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

Abstract

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 fairywrens, 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.

Intra-sexual selection in cooperative mammals and birds: why are females not bigger and better armed?

“…Mate choice may generate reproductive variance in the chosen sex that selects instead for the exaggeration of attractive traits, which often differ from those that yield success in competition [4,18,94]. However, as female mammals and birds are generally expected to be the choosier sex, where mate choice does play a role it might be expected to contribute differentially to reproductive variance among males [24,92,95], leaving any association between female-biased reproductive variance and male-biased size dimorphisms if anything more perplexing.…”

“…At least two processes could leave the mean relatedness among competing females higher than that among competing males. First, males frequently contest extra-group parentage (with non-relatives) [89,92,95,130,131], whereas females rarely do (though see [93]). Second, the strong female philopatry seen in many cooperative mammals [122,123,132] can be expected to exacerbate this sex difference, leaving females frequently contesting within-group reproduction near-exclusively with close relatives [28,45,133,134], whereas males disperse and contest dominance with unrelated males in other groups ( [45,134,135]; though coalitional dispersal can also leave relatives competing post-dispersal).…”

Timing as a sexually selected trait: the right mate at the right moment

“…In superb fairy wrens (Malurus cyaneus) the earlier a male moults into breeding plumage, the more likely he is to increase his fitness by extra-pair paternity [128][129][130]. Indeed, a multi-year study found strong evidence for directional selection in promoting early moult in males [131].…”

Experimental evidence that extra-pair mating drives asymmetrical introgression of a sexual trait

“…weight, tarsus length, tail length) from all captured birds, and also collected feather samples (six to 12 feathers from the orange back patch) from all males in nuptial plumage. Daily monitoring of individuals allowed us to quantify the date at which males completed their nuptial moult, which is an important indicator of age and reproductive success in a congener, the superb fairy-wren (Malurus cyaneus) [31], and may be in the red-backed fairy-wren as well [17]. We were able to classify adult birds into two age classes-second-year and after-secondyear-based on degree of skull ossification [32], which was important because, in a different population of red-backed fairywrens, older males tend to foray off their territories (probably seeking extra-pair copulations) more often than do younger males (A. L. Potticary et al 2013, unpublished data).…”