The largest and most well-endowed male sheep can copulate as many as 13 times a day, but in the end they can't compete with their smaller counterparts, according to a report in the 8 February Nature.

From a study of feral Soay sheep in the wild, Brian Preston of the University of Stirling and his colleagues found that although large rams are initially more successful at mating, and father more lambs overall, they eventually become depleted of sperm (Nature 2001, 409:681-682). Towards the end of the breeding season it is the smaller males who, having exerted themselves less, sire more lambs. The scientists also found an effect of copulation frequency on sperm quality. Rams that copulate more frequently produce fewer healthy sperm at the end of the breeding season. It seems too much sex can have ram-ifications.