The parasite gives wolves what it takes to be pack leaders
According to an analysis of more than 200 North American wolves, wolves infected with the common parasite are more likely to lead the pack than uninfected animals.1. Infected animals are also more likely to leave their home packs and go off on their own.
parasite, Toxoplasma gondii, makes its hosts brave – a mechanism that increases its survival. For sexual reproduction, T. gondii it must reach the cat’s body, usually when one eats its host. This becomes much more likely if the parasite alters the host’s behavior, rendering it mindless. Research results are mixed, but in rodents infection is generally correlated reduced fear of cats and increased exploratory behavior. Physical and behavioral changes have also been found in humans: testosterone and dopamine production are increased and more risk-taking is taken.
Warm-blooded mammals can catch the parasite by eating an infected animal or ingesting the forms T. gondii excreted in the feces of infected cats. After a period of acute infection, semi-dormant cysts are formed in muscle and brain tissue that last for the rest of the host’s life. Up to one third of people may be chronically infected.
Unique data set
T. gondii it is known to infect wildlife, but few studies have examined its behavioral effects. In one paper, lions were more likely to eat infected hyenas in Kenya2. 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 prey (Puma concolor), which are known to carry the parasite. Wolves can become infected by eating cats – or their feces.
The team examined 256 blood samples from 229 wolves, which were carefully monitored throughout their lives, and their life histories and social status were recorded. Meyer and Cassidy found that infected wolves were 11 times more likely than uninfected wolves 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 breed.
“We got that score and we were just staring at each other with our mouths hanging open,” says Meyer. “This is a lot bigger than we thought it would be.” The paper was published today in Communications Biology.
Dan Macnulty, a wolf biologist at Utah State University in Logan, says the study “provides compelling evidence of the profound impact that pathogens can have on the ecology and behavior of wildlife populations.” He adds that it shows the enormous value of long-term studies of wolves and other wildlife in Yellowstone National Park.
In the future, the team hopes to look at whether infection can make wolves more likely to breed successfully—and what the effects of low or high infection rates might be on ecosystems. Wolf populations with high rates T. gondii the infection could spread more quickly across the landscape as individual wolves decide to disperse. Aggressive and risky pack leaders can affect how entire packs behave—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 may play a much bigger role than anyone generally gives them credit for,” he says.
However, wolves are notorious for killing cougars, so even brave, risk-taking wolves infected with the parasite are unlikely to end up as a cougar’s lunch, Meyer says. He speculates that in the past infected wolves may have been more likely victims of 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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