When too much sex is exhausting

New Scientist, 10 April 1999

BIG, tough rams may get more sex, but they're victims of their own success. They run out of sperm before the mating season is over, so their scrawny competitors end up fathering most late-born lambs, researchers say.

Brian Preston of the University of Stirling and his colleagues studied a flock of wild Soay sheep on the Scottish island of St Kilda over three years. They observed the behaviour of more than 100 sheep during the mating season, which runs for five weeks in November and December.

During this time, the heaviest males copulated most. Some had sex more than ten times a day thanks to their skills at fending off other males. But blood samples later revealed that although big sheep fathered most offspring overall, small sheep fathered just as many lambs as large ones in the last two weeks of mating.

"The lighter males were gaining in paternity toward the end of the rut," Preston concluded at a meeting of the Association for the Study of Animal Behaviour in Newcastle last week. He believes this happens because the promiscuous large sheep simply run out of sperm.