Maternal programming: costs, benefits and constraints of maternal hormone transfer

Abstract: Avian mothers convey significant quantities of androgens (i.e. ?male? steroids) to their eggs, which are known to positively influence embryonic development, post-natal growth and competitive behaviour of the offspring. However, there is large inter-female variation in the amount of androgens transferred to the eggs. Recent studies suggest that this variation may be maintained because the positive effects on growth and behaviour are counterbalanced by negative effects on the immune function of the offspring. This thesis deals with the causal and functional aspects of maternal androgen transfer in birds. In a study on a wild pied flycatcher (Ficedula hypoleuca) population, I investigated female plasticity in androgen transfer with respect to the timing of breeding. Eggs of replacement clutches produced late in the season contained on average lower levels of androgens compared to first clutches produced early in the season. Within clutches, androgen concentration increased over the laying sequence in the first clutch, but decreased or remained more constant over the laying sequence in the replacement clutch. Differential androgen transfer with respect to timing of breeding is discussed as an adaptive female tactic to optimize reproductive output. To examine the effects of egg androgens on offspring phenotype, I compared offspring hatched from eggs with experimentally elevated androgen levels with offspring hatched from eggs with sham-manipulated androgen levels (controls). I found that exposure to elevated egg levels of testosterone (T) increased metabolism in zebra finch (Taeniopygia guttata) chicks. Extra egg T also enhanced growth in male zebra finch chicks, but at the same time reduced their ability to resist oxidative stress during the early growth phase. These results are the first to demonstrate that high levels of egg androgens can entail costs in terms of increased energy expenditure and increased susceptibility to oxidative stress. To evaluate whether there are any long-term effects of egg T on offspring physiology and behaviour, I followed a cohort of zebra finches hatched from manipulated eggs. Although no effects of egg T on adult immunity were found, there appeared to be a positive effect of high levels of egg T on oxidative stress resistance in adult males when they were subjected to an immunization with a pathogen. Furthermore, egg T influenced adult behaviour in males and females. High levels of egg T facilitated the ability of adult zebra finches to habituate to a novel situation. The effects of egg androgens on growth and immunity were also investigated in a study on wild jackdaws (Corvus monedula). In this study, there was a clear negative effect of high egg androgen levels on nestling immunity. The study shows that the effect is present during different developmental periods of the nestlings, and most interestingly, still present just before fledging. The results from these experimental studies add to our understanding of how transgenerational transmission of hormones affect the formation of the offspring phenotype.