Stallions in a Swiss study were found to increase their sperm numbers in response to exposure to other stallions ahead of mares.
The researchers set out to examine sperm changes in response to stallions being housed in different social environments.
Stallions were sequentially exposed, over a period of eight weeks each, to other stallions in neighboring boxes separated by grills. They also were housed for eight weeks beside a mare, which was replaced by another mare at the four-week mark.
Ejaculate samples were collected after the eight-week cycles to allow semen characteristics to be assessed.
Dominik Burger, Guillaume Dolivo and Claus Wedekind, writing in the peer-reviewed open-access journal, PLOS ONE, found that sperm numbers were highest 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.
Sperm numbers were about in between after exposure to stallions, regardless of the order of exposure.
“When controlling for these order effects, sperm numbers did not seem to be affected by the exposure to other stallions or to mares only.”
One of three sperm velocity measures – curvilinear velocity – was consistently higher in stallions that were first exposed to stallions and then to mares.
The changes noted in sperm appeared to be related to blood testosterone levels, they wrote.
“We conclude that ejaculate characteristics are plastic traits affected by the social environment in horses.”
The order of exposure to stallions and mares significantly affected ejaculate characteristics, they reported.
The trio had set out to examine an element of what is known as sperm competition theory, which predicts semen characteristics can be affected by the social suroundings.
The researchers chose stallions for the study, as they typically showed “behavioral plasticity” in response to different social environments, which they hypothesized may well be reflected in plasticity in semen parameters.
“We therefore compared two environments that are likely to create extreme differences in predicted optimal semen investments: exposure to a group of other stallions versus exposure to one mare only.”
The researchers used 12 breeding Franches-Montagnes stallions with stud records showing normal fertility, and six mares, comprising three Franches-Montagnes and three Warmblood horses. The study was carried out at the Swiss National Stud Farm.
Previously, all stallions had been stabled in individual stud housing systems.
Blood samples were taken weekly from the stallions during the study for testosterone analysis.
“We found that blood testosterone levels were useful predictors of mean sperm number after exposure to mares but not after exposure to stallions,” the study team reported.
“This suggests that testosterone levels during exposure to a mare can be a proxy for a stallion’s mate preference and his willingness to invest into costly semen production.
“In conclusion, as predicted from sperm competition theory, stallions are able to adjust ejaculate characteristics to the social environment they experience.
“Blood testosterone can be a useful predictor of ejaculate characteristics under some circumstances, but probably not under others.”
Burger and Dolivo are with the Swiss Institute of Equine Medicine, Agroscope and University of Berne; Wedekind is with the Department of Ecology and Evolution, Biophore, at the University of Lausanne.
Burger D, Dolivo G, Wedekind C (2015) Ejaculate Characteristics Depend on Social Environment in the Horse (Equus caballus). PLoS ONE 10(11): e0143185. doi:10.1371/journal.pone.0143185
The full study can be read here.
The study was published a Creative Commons License.