Do androgens link morphology and behaviour to produce phenotype-specific behavioural strategies?

Morphological and behavioural traits often covary with each other, and the links between them may arise from shared physiological mechanisms. In particular, androgens such as testosterone have emerged as prime candidates for linking behaviour and morphology due to the environmental sensitivity and pleiotropic effects of these hormones. In this study we investigated the hypothesis that androgens simultaneously relate to morphological and behavioural variation, thereby producing the integrated reproductive phenotypes of male red-backed fairy-wrens, Malurus melanocephalus. Males of this species can adopt one of three discrete breeding phenotypes: breeding in red/black plumage, breeding in brown plumage, or remaining as nonbreeding brown natal auxiliaries. Although the expression of morphological traits in this species is regulated by androgens and phenotypes differ in baseline androgen levels (red/black breeder > brown breeder > auxiliary), injection with GnRH failed to expose phenotype-specific constraints on androgen production. Observations of territoriality, nestling feeding and extraterritorial forays revealed phenotype-specific patterns of mating and parental effort, yet these were largely related to age and were not correlated with baseline or GnRH-induced androgen levels, or the androgen change between these points. While these findings support the idea that morphological and behavioural traits are linked via phenotypic correlations, they do not support the hypothesis that behavioural differences arise from variation in circulating androgens or the capacity to produce them.