In mammals, sperm morphology varies greatly in size and shape across species. Most mice and rats have a distinctive sperm morphology with the head having a highly decurved apical hook into which the nucleus, acrosome and perforatorium extend and long tail with most Australian old endemic rodents have two further processes that extend from the sperm head upper concave surface, the ventral processes. Nevertheless a few species of mice and rats have evolved highly divergent sperm morphologies that lack both an apical hook and ventral processes as well as having considerably shorter tails. What is the functional significance of the sperm head apical hook, and ventral processes where they occur, and why have a few species evolved such highly divergent sperm morphologies? Here the hypothesis is tested that sperm head shape and tail length are sexually selected traits that are determined by differences in intensity of intermale sperm competition.
The results show that in species where the sperm head has an apical hook, large relative testes mass (RTM) invariably occurs. By contrast where the sperm head lacks this extension, a greatly reduced perforatorium, often a large acrosome, and low RTM, are present suggesting depressed levels of intermale sperm competition and hence a monogamous mating system. It is suggested that, high levels of intermale sperm competition select for a streamlined sperm head with an apical hook and large perforatorium to facilitate egg coat penetration at fertilisation, whereas depressed levels of intermale sperm competition result in a less streamlined sperm head with reduced perforatorium but greater acrosomal enzyme release to aid in sperm passage through the zona pellucida at this time. It is concluded that sperm morphology, at least in the murine rodents, is determined in large part by post-copulatory sexual selection, and hence breeding system, of the species concerned.