Reproductive thermoregulation in marine mammals: mechanisms, adaptations and evolutionary implications

Abstract

Marine mammals inhabit thermally challenging environments in which maintaining internal body temperature is essential for survival, metabolic function, and reproductive success. Unlike terrestrial mammals, many marine male species lack external reproductive structures (e.g. scrotum), raising questions about how reproductive temperatures are regulated in the absence of conventional cooling mechanisms. This review synthesizes current knowledge of reproductive thermoregulation in marine mammals, with a focus on cetaceans and pinnipeds, and highlights key anatomical and physiological adaptations. These include countercurrent heat exchange systems, specialized vascular networks surrounding reproductive organs, and behavioral strategies that facilitate localized cooling while conserving overall body heat. The review also addresses thermal protection of the developing fetus, illustrating how vascularization and insulation help maintain optimal intrauterine conditions. In addition, it examines evolutionary trade-offs associated with internal testes and streamlined body forms, ecological pressures imposed by aquatic environments, and the integration of thermoregulation with diving physiology. Finally, implications for conservation are discussed, particularly in the context of climate change, rising ocean temperatures and human-induced habitat disturbances. By integrating anatomical, physiological, ecological and evolutionary perspectives, this article underscores how marine mammals have evolved highly efficient thermoregulatory strategies, offering insights into reproductive biology and species resilience in dynamic aquatic ecosystems.