Yolk androgen deposition in two passerine species: Do females play favorites?

Abstract

Eggs of oviparous species contain variable amounts of androgens, such as testosterone (T), dihydrotestosterone (DHT), and androstenedione (A4). Previous studies in birds show that yolk androgens of maternal origin have both immediate and long-term effects on offspring growth, immune function, behavior, and mortality. Additionally, female birds alter patterns of androgen deposition into yolk in a species-specific manner according to social interactions, environmental conditions, and incubation patterns. Thus, the deposition of yolk androgens has been proposed as an adaptive mechanism by which female birds can promote the survival of certain offspring, and thus maximize reproductive success. I assessed deposition patterns of yolk androgens as well as the effects of high yolk androgen concentrations on offspring in two passerine species with very different life history traits. House finches (Carpodacus mexicanus) hatch asynchronously and females choose mates based on plumage coloration. In a correlational study, female house finches deposited significantly more androgens into eggs sired by unattractive males, and into eggs laid later in the laying sequence, but only when mated to unattractive males. Injections of testosterone into house finch eggs resulted in offspring that were larger at hatch and had a larger T cell immune response compared to control nestlings. The eastern bluebird (Sialia sialis) differs from the house finch in that individuals of both sexes are extremely aggressive and territorial. Presentation of an “intruder challenge” drove females to deposit significantly more androgens into eggs, but concentrations of androgens in female plasma were significantly lower in stimulated females than in controls. Additionally, hormone profiles of egg yolks did not simply reflect the content of female plasma during follicular development. Finally, injections of testosterone into bluebird eggs increased embryonic mortality, stimulated weight gain during the nestling period, and decreased T cell immunity of offspring. Strategies involving the deposition of yolk androgens as well as the effects of such strategies on offspring are likely adaptive but are complex and species-specific.