MORTALITY COSTS OF SEXUAL DIMORPHISM IN BIRDS

Sexually selected traits provide a mating advantage to the bearer but they should also exact a cost through natural selection. Whereas the mating benefits from such traits have been well documented, the costs have been difficult to demonstrate. In this analysis of mortality patterns across 28 North American passerine bird species, we show that sex-biased mortality (log10 male mortality - log10 female mortality) is positively correlated with both sexual size dimorphism and male plumage brightness. Male (but not female) mortality is positively correlated with sexual size dimorphism, suggesting a cost to male-male competition. Female (but not male) mortality is negatively correlated with male brightness, and we argue from this that the evolution of male brightness has been constrained by mortality costs. Thus sexual dimorphism in body size and plumage colour within bird species appears to be influenced by the opposing forces of sexual selection, acting to increase dimorphism, and adult mortality rates, which constrain the evolution of these traits. Differences in the expression of ornamental traits across species may be explained not only by variation in the mating benefits that accrue from ornaments, as is so often assumed, but also by variation in the costs of these traits.