Data from: Sperm competition and the evolution of pre-copulatory weapons: increasing male density promotes sperm competition and reduces selection on arm strength in a chorusing frog

Sperm competition theory assumes a trade-off between pre-copulatory traits that increase mating success and post-copulatory traits that increase fertilization success. Predictions for how sperm competition might affect male expenditure on these traits depend on the number of competing males, the advantage gained from expenditure on weapons, and the level of sperm competition. However, empirical tests of sperm competition theory rarely examine pre-copulatory male expenditure. We investigated how variation in male density affects pre-copulatory sexual selection on male weaponry and the level of sperm competition in the chorusing frog Crinia georgiana, where males use their arms as weapons in male-male combat. We measured body size and arm girth of 444 males, and recorded their mating success in the field. We found density-dependent selection acting on arm girth. Arm girth was positively associated with mating success, but only at low population densities. Increased male density was associated with higher risk and intensity of sperm competition arising from multi-male amplexus, and a reversal in the direction of selection on arm girth. Opposing patterns of pre- and post-copulatory selection may account for the negative covariation between arm girth and testes across populations of this species.