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Physiological costs and age constraints of a sexual ornament: an experimental study in a wild bird

Citation

McQueen, Alexandra et al. (2020), Physiological costs and age constraints of a sexual ornament: an experimental study in a wild bird, Dryad, Dataset, https://doi.org/10.5061/dryad.bg79cnp97

Abstract

Sexual ornaments are often considered honest signals of quality because potential costs or constraints prevent their display by low-quality individuals. Testing for potential physiological costs of ornaments is difficult, as this requires experimentally forcing individuals to produce and display elaborate ornaments. We use this approach to test whether a sexually selected trait is physiologically costly to male superb fairy-wrens (Malurus cyaneus). Male fairy-wrens moult from brown to blue breeding plumage at different times of the year, and females strongly prefer the few males that are blue early, during winter. We used short-acting testosterone implants to stimulate males to produce ‘early-blue’ plumage and assessed costs during and after moult using a panel of physiological indices. Testosterone-implanted, T-males moulted in winter and produced blue plumage six weeks before control-implanted, C-males. T-males moulted while in lower body condition, had lower fat reserves, and were more likely to be parasitised by lice. However, we detected no negative effects on immune function, blood parasites, exposure to stressors, or survival. Juvenile males never naturally display early-blue plumage, but we found no evidence for increased costs paid by juvenile T-males. Instead, juvenile T-males moulted later than adult T-males, suggesting that age presents an absolute constraint on ornament exaggeration that cannot be fully overcome by testosterone treatment. Together, these small costs and large, age-related constraints may enforce signal honesty, and explain female preference for early-blue males.

Funding

Holsworth Wildlife Research Endowment

Monash University

Australian Research Council, Award: FT10100505

Australian Research Council, Award: DP150103595