title: Quality of seminal fluids varies with type of stimulus at ejaculation.
abstract: The theory of ejaculate economics was mainly built around different sperm competition scenarios but also predicts that investments into ejaculates depend on female fecundity. Previous tests of this prediction focused on invertebrates and lower vertebrate, and on species with high female reproductive potential. It remains unclear whether the prediction also holds for polygynous mammals with low female reproductive potential (due to low litter size and long inter-birth intervals). We used horses (Equus caballus) to experimentally test whether semen characteristics are adjusted to the oestrous cycle of the mare a stallion is exposed to during few moments before ejaculation. We analysed 122 weekly semen samples collected from 16 stallions during exposure to either an oestrous or a dioestrous mare. Semen volume and the rate of motile sperm were higher when stallions were exposed to an oestrous than to a diestrous mare, while total sperm counts and sperm velocity remained unchanged. Sperm collected after exposure to an oestrous mare also showed reduced oxidative degeneration of cell membranes over a period of 48 hours. We conclude that stallions invest more into their seminal fluids when the chance of fertilization is elevated, and that this adjustment of ejaculate quality can happen very quickly.
authors.affiliation: Swiss Institute of Equine Medicine ISME, Agroscope and University of Berne, Avenches, Switzerland.
authors.forename: E
authors.initials: E
authors.lastname: Jeannerat
authors.affiliation: Clinic of Reproductive Medicine, University of Zurich, Zurich, Switzerland.
authors.forename: F
authors.initials: F
authors.lastname: Janett
authors.affiliation: Unit for Reproductive Medicine - Clinic for Horses, University of Veterinary Medicine Hanover, Hanover, Germany.
authors.forename: H
authors.initials: H
authors.lastname: Sieme
authors.affiliation: Department of Ecology and Evolution, Biophore, University of Lausanne, Lausanne, Switzerland.
authors.forename: C
authors.initials: C
authors.lastname: Wedekind
authors.affiliation: Swiss Institute of Equine Medicine ISME, Agroscope and University of Berne, Avenches, Switzerland.
authors.forename: D
authors.initials: D
authors.lastname: Burger
mesh_terms: D000818:Animals; D004542:Ejaculation / Q000502:physiology*; D004971:Estrus / Q000502:physiology*; D005260:Female; D006736:Horses; D008297:Male; D012661:Semen / Q000166:cytology / Q000502:physiology*; D055101:Semen Analysis / Q000379:methods* / Q000662:veterinary; D012726:Sexual Behavior, Animal; D013076:Sperm Count / Q000662:veterinary; D013081:Sperm Motility / Q000502:physiology*