title: Greater sperm complexity in the Australasian old endemic rodents (Tribe: Hydromyini) is associated with increased levels of inter-male sperm competition.
abstract: Spermatozoa exhibit considerable interspecies morphological variation across mammals, especially among murid rodents. In Australasia, most murids in the tribe Hydromyini have a spermatozoon with a highly complex head exhibiting an apical hook, characteristic of most murids, and two projections that extend from its upper concave surface, the ventral processes. In the present study we performed a phylogenetically controlled comparison of sperm morphology across 45 species of hydromyine rodents to test the hypothesis that the length and angle of both the apical hook and ventral processes, as well as the length of the sperm tail, increase with relative testes mass as a proxy for differences in levels of inter-male sperm competition. Although both sperm head protrusions exhibited considerable variation in their length and angle across species, only the angles increased significantly in relation to relative testes mass. Further, the length of the sperm flagellum was positively associated with relative testes mass. These results suggest that, in hydromyine rodents, the angle of the apical hook and ventral processes of the sperm head, as well as the sperm tail length, are likely to be sexually selected traits. The possible functional significance of these findings is briefly discussed.
authors.affiliation: Discipline of Anatomy and Pathology, Faculty of Health Sciences, The University of Adelaide, SA 5005, Australia.|Robinson Research Institute, School of Medicine, Faculty of Health Sciences, The University of Adelaide, SA 5005, Australia.
authors.forename: Hanna J
authors.initials: HJ
authors.lastname: McLennan
authors.affiliation: Department of Evolutionary Biology and Environmental Studies, University of Zurich, 8057 Zurich, Switzerland.
authors.forename: Stefan
authors.initials: S
authors.lastname: Lüpold
authors.affiliation: School of Biosciences, University of Melbourne, Parkville Campus, Melbourne, Vic. 3010, Australia.
authors.forename: Pete
authors.initials: P
authors.lastname: Smissen
authors.affiliation: Natural Sciences Department, Museum Victoria, Melbourne, Vic. 3001, Australia.
authors.forename: Kevin C
authors.initials: KC
authors.lastname: Rowe
authors.affiliation: Discipline of Anatomy and Pathology, Faculty of Health Sciences, The University of Adelaide, SA 5005, Australia.|Robinson Research Institute, School of Medicine, Faculty of Health Sciences, The University of Adelaide, SA 5005, Australia.|Corresponding author. Email: bill.breed@adelaide.edu.au.
authors.forename: William G
authors.initials: WG
authors.lastname: Breed
keywords: Muridae; apical hook; hydromyine rodents; spermatozoa; ventral processes