title: Female major histocompatibility complex type affects male testosterone levels and sperm number in the horse (Equus caballus).
abstract: Odours of vertebrates often contain information about the major histocompatibility complex (MHC), and are used in kin recognition, mate choice or female investment in pregnancy. It is, however, still unclear whether MHC-linked signals can also affect male reproductive strategies. We used horses (Equus caballus) to study this question under experimental conditions. Twelve stallions were individually exposed either to an unfamiliar MHC-similar mare and then to an unfamiliar MHC-dissimilar mare, or vice versa. Each exposure lasted over a period of four weeks. Peripheral blood testosterone levels were determined weekly. Three ejaculates each were collected in the week after exposure to both mares (i.e. in the ninth week) to determine mean sperm number and sperm velocity. We found high testosterone levels when stallions were kept close to MHC-dissimilar mares and significantly lower ones when kept close to MHC-similar mares. Mean sperm number per ejaculate (but not sperm velocity) was positively correlated to mean testosterone levels and also affected by the order of presentation of mares: sperm numbers were higher if MHC-dissimilar mares were presented last than if MHC-similar mares were presented last. We conclude that MHC-linked signals influence testosterone secretion and semen characteristics, two indicators of male reproductive strategies.
authors.affiliation: Swiss Institute of Equine Medicine, Agroscope and University of Berne, Avenches, Switzerland.
authors.forename: D
authors.initials: D
authors.lastname: Burger
authors.affiliation: Swiss Institute of Equine Medicine, Agroscope and University of Berne, Avenches, Switzerland.
authors.forename: G
authors.initials: G
authors.lastname: Dolivo
authors.affiliation: Department of Clinical Research, Vetsuisse Faculty, University of Berne, Berne, Switzerland.
authors.forename: E
authors.initials: E
authors.lastname: Marti
authors.affiliation: Unit for Reproductive Medicine-Clinic for Horses, University of Veterinary Medicine Hannover, Hannover, Germany.
authors.forename: H
authors.initials: H
authors.lastname: Sieme
authors.affiliation: Department of Ecology and Evolution, Biophore, University of Lausanne, Lausanne, Switzerland claus.wedekind@unil.ch.
authors.forename: C
authors.initials: C
authors.lastname: Wedekind
mesh_terms: D000818:Animals; D005260:Female; D006736:Horses / Q000276:immunology / Q000502:physiology*; D008285:Major Histocompatibility Complex / Q000502:physiology*; D008297:Male; D009812:Odorants; D012903:Smell / Q000502:physiology; D013076:Sperm Count / Q000662:veterinary*; D013081:Sperm Motility; D013739:Testosterone / Q000097:blood*
chemical_list: D013739:Testosterone
keywords: equine leucocyte antigen; kin recognition; life history; major histocompatibility complex-linked odours; male reproductive strategy; male–male interactions