There’s nothing like a little bit of competition to get the blood pumping. And in the case of stallions, such competition appears to get the sperm going, too. In a new study, Swiss researchers revealed that stallions have better-quality semen when housed next to other stallions before being housed next to a mare.

Stallions that spent eight weeks in a stallion barn—housed in individual stalls next to other breeding stallions—and then several weeks stabled in an individual stall next to a mare experienced an increase in sperm counts and velocities, said Dominik Burger, DVM, a scientist at the Swiss Institute of Equine Medicine (University of Bern and Agroscope) in Avenches.

The same stallions housed next to a mare without first spending a few weeks housed with other stallions had comparatively lower sperm counts and velocities, Burger said.

“Ejaculate characteristics are plastic (meaning they can change), and here we confirm that sperm quantity and quality react to the social environment of stallions,” Burger said.

In their study, Burger and his fellow researchers investigated social effects on the sperm quantity and quality of 12 healthy, fertile Franches-Montagnes stallions at the Swiss National Stud in Avenches. The stallions first spent eight weeks in one of two situations: in an exclusive stallion barn, each in an individual stall separated from other stalls by bars, or in a mixed barn, where each stallion was stalled next to a single mare without other stallions present. Then the housing situation was reversed for eight weeks. The researchers tested each stallion in all possible combinations, and the horses