A little competition can help increase performance. In the case of stallions, a little competition also makes for a better semen. In a new study, Swiss researchers have found that stallions have better semen quality when they are kept next to other stallions before they are placed next to mares.
Stallions that were kept in a stallion stable for eight weeks (individual stables alongside other breeding stallions) and then kept in an individual stable next to a mare for several weeks had a higher concentration of sperm cells and more motility, said Dominik Burger, DVM, a scientist at the Swiss Institute of Equine Medicine in Avenches.
As soon as the same stallions were stabled next to a mare without having been next to other stallions for a number of weeks, they had a relatively lower concentration of sperm cells and less motility. “Semen properties are flexible and now we can determine that the quality and quantity of semen respond to the social environment of stallions,” Burger said.
In the study, Burger and his colleagues looked at the social effects on semen quality and quantity in 12 healthy, fertile Freiberger stallions at the Swiss National Stud in Avenches. The stallions first spent eight weeks in one of two situations: in individual stables (separated by bars) in a stallion stable or in a mixed stable where each stallion was stabled next to a mare, without the presence of other stallions. Then the situation was reversed for another eight weeks. The researchers tested every stallion in all possible combinations. The horses continued to perform their normal breeding duties according to a standardized protocol. The eight-week period was an important period to go through the entire cycle of semen production. In this way, the researchers could see what the social effects were from the early stages of semen development. The team recorded the testosterone level, sperm count and cell motility of each stallion at various times.
Interestingly, testosterone levels were linked to semen quantity in some cases. The stallions kept in the stallion stable showed no significant difference. However, when the stallions were kept next to a mare, the testosterone level seemed to give an indication of the amount of semen cells that were ejaculated. This information can help the breeder choose a mare that will increase the stallion’s sperm count. Stallions have a preference for a certain mare. So by testing the testosterone level after a stallion has stood next to a certain mare, you can see what the difference in semen quantity is per mare.
“Our results are consistent with the ‘competitive sperm theory’. That means, to put it simply, in species where a female can have multiple mates, the semen itself has to compete against the semen of other males to get to the ovum first,” Burger said. “Stallions are able to respond to their social environment by adjusting the quality of their semen, potentially leading to better fertility.”
The study “Ejaculate Characteristics Depend on Social Environment in the Horse (Equus caballus),” is published in PLoS One.