Cool sperm: why some placental mammals have a scrotum

Throughout the Cenozoic, the fitness benefits of the scrotum in placental mammals presumably outweighed the fitness costs through damage, yet a definitive hypothesis for its evolution remains elusive. Here, I present an hypothesis (Endothermic Pulses Hypothesis) which argues that the evolution of the scrotum was driven by Cenozoic pulses in endothermy, that is, increases in normothermic body temperature, which occurred in Boreotheria (rodents, primates, lagomorphs, carnivores, bats, lipotyphylans and ungulates) in response to factors such as cursoriality and climate adaptation. The model argues that stabilizing selection maintained an optimum temperature for spermatogenesis and sperm storage throughout the Cenozoic at the lower plesiomorphic levels of body temperature that prevailed in ancestral mammals for at least 163 million years. Evolutionary stasis may have been driven by reduced rates of germ-cell mutations at lower body temperatures. Following the extinction of the dinosaurs at the Cretaceous-Palaeogene boundary 65.5 mya, immediate pulses in endothermy occurred associated with the dramatic radiation of the modern placental mammal orders. The fitness advantages of an optimum temperature of spermatogenesis outweighed the potential costs of testes externalization and paved the way for the evolution of the scrotum. The scrotum evolved within several hundred thousand years of the K-Pg extinction, probably associated initially with the evolution of cursoriality, and arguably facilitated mid- and late Cenozoic metabolic adaptations to factors such as climate, flight in bats and sociality in primates.