Preston BT, Stevenson IR, Pemberton JM, Wilson K, 2001. Dominant rams lose out by sperm depletion. Nature 409: 681-682.

Male traits such as large body size and weaponry are thought to have evolved to aid males contesting for access to females. But in species where females copulate with many males in a single reproductive cycle, sperm also compete in the female’s reproductive tract to fertilize ova. Both types of competition occur in the romiscuously mating feral Soay sheep (Ovis aries) found on the island group of St Kilda, Scotland. Here we show that constraints on sperm production mean that those males that are most successful in overt contests can become ineffectual in covert sperm competition.