After studying 25 years of data, they found that the wolves infected with the parasite Toxoplasma gondii were more likely to engage in risky behaviors, such as leaving the pack they were born into or becoming pack leaders.
In order to survive, the single-celled parasite must be ingested by cats because they have high levels of an acid in their stomachs that allows the parasite to reproduce inside the cat's intestines. The parasite's oocysts are then expelled through the cat's feces and can infect other animals that eat it.
Because the parasites need to find their way back to a feline, they burrow into the brain of their new hosts and alter their behavior to take more risks that increase the likelihood of the infected animal being eaten by a cat.
"This study is a rare demonstration of a parasite infection influencing behavior in a wild mammal population," the researchers wrote, per Gizmodo. "These two life history behaviors represent some of the most important decisions a wolf can make in its lifetime and may have dramatic impacts on gray wolf fitness, distribution, and vital rates."
The researchers are still trying to understand how the parasite works and why it causes wolves to change their behavior.