The Gerbil Spermatozoon – interspecific variation in morphology does not invariably follow murine rodent evolutionary trends in response to predicted intermale sperm competition

In various groups of mammals, the intensity of intermale sperm competition relates to relative testes mass (RTM) with some evidence suggesting that this may also be the case for some aspects of sperm form. In murid rodents, a large RTM generally correlates with a streamlined sperm head, long apical hook and long tail with most data coming from species in the subfamily Murinae. In this study, RTM and sperm form are compared across 15 species of gerbils, seven from the Tribe Taterillini and eight from the Tribe Gerbillini. Marked interspecific differences in RTM and sperm morphology were observed. However, the Gerbilliscus species with the largest RTM do not have a sperm head with an apical hook nor a longer sperm tail than other species with smaller RTM whereas, by contrast, in the Tribe Gerbillini, species where the sperm head lacks a hook have a relatively small testes mass. We thus suggest that in gerbils, unlike in murine rodents, high levels of postcopulatory sexual selection have not invariably resulted in the evolution of a spermatozoon with a long apical hook and long sperm tail. The possible reasons for this are briefly discussed.