Ejaculate Characteristics Depend on Social Environment in the Horse (Equus caballus).
Authors: Burger Dominik, Dolivo Guillaume, Wedekind Claus
Journal: PloS one
Summary
# Editorial Summary: Social Environment and Stallion Ejaculate Quality Sperm competition theory predicts that males should invest differentially in reproductive effort depending on perceived mating opportunities, yet evidence for this plasticity in large domesticated animals remains limited. Burger and colleagues exposed stallions to contrasting social environments over 8-week periods—either to competitor males or to mares alone—and analysed resulting ejaculate characteristics alongside blood testosterone levels. Stallions previously exposed to rival males before mating with mares produced significantly higher sperm concentrations and demonstrated elevated curvilinear velocity (a measure of sperm swimming speed), with both traits correlating positively to testosterone levels during the relevant exposure period. Conversely, stallions encountering mares without prior male competition showed the lowest sperm counts, suggesting that perceived sperm competition cues trigger physiological upregulation of reproductive investment. These findings indicate that stallion semen quality is a plastic trait responsive to social context rather than a fixed characteristic, with practical implications for breeding programmes: reproductive performance may be influenced by stable management practices, peer housing arrangements, and the sequencing of exposure to competitors versus potential mates.
Read the full abstract on PubMed
Practical Takeaways
- •Social grouping of stallions affects reproductive output—exposure to other stallions may enhance subsequent fertility when mares are present, likely through testosterone-mediated mechanisms
- •Stallion management protocols should consider sequential social exposure patterns if maximizing semen quality and breeding outcomes is a priority
- •Housing arrangements and social context matter for stallion reproductive physiology; isolated or mare-only exposure may reduce ejaculate quality compared to mixed-sex exposure sequences
Key Findings
- •Stallions exposed to other stallions followed by mares produced significantly higher sperm numbers than those exposed to mares first
- •Curvilinear sperm velocity was consistently elevated in stallions exposed to stallions then mares
- •Sperm number after mare exposure and sperm velocity after stallion exposure both correlated positively with blood testosterone levels
- •Ejaculate characteristics are plastic traits modulated by social environment rather than fixed traits