What makes a wolf decide to strike out on its own or assert leadership of its pack? That question has long intrigued scientists. Now, a new study shows that gray wolves infected with the parasite Toxoplasma gondii are more likely to become top dog than uninfected wolves.

The finding forces us to think more broadly about what influences how animals act, says study co-author Kira Cassidy, a wildlife biologist with the Yellowstone Wolf Project, a Montana-based nonprofit that oversees research on the predators in Yellowstone National Park.

“We know that behavior is influenced by all sorts of factors, including past experiences, genetics, current circumstances, and social context,” she says. “Now we can add parasites to