A parasite best known for making mice lose their fear of cats might also provoke risk-taking behavior in gray wolves.
The result, described in the Nature journal Communications Biology, suggests the tiny microbe might exert a large influence over whole groups, populations or ecosystems.
As an adult, the protozoan parasite Toxoplasmosis gondii settles down in domestic and wild felines, but it spends much of its youth in a wide array of warm-blooded animals.
Using 26 years of blood tests and data on wolf behavior in Yellowstone National Park, researchers found wolves whose ranges overlapped with cougar hotspots were more likely to contract toxoplasmosis.
Once infected, they were 11 times more likely to leave their packs and 46 times more likely to become pack leaders elsewhere.
The authors suggest infected wolves might lead their packs into cougar territory, exposing them to further infection and continuing the cycle.
