Sometimes It Pays to Be a Weakling | Science/AAAS

Sometimes it’s better not to be the best. Take rams. Those with bigger horns get the girl more often—but they also die younger… The tradeoff helps explain a long-standing puzzle about why the best genes for mating don’t take over…

…Fending off potential rivals or vying for a female’s attention has driven males to be ever more extreme—bigger, stronger, more colorful. This process, called sexual selection, should also affect the genes, so that only versions of genes that lead to these enhanced qualities should exist. But this isn’t what happens in the real world. There are plenty of small, weak males among the supermen.

Sheep are a good example. Males with the biggest horns get the ewe more often—they tussle with each other over mates and big horns provide an advantage. Yet some guys still have stubby horns. “It has always been a bit of a mystery as to why there would be so much variation in horn type,” says evolutionary biologist Susan Johnston of the University of Edinburgh in the United Kingdom.

Johnston realized that she might be able to resolve this paradox by analyzing long-term data about wild sheep living on an island off Scotland’s coast...