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

The study shows that the characteristics of a horse’s semen, including sperm numbers and velocity, change depending on the horse’s social environment, particularly the sequence of exposure to other stallions and mares.

Objective of the Study

The study had two main goals:

• To provide evidence to support sperm competition theory, which predicts that semen characteristics would be influenced by the animal’s social environment
• To investigate the within-subject plasticity in horses in response to different social environments

Methods

To achieve these goals, the researchers:

• Used a polygamous horse species (Equus caballus) as the subject of their study
• Exposed these horses to different social environments. Specifically, the horses were exposed, for eight weeks each, first to other stallions and then to mares, or vice versa
• Collected and analyzed the horses’ ejaculates during these exposure periods to determine semen characteristics

Findings

The results of the study were as follows:

• The highest sperm numbers were found in stallions that were first exposed to other stallions and then to mares
• The lowest sperm numbers were seen in stallions that were exposed to mares but had yet not been exposed to other stallions
• One of the three sperm velocity measures (curvilinear velocity) was consistently higher in stallions that were first exposed to stallions and then to mares
• The number of sperm after exposure to mares, and curvilinear sperm velocity after exposure to other stallions, both showed a positive correlation with average blood testosterone levels during the corresponding exposure period

Conclusion

Based on the findings, the researchers concluded that:

• Ejaculate (semen) characteristics are plastic traits – traits that can change according to the environment – in horses
• Social environment, particularly the order of exposure to mares and stallions, influences these characteristics
• The study’s results support the sperm competition theory