How did non-human populations inform the human “population crisis” of the 1960s and 1970s? This project reexamines the history of human population control to illustrate its overlaps, influences, and reliance on fertility control efforts in non-human species during the era of international development. While historians have documented the history of population control, they have typically considered the populations in question to be exclusively human. My work reframes the population crisis as a series of scientific interventions into the reproduction of many different species populations, from human beings to stray cats to tsetse flies. Drawing together histories of population control, environmentalism, and agriculture in the Cold War, I explore how interactions between species shaped scientific assessments of “overpopulation.” I examine how the scientific control of fertility across species lines arose as a central tenet of modernity for international organizations and governments during the late twentieth century.