Ejaculate Characteristics Depend on Social Environment in the Horse (Equus caballus)

• Published in: PloS one Vol. 10; no. 11; p. e0143185
• Main Authors: Burger, Dominik, Dolivo, Guillaume, Wedekind, Claus
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 24.11.2015; Public Library of Science (PLoS)
• Subjects: Analysis; Animal behavior; Animal reproduction; Animals; Competition; Ejaculation; Environmental aspects; Equus caballus; Evolution; Exposure; Females; Hormones; Horses; Hypotheses; Male; Mammals; Medical research; Plasticity; Semen; Sexual behavior; Sexual Behavior, Animal; Social ecology; Social Environment; Sperm; Sperm competition; Sperm Count; Sperm Motility; Spermatogenesis; Testosterone; Testosterone - blood; Velocity; Switzerland
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0143185

Sperm competition theory predicts semen characteristics to be affected by the social environment. We used the polygamous horse (Equus caballus) to experimentally study within-subject plasticity in response to different social environments. Stallions were sequentially exposed, over a period of 8 weeks each, to other stallions and then singly to mares, or vice versa (in adjacent boxes separated by grills). Ejaculates were collected to determine semen characteristics. Highest sperm numbers were found in stallions that were first exposed to other stallions and then to mares, while lowest sperm numbers were observed in stallions that had been exposed to mares but not yet to other stallions. One of three sperm velocity measures (curvilinear velocity) was consistently elevated in stallions that were first exposed to stallions and then to mares. Sperm number after exposure to mares and curvilinear sperm velocity after exposure to stallions were both positively correlated to average blood testosterone levels during the corresponding period of exposure. We conclude that ejaculate characteristics are plastic traits affected by the social environment in horses.