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Hopping mice (Genus Notomys) are a monophyletic group of Australian Old Endemic murid rodents in the Tribe Hydromyini four of which, Notomys alexis, N. mitchellii, N. fuscus and N cervinus, occur in arid or semiarid environments. Here, we compare the size and morphology of their testes, spermatozoa, accessory sex glands and external genitalia across the species, and from these data, we draw conclusions as to their likely breeding systems. We show that both wild caught and laboratory bred adult N. alexis, N. fuscus and N. mitchellii individuals have extremely small testes, but their spermatozoa vary in morphology across the species. The ventral prostate is the only large accessory sex gland present, and their glans penis has very large spines. By contrast N. cervinus individuals have significantly larger testes with their spermatozoa being less variable but generally more complex in morphology, their seminal vesicles and coagulating glands are much larger, whereas the glans penis has only small spines. Since these reproductive traits in N. cervinus are more similar to those of most other hydromyine rodents in the genus Pseudomys, they are likely to reflect the retention of the ancestral condition within the Notomys lineage whereas the divergent reproductive traits of the other species of tiny testes, variable sperm, small seminal vesicles and coagulating glands and large penile spines are probably more recently derived traits. It is suggested that N. cervinus with its relatively larger testes mass and greater abundance of sperm of more consistent morphology has retained a mating system that may potentially involve some intermale sperm competition whereas, by contrast, the reproductive anatomy of the other three Notomys species suggests that these species exhibit monogamy. [ABSTRACT FROM AUTHOR]

Abstract: This study explores the potential effects of interspecific differences in breeding systems on testis organisation and sperm morphology of native murid rodents. It poses the question – what are the effects of depressed levels of intermale sperm competition, as indicated by small relative testes mass, on the morphology of testes and spermatozoa in murine rodents? Species from three separate murine tribes, those of the Hydromyini, Rattini and Arvicanthini, were investigated with low relative testes mass being used as a proxy for low, or non‐existent, levels of intermale sperm competition. The findings show that in only one of the tribes with low relative testes mass, that of the Hydromyini, was there a reduced testicular area occupied by seminiferous tubules, but in two of the tribes significantly smaller seminiferous tubule diameters were present. In all three tribes, most species with low relative testes mass had a highly derived sperm morphology that, unlike in species with large relative testes mass, had spermatozoa where the head lacked an apical hook, was highly variable in form, often had a very large acrosome overlying the apical region of the nucleus which suggests divergent processes of sperm–egg interaction at the time of fertilisation, and a significantly shorter tail. It is therefore hypothesised that, unlike in species with a large relative testes mass where high levels of intermale sperm competition maintain a streamlined sperm head with an apical hook to aid in zona penetration, in most species of murine rodents with low relative testes mass there is greater reliance on enzymatic digestion of the extracellular matrix around the egg, and especially that of the zona pellucida, to facilitate sperm penetration at the time of fertilisation. [ABSTRACT FROM AUTHOR]