The Australian Hydromyine Rodents-Their Interspecific Diversity and Reproductive Biology: A Review

Australia has a diversity of mammals, with approximately 60 extant species being native rodents in the Family Muridae. Two major groups occur, the Rattus species and the Hydromyini. Investigations on their reproductive biology have shown that all species of Rattus have large scrotal testes which produce abundant spermatozoa. The sperm head contains a single apical hook in which a nuclear extension occurs. In female Rattus, the litter size varies from about 4 to 12 across the species. In sexually mature male hydromyine rodents, marked interspecific differences occur in relative testes mass from about 0.5% to 3% body mass in most species to only 0.1% body mass in most Notomys. Most male Notomys also have a highly divergent reproductive tract, with the penis containing large spines, whereas the female has a small, highly muscular vagina which facilitates locking between the pair at the time of mating. The hydromyine sperm morphology also varies markedly across the species, with most having a complex sperm head with an apical hook together with two ventral processes that contain actin. In a few species, a simpler sperm head occurs with only one, or no, apical hook. Clearly, the hydromyine rodents, unlike the Rattus genus, exhibit a diverse array of reproductive biological traits in both males and females, the full evolutionary significance of which has yet to be ascertained.