temperatures. Indeed, experimental reflection of one epididymis into the warmer temperature of an abdominal location reduced sperm storage capacity by 75% compared to the contralateral epididymis that remained in the scrotum. Significantly, cooler scrotal temperatures reduces oxidative respiration of sperm, thereby increasing oxygen availability to store more sperm per unit volume of duct, which has informed the evolution of descended testes in mammals. Trends in the evolution of the epididymis from testicond reptiles and monotremes to scrotal mammals A histologically distinct initial segment of the epididymis is widely observed in many species of reptiles and even as far back as sharks. A large initial segment is also present in the epididymis of the testicond monotreme echidna. Furthermore, the scrotal epididymis of metatherian and eutherian mammals nearly all exhibit an initial segment which may contain histologically distinct sub-zones therein. Hence, the initial segment of the epididymis is well conserved in testicond vertebrates (reptiles, monotremes) and in scrotal metatherian and eutherian mammals (Figure 1). The status of the histologic middle segment of the epididymis in reptiles is incompletely defined (Figure 1A). Considering the wide variation in the anatomical structures of the four orders (Crocodilia, Sphenodontia, Squamata, Testudines) of reptilian epididymides and the paucity of histologic studies that correlate anatomical structure to histology, the evolution of the middle segment in reptiles, if present, remains to be delineated. In contrast, extensive studies of the echidna epididymis show that the monotreme epididymis lacks a middle segment. It is only in metatherian and eutherian mammals that a middle segment has been extensively documented. Whereas the initial segment of the epididymis often contains histologically distinct sub-zones therein, the downstream zones that collectively constitute the middle segment most likely evolved from the upstream sub-zones of the initial segment. The histologic terminal segment is the sperm storage region of the epididymis in reptiles, monotremes and both metatherian and eutherian mammals (Figure 1). The testicond epididymis (reptiles and monotremes) has a limited sperm storage capacity compared with the scrotal epididymis (metatherian and eutherian mammals), which has a much larger terminal segment to accommodate increased sperm storage. It is the cooler temperature of the scrotal epididymis that reduces oxidative respiration of sperm in the terminal segment, thereby increasing oxygen availability to store more sperm per unit volume of duct, thus informing the evolution of descended testes in mammals. Whereas different and multiple histologic sub-regions may or may not occur within any