Wolf pack leadership has long been of great interest to many scientists. What makes a wolf decide to ＿＿（41）＿＿ out on its own and assert leadership of its pack? That question has long 42 scientists. Now, a new study shows that gray wolves infected with the parasite Toxoplasma gondii are more likely to become top dogs than uninfected wolves. The finding forces us to think more broadly about what influences how animals act, says Kira Cassidy, a ＿＿（43）＿＿ biologist. Toxoplasma gondii is a single-celled parasite that infects at least one-third of the world's human population at any given time with the disease toxoplasmosis. ＿＿（44）＿＿ the infection is usually mild, it can be fatal to the young or immune-suppressed. It's famous for its ability to ＿＿（45）＿＿ its hosts, most notably in making rodents reckless around house cats. Toxoplasma can only reproduce in the intestines of domestic or wild cats, yet it is widespread in nature, and can infect any warm-blooded animal.