes steal prey from them (Puma concolor) known to carry the parasite. Wolves could become infected by eating the cats — or their droppings.

The team examined 256 blood samples from 229 wolves that had been carefully monitored throughout their lives, recording their life histories and social status. Meyer and Cassidy found that infected wolves were 11 times more likely than uninfected wolves to leave their birth families to form a new pack and 46 times more likely to become pack leaders — often the only wolves in the pack to breed .

“We got this result and just stared at each other with our mouths open,” says Meyer. “This is a lot bigger than we thought.” The work appears today in communication biology.

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

ecosystem effects

Going forward, the team hopes to study whether infection increases the likelihood that wolves will successfully reproduce – and what impact low or high infection rates might have on ecosystems. Wolf populations with high rates of T. gondii Infection can spread more quickly across a landscape when individual wolves make the decision to disperse. Aggressive and risk-taking pack leaders could influence the behavior of entire packs – possibly 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 important players in ecosystems. “Parasites could play a much larger role than is generally given credit for,” he says.

However, wolves have been known to kill cougars, so even brave, risk-taking wolves infected with the parasite are unlikely to end up as lunch for a cougar, Meyer says. He speculates that in the past infected wolves may have been more likely to have been preyed upon by American lions (Panthera atrox), massive big cats weighing about 200 kilograms that roamed North America until they became extinct over 11,000 years ago.