the monotremes (short beaked echidna, long beaked echidna, platypus) are testicond seasonal breeding mammals that exhibit some characteristics of the reproductive tract found in reptiles (e.g. testicond, presence of a cloaca). the fully developed monotreme epididymis exhibits two anatomical regions, similar to some reptiles. the two anatomical regions of the monotreme epididymis closely correspond to just two histologic regions (figure 1b), an initial segment and a terminal segment. structural differentiation of the epididymis into just an initial segment and terminal segment, with no intervening middle segment, has also subsequently been observed as far back as the epididymis of sharks. in the monotreme echidna, the initial segment, where sperm undergo maturation, is much larger than the terminal segment (figure 1b), the later segment being the sperm storage region of the epididymis. in the monotreme echidna, the proportion (26% of total) of mature sperm stored intra-abdominally in the terminal segment of the epididymis is considerably less than the proportion of mature sperm stored in the epididymis of many eutherian mammals (50-75% of total) with descended testes. hence, both reptiles and the monotreme echidna appear to have relatively limited sperm storage capacity in the testicond epididymis compared to mammals with the epididymis located in an extra-abdominal scrotum. this reduced sperm storage capacity of the monotreme testicond epididymis is further supported by observations that the sperm storage region of the epididymis of a testicond mammal (echidna) and a scrotal mammal (rat) are respectively 4% and 8% of the total length of the duct. significantly, the low intra-abdominal sperm storage capacity of the echidna epididymis helped inform the role of the epididymis as a prime mover in the evolution of descended testes in mammals whereby lower extra-gonadal temperatures within the scrotal cauda epididymis reduces oxidative respiration of sperm, which enhances oxygen availability, thereby allowing greater epididymal sperm storage in the cooler scrotum of mammals.