It's the sperm, stupid

A Scottish study shows that while larger sheep may mate more often, they don't sire more offspring than the little guys.

By Jack Boulware

Feb 16, 2001 | In any given social circle, you'll find certain men who are, for lack of a better term, complete pussy-hounds. These studs sling their seed all over town with a variety of women, as if following a primal urge to make as many babies as possible. And it may be that these studs have so many ejaculations that it decreases their sperm production. Their testosterone and libido levels drop, and before you know it, the little guy with thick glasses and moles starts getting all the action. According to a just-released study, this is exactly what happens in the world of feral sheep.

On the St. Kilda group of islands off the coast of Scotland, a flock of 1,400 Soay sheep runs wild and free of human exploitation. The sheep's testicles are extremely large for their body weight, which produces a swinger's orgy of frenzied sheep lust. During a two-day period of rutting, females might have sex with up to seven males, and males might mate up to 13 times a day. The biggest rams with largest testes get most of the chicks, as you might expect, but all the action leaves them exhausted, and drained of sperm. Initially, they might get their rocks off more often and father more lambs, but by the end of the orgy they don't sire any more offspring than smaller, wimpier rams. In other words, the little ram gets less action, but he shoots more bullets and therefore leaves a nice legacy.

Scottish ecologist Ian Stevenson of the University of Stirling is excited about the results of his team's study. "This is the first time that sperm depletion has been shown to undermine overt competition," Stevenson told the Nature news service. "It's not as bad to be a small male as you might think."

Matthew Gage, a behavioral ecologist at the University of Liverpool, agrees that the level of sperm in a male is more important than previously thought. "We're finding more and more that sperm production has a cost," Gage says. He suspects that these effects of sperm depletion may be common in other mammals, "but we need to look at it in more detail; this work is a good first step."

As high school coaches so often tell their team members, it's not the size of the dog in the fight, it's the size of the fight in the dog.

• View blog reactions