Maternal hormones meet environmental variability: Context-dependent effects of maternal hormones in avian egg yolks

In the past few decades, maternal effects have been widely recognized as an important way through which mothers can modify offspring phenotypes above and over direct genetic effects. As a wide variety of animals are prenatal exposed to maternal hormones, accumulating evidences also suggest that maternal hormones is an important mediator of maternal effects. In birds, previous studies have already shown evidence that maternal hormones in egg yolks can influence offspring development, growth, physiology, and behaviour, influencing their fitness. Nevertheless, the adaptive significance of prenatal maternal hormone exposure is still unclear because of the great discrepancy in the literature. A very likely explanation to the discrepancy is that because maternal hormones bring costs as well as benefits and the final outcome would be modulated by the environmental and/or biological contexts. Focusing on the most extensively studied maternal hormone – testosterone, and using rock pigeons (Columba livia) as a model species, I experimentally investigated whether the effects of maternal yolk testosterone would depend on post-hatching food conditions, offspring age, and egg composition. The results indeed suggested food-dependent effects of maternal testosterone. Maternal testosterone also showed long-term effects far into adulthood and did not wane over age. No strong evidence of interaction between yolk testosterone and another class of yolk hormones – thyroid hormones – were found, but subtler interactions might be still hidden and require more studies. These results provide explanation to the literature discrepancy and strongly suggest the importance to take context into account in future studies of maternal hormones.