Sexual selection theory encompasses both overt precopulatory mate securing tactics and covert postcopulatory gametic competition. As a consequence, copulation success need not translate into fertilization success. In mammals, one of the key adaptations to sperm competition is the insemination of large numbers of sperm, which is associated with an evolutionary increase in testicular size. Based on the lottery principle, males that inseminate most sperm tend to fertilize most ova. However, the production of sperm and seminal fluid is costly, because frequently copulating males haveto replenish depleted supplies. Thus, there might be a tradeoff between pre- and postcopulatory success, with frequently successful maters losing out in sperm competition.This appears to be the case in feral Soay sheep (Ovis aries) on the island group of St Kilda, UK, detailed in a new study by B.T. Preston et al.
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