Parasite gives wolves what it takes to be pack leaders

Wolves infected with a common parasite are more likely than uninfected animals to lead a pack, analysis of more than 200 North American wolves finds¹. Infected animals are also more likely to leave their pack and fend for themselves.

the parasite, Toxoplasma gondii, makes its hosts bold – a mechanism that increases its survival. reproduce sexually, T. gondii must reach a cat’s body, usually when its host is eaten by it. This becomes much more likely if the parasite changes the behavior of the host, making it reckless. Research results are mixed, but in rodents, infection is generally correlated with decreased cat fear and increased exploratory behavior. Physical and behavioral changes have also been seen in people: testosterone and dopamine production are increased and more risks are taken.

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

Single data set

T. gondii is known to infect wildlife, but few studies have examined its behavioral effects. In a work, infected hyenas in Kenya have become more likely to be eaten by lions². Connor Meyer and Kira Cassidy, wildlife ecologists at the University of Montana at Missoula, thought of a rare opportunity to link infection to the behavior of wild wolves: data on gray wolves (Canis lupus) collected extensively in Yellowstone National Park, Wyoming, for nearly 27 years. Some Yellowstone wolves live near cougars and sometimes steal their prey (concolor puma), which are known to carry the parasite. Wolves could become infected by eating cats – or their droppings.

The team examined 256 blood samples from 229 wolves, which had been carefully monitored throughout their lives, and recorded their history and social status. Meyer and Cassidy found that infected wolves were 11 times more likely than uninfected wolves to leave their biological families to start a new pack, and 46 times more likely to become pack leaders – often the only wolves in the pack who breed.

“We got this result and just stared at each other with our mouths open,” Meyer says. “It’s a lot bigger than we thought.” The book is published today in Communications Biology.

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

Ecosystem effects

Going forward, the team hopes to determine whether infection might make wolves more likely to breed successfully — and what the ripple effects of low or high infection rates in ecosystems might be. Wolf populations with high rates of T. gondii infection can spread more rapidly across a landscape as wolves choose to disperse. Aggressive, risk-taking pack leaders could influence how entire packs act, 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 could have a much bigger role than anyone usually assigns to them,” he says.

Wolves are known to kill cougars, however, even bold, risk-taking wolves infected with the parasite probably won’t end up lunch for a cougar, Meyer says. He speculates that in the past, infected wolves might have been more likely to be preyed upon by American lions (Panthera atrox), huge feline predators weighing around 200 kilograms, that roamed North America until their extinction more than 11,000 years ago.