Interspecific Variation of Testis Size and Epididymal Sperm Numbers in Australasian Rodents with Special Reference to the Genus Notomys

When testis mass is plotted against body mass for Australasian rodents, an allometric relationship is found to occur. Nevertheless, considerable interspecific variability in testis mass as a percentage of body mass is evident for the different species, with the smallest relative size of testes in the Australian rodents being present in most Notomys and several Pseudomys species. In other Pseudomys, and nearly all species of the other genera, the relative size of testes is considerably greater. Notomys and Pseudomys with small testes tend to have a lower relative volume of seminiferous tubules to the total testicular mass than species with relatively large testes. These species also generally have small cauda epididymides and a less dense sperm population in this region. The data thus clearly indicate far fewer sperm are produced, and stored, in the male reproductive tract of these animals. The causative reason(s) for the differences in relative testis size are discussed and the possibility that it relates to variation in breeding system, and hence potential intermale sperm competition, is considered. However, the scant data available do not indicate an obvious association among these parameters. It is thus suggested that, in Notomys at least, the relatively small testes may relate to the other divergent features of the reproductive tract one of the consequences of which may be a highly efficient sperm-transport system.