Sperm competition and the coevolution of pre- and postcopulatory traits: Weapons evolve faster than testes among onthophagine dung beetles.

Reproductive competition generates episodes of both pre- and postcopulatory sexual selection. Theoretical models of sperm competition predict that as the fitness gains from expenditure on the weapons of male combat increase, males should increase their expenditure on weapons and decrease their expenditure on traits that contribute to competitive fertilization success. Although traits subject to sexual selection are known to have accelerated evolutionary rates of phenotypic divergence, it is not known whether the competing demands of investment into pre- and postcopulatory traits affect their relative rates of evolutionary divergence. We use a comparative approach to estimate the rates of divergence in pre- and postcopulatory traits among onthophagine dung beetles. Weapons evolved faster than body size while testes mass and sperm length evolved more slowly than body size, suggesting that precopulatory competition is the stronger episode of sexual selection acting on these beetles. Although horns evolved faster than testes, evolutionary increases in horn length were not associated with evolutionary reductions in testes mass. Our data for onthophagines support the notion that in taxa where males are unable to monopolize paternity, expenditure on both weapons and testes should both be favored.