Puberty and spermatogenesis Puberty is reached inmost bats in the year following birth. However, in some vespertilionids some individuals achieve sexual maturity in their first autumn, while some horseshoe bats become sexually mature only after several years. Both testes and epididymides are covered with a sheath of peritoneum - the tunica vaginalis. In juvenile and sexually immature male vespertilionids the tunica round the cauda epididymidis (= tail of the epididymis) can typically be seen through the skin as a densely pigmented sheath (figure a. Increase in the size of the testes, associated with growth of the seminiferous tubules and spermatogenesis, can be seen through the skin. After their release from the testes, spermatozoa pass through the epididymides to the caudae, which become distended between the layers of skin forming the interfemoral membrane. The rapid shrinkage of the testes at the end of spermatogenesis and the correspondingly rapid swelling of the caudae is very striking in captive bats. As a result of this swelling, the tunica vaginalis over the epididymis becomes stretched and the black pigment cells (melanocytes) separate so that the distended epididymal tubules appear white through the skin (figure b. After this initial separation has occurred, the melanocytes seldom return to their former density (in pipistrelles), so the apparent reduction in pigmentation, accompanied by varying degrees of distension of the epidydimis, can be used as a criterion of sexual maturity (although this is not proven in all genera. Where testicular swelling is apparent but the cauda is still heavily pigmented, the individual is probably undergoing its first spermatogenesis and is therefore described as pubertal. Not only do immature bats have pigmented tunicae, but their testes are also smaller than those of individuals that have experienced spermatogenesis. This maybe seen when the testes are examined through the skin. The distinction between those males that have lost most of their epididymal spermatozoa and immature individuals is complicated when fat is deposited within the tunica vaginalis around the convoluted tubule of the epididymis and causes this membrane to appear stretched. In very fat hibernating bats, both testes and epididymis maybe completely obscured from view.