On the origin of descended scrotal testicles: The activation hypothesis

Abstract

Male gonads contained in an unprotected skin sack located outside the body cavity are a peculiar mammalian anomaly. We advance the hypothesis that descended scrotal testicles in humans and many other mammals evolved to provide a situation specific means of activating sperm. As a result of consistent temperature differences between the male and female reproductive tracts (i.e., scrotal temperatures are typically maintained at 2-3°C below body temperature), we propose that the rise to body temperature that accompanies insemination into the vagina serves as one of several triggers for the activation of sperm. We explore some of the features of mammalian reproduction and behavior that are consistent with this hypothesis, make some testable predictions, and examine the psychological and behavioral adaptations that have evolved to protect otherwise vulnerable scrotal testicles from damage.