Dr. Paul Vasey has spent much of his life studying sexual behavior across species and across cultures. For two decades, he observed Japanese macaques—a species of monkey—in mountains outside of Kyoto. During his time in Japan, Dr. Vasey consistently observed female macaques sexual mounting other females. If fact, he estimates that only 20% of these female monkeys are exclusively heterosexual. This results in an intersexual mate competition, whereby males must compete with both female and male members of their species to find a female mate. Fascinated, Dr.