title: Sperm competition and the evolution of sperm design in mammals.
abstract: BACKGROUND
  The influence of sperm competition upon sperm size has been a controversial issue during the last 20 years which remains unresolved for mammals. The hypothesis that, when ejaculates compete with rival males, an increase in sperm size would make sperm more competitive because it would increase sperm swimming speed, has generated contradictory results from both theoretical and empirical studies. In addition, the debate has extended to which sperm components should increase in size: the midpiece to accommodate more mitochondria and produce more energy to fuel motility, or the principal piece to generate greater propulsion forces.
  
  
  RESULTS
  In this study we examined the influence of sperm competition upon sperm design in mammals using a much larger data set (226 species) than in previous analyses, and we corrected for phylogenetic effects by using a more complete and resolved phylogeny, and more robust phylogenetic control methods. Our results show that, as sperm competition increases, all sperm components increase in an integrated manner and sperm heads become more elongated. The increase in sperm length was found to be associated with enhanced swimming velocity, an adaptive trait under sperm competition.
  
  
  CONCLUSIONS
  We conclude that sperm competition has played an important role in the evolution of sperm design in mammals, and discuss why previous studies have failed to detect it.
authors.affiliation: Reproductive Ecology and Biology Group, Museo Nacional de Ciencias Naturales (CSIC), José Gutiérrez Abascal 2, 28006 Madrid, Spain.
authors.forename: Maximiliano
authors.initials: M
authors.lastname: Tourmente
authors.forename: Montserrat
authors.initials: M
authors.lastname: Gomendio
authors.forename: Eduardo R S
authors.initials: ER
authors.lastname: Roldan
mesh_terms: D000818:Animals; D005075:Biological Evolution; D048429:Cell Size; D006801:Humans; D008297:Male; D008322:Mammals / Q000145:classification / Q000235:genetics / Q000502:physiology*; D010802:Phylogeny; D013081:Sperm Motility; D013094:Spermatozoa / Q000145:classification / Q000166:cytology*