But gradually, clue by clue, he and his collaborators began to piece together the parasite’s modus operandi. T. gondii travels not only to the brain but also to the testicles, where it bumps up production of testosterone. What’s more, female rats are far more receptive to copulating with infected males. “It’s a very strong effect,” Ajai Vyas, Sapolsky’s postdoc at the time of the discovery, told me. “Seventy-five percent of the females would rather spend time with the infected male.”
