Elevated yolk androgen levels benefit offspring development in a between-clutch context

Abstract

The field of androgen deposition in avian eggs and its consequences for offspring development has received a lot of attention in recent research. However, although variation within clutches in yolk androgens is relatively well understood, the adaptive significance of patterns of variation between clutches remains rather unclear. Furthermore, it has been hypothesized that yolk androgens act as a means of an adaptive maternal effect to adjust offspring to a given posthatching environment. Thus, the consequences of maternal yolk androgens for offspring development are likely to depend on the specific environment of a given brood. We experimentally manipulated yolk androgen concentrations in spotless starling eggs, using a between-brood design in which full broods were manipulated applying either an androgen or sham treatment, in order to test the effects of between-brood variation in yolk androgen levels. We also included in the analysis several female characters that have been shown to affect androgen deposition. Androgen-chicks tended to gain more mass, a similar effect to that shown in previous studies where direct competition between chicks belonging to different treatments was allowed, but did not gain a survival benefit. Androgen-chicks had wider beak flanges, an effect that has not been described previously and could play an important role in food acquisition. In addition, androgen-chicks had higher endogenous plasma levels of androgens, which could induce higher begging intensity. We hypothesize that these effects are an important mechanistic link for our understanding of how yolk androgens exert their effects on offspring development after hatching. Contrary to our predictions, we found no evidence that the observed effects depended on the environment under which it was investigated. Because high yolk androgen levels seem to be beneficial for nestlings in this species, we hypothesize that yolk hormone deposition could be costly for females or alternatively that potential negative effects later in adulthood may constrain maternal hormone allocation.