Red-backed Fairy-wren Research in Australia
Over a century ago, Charles Darwin provided a robust explanation for behavioral and morphological differences between the sexes, the theory of sexual selection. But how can we explain variation, and even discrete morphs, within a single sex? Since 1997, I have addressed this and related questions in the Red-backed Fairy-wren Malurus melanocephalus. In this small Australian passerine, males breed in dull, female-like plumage or in bright, nuptial plumage. My Ph.D. research was the first detailed study of the species.
By combining microsatellite screening, mate-choice experiments, and behavioral observation, I showed that bright and dull males follow discrete behavioral strategies. Bright males actively pursue extra-pair copulations, resulting in increased reproductive success (Evolution 56:1673-1682). Dull males, conversely, invest in mate guarding and parental care, and experience reduced aggression, which may increase within-brood paternity and survival (Emu 103:87-92, MS in prep)
Male phenotype is not a fixed trait in Red-backed Fairy-wrens. Rather, it varies in relation to age and social status. Currently, I am investigating the evolutionary and mechanistic underpinnings of this switch in collaboration with Dr. Michael Webster and Dr. Hubert Schwabl of Washington State University. I have shown that male bill color rapidly darkens following a switch in social or breeding status (Journal of Avian Biology, in review). This led us to hypothesize that male phenotype is socially mediated. I tested this in early 2006 by experimentally manipulating male social status and documenting phenotypic changes relative to controls. As predicted, males experimentally switched from subordinate non-breeders to dominant breeders developed darker bills, brighter plumage, and larger sperm storage organs (MS in prep.). Currently, we are testing the role that testosterone plays in these patterns of plumage change.
Male Red-backed Fairy-wrens also exhibit an unusual case of reversed sexual dimorphism in tail length. With Dr. John Swaddle of the College of William and Mary and Dr. Stephen Pruett-Jones of University of Chicago, I reported that directional selection has likely been acting to reduce tail length in bright males, but only during the breeding season (Behavioral Ecology 11:345-349). Further experimental and molecular analyses suggested that reduced tail length is a sexually selected trait in bright males associated with intra-sexual social dominance and within-pair reproductive success (Behavioral Ecology, in review). Thus, tail length provides an interesting contrast to plumage brightness, which appears to increase male fitness through female choice and extra-pair copulations.
A bright breeding male
A dull male with darkening bill