This project investigates the transnational and colonial entanglements that made the science of behavioral endocrinology possible. From the 1920s to the 1960s, American behavioral endocrinologists sought to simultaneously collect living specimens to establish breeding colonies of standardized organisms and create field sites to study the sexual behavior of these same species “under natural conditions.” Their experimental spaces were populated by animals deemed exotic (tilapia, jungle fowl), almost human (chimpanzees, monkeys), domesticated (cats, dogs), and vermin (rats, pigeons). Research required the circulation of materials and knowledge from breeders, agricultural workers, pet-keepers, and dealers in imported goods. The boundaries of this scientific discipline became structured by a culture of gift giving of off-prints, motion pictures, organisms, and synthesized hormones. Attending to these relations brings to the fore the materiality and agency of these experimental animals as commodities, gifts, and sexual subjects entangled in colonial, national, and institutional networks of exchange. Behavioral endocrinology's heyday coincided with dramatic reconfigurations in the sexual conduct and attitudes of ordinary Americans. These experiments nonhuman animals helped materialize a sexual plenum by normalizing masturbatory, non-monogamous, and same-sex contacts. By the 1970s, members of the women’s, gay, and animal liberation movements each in turn challenged the central tenets of this liberal interpretation of human and animal sexuality. They criticized the cruelty of experimental designs, the lack of attention to female agency, and conflation of behavior with identity. In short, experiments on animal bodies became a contested site for debating the nature of sex in postwar America.