title: By hook or by crook? Morphometry, competition and cooperation in rodent sperm.
abstract: BACKGROUND
  Sperm design varies enormously across species and sperm competition is thought to be a major factor influencing this variation. However, the functional significance of many sperm traits is still poorly understood. The sperm of most murid rodents are characterised by an apical hook of the sperm head that varies markedly in extent across species. In the European woodmouse Apodemus sylvaticus (Muridae), the highly reflected apical hook of sperm is used to form sperm groups, or "trains," which exhibited increased swimming velocity and thrusting force compared to individual sperm.
  
  
  RESULTS
  Here we use a comparative study of murine rodent sperm and demonstrate that the apical hook and sperm cooperation are likely to be general adaptations to sperm competition in rodents. We found that species with relatively larger testes, and therefore more intense sperm competition, have a longer, more reflected apical sperm hook. In addition, we show that sperm groups also occur in rodents other than the European woodmouse.
  
  
  CONCLUSIONS
  Our results suggest that in rodents sperm cooperation is more widespread than assumed so far and highlight the importance of diploid versus haploid selection in the evolution of sperm design and function.
authors.affiliation: Department of Animal and Plant Sciences, University of Sheffield, Sheffield, United Kingdom. s.immler@sheffield.ac.uk
authors.forename: Simone
authors.initials: S
authors.lastname: Immler
authors.forename: Harry D M
authors.initials: HD
authors.lastname: Moore
authors.forename: William G
authors.initials: WG
authors.lastname: Breed
authors.forename: Tim R
authors.initials: TR
authors.lastname: Birkhead
mesh_terms: D000818:Animals; D005075:Biological Evolution; D008297:Male; D051379:Mice; D051381:Rats; D012377:Rodentia; D013077:Sperm Head / Q000648:ultrastructure*; D013094:Spermatozoa / Q000166:cytology / Q000502:physiology; D013737:Testis / Q000033:anatomy & histology