According to an analysis of more than 200 North American wolves, wolves infected with a common parasite are more likely than uninfected animals to lead a pack.1. Infected animals are also more likely to leave their home pack and strike on their own.
the parasite, Toxoplasma gondii, makes its hosts bold – a mechanism that increases its survival. To reproduce sexually, T. gondii must reach a cat’s body, usually when its host is eaten by a cat. That becomes much more likely if the parasite changes the host’s behavior, making it reckless. Research results are mixed, but in rodents, infection generally correlates with reduced fear of cats and more exploratory behavior. Physical and behavioral changes have also been observed in humans: the production of testosterone and dopamine is increased and more risks are taken.
Warm-blooded mammals can contract the parasite by eating an infected animal or by ingesting forms of the parasite T. gondii feces 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 rest of the host’s life. Up to a third of people may be chronically infected.
Unique data set
T. gondii is known to infect wildlife, but few studies have examined its behavioral effects. In one work, infected hyenas in Kenya became more likely to be eaten by lions2. Connor Meyer and Kira Cassidy, wildlife ecologists at the University of Montana in Missoula, thought of a rare opportunity to link infection to behavior in wild wolves: Gray wolf data (Wolf) collected intensively for nearly 27 years in Yellowstone National Park, Wyoming. Some wolves in Yellowstone live near and sometimes steal prey from cougars (Puma color), known to carry the parasite. Wolves can become infected by eating the cats – or their feces.
The team looked at 256 blood samples from 229 wolves, who 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 natal family to start a new pack, and 46 times more likely to become pack leaders — often the only wolves in the pack to breed .
“We got that result and we just stared at each other open-mouthed,” says Meyer. “This is much bigger than we thought it would be.” The work was published today in Communication Biology.
Dan Macnulty, a wolf biologist at Utah State University in Logan, says the study provides “convincing evidence of the profound impact pathogens can have on the ecology and behavior of wildlife populations.” He adds that it demonstrates the immense value of the long-term study of wolves and other animals in Yellowstone National Park.
In the future, the team hopes to see whether infection makes wolves more likely to reproduce successfully – and what the ripple effects of low or high infection rates might be in ecosystems. Wolf populations with high percentages of T. gondii infection can spread more quickly across a landscape if individual wolves make the choice to spread. Aggressive, risk-taking pack leaders can 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 may play a much bigger role than anyone generally admits,” he says.
Wolves are known for killing cougars, so even daring, high-risk wolves infected with the parasite probably won’t end up as lunch for a cougar, Meyer says. He speculates that infected wolves were more likely to be preyed upon by American lions in the past (Panthera atrox), huge feline predators weighing about 200 kilograms, that prowled North America until they became extinct more than 11,000 years ago.