Swingin' in the rain: condition dependence and sexual selection in a capricious world

Signals used in mate attraction are predicted to be highly condition dependent, and thus should be sensitive to environmental contributions to condition. However, the effects of temporal fluctuations in the environment on sexual selection in long-lived animals have been largely ignored. Female superb fairy-wrens, Malurus cyaneus, use the time that males moult into nuptial plumage prior to the onset of the breeding season to distinguish between the extra-group sires that dominate paternity. Although moult varies predictably with age, and shows marked differences between males, the phenotypic distribution also changes radically with climate; so after dry summers few males can attempt early moult. We use the recently introduced de-lifing technique to examine sexual selection gradients over 15 years of selection. Overall, there was strong evidence of directional sexual selection for early moult. However, sexual selection was much stronger when the conditions were favourable (rainfall was high), and selection was undetectable in some years. The contribution of early moulting males to population growth increased when many males moulted early, decreased when early moulting males suffered disproportionate mortality and decreased when females lacked subordinate helpers, forcing them to cede paternity to their social partner. These data suggest that short-term and laboratory studies of mate choice and sexual selection may misrepresent or underestimate the complexity of the sexual selection landscape.