Prenatal thyroid hormone exposure increases growth but not oxidative stress in a wild passerine species

Hormones transferred from mothers to their offspring are thought to be a maternal tool for mothers to prepare their progeny for expected environmental conditions, thus increasing fitness. Thyroid hormones (THs) are crucial across vertebrates for embryonic and post-natal development and metabolism. Nevertheless, the studies that investigated the consequences of maternal hormones have mostly focused on steroid hormones and ignored maternally-derived thyroid hormones. In this study, we experimentally elevated yolk thyroid hormones in a wild population of a migratory passerine, the European Pied flycatcher Ficedula hypoleuca. We injected eggs with a mixture of T4 and T3 within the natural range of the species to assess its effects on hatching success, nestling survival, growth and oxidative status (antioxidant enzyme activity, lipid peroxidation and oxidative balance). We found no effects of yolk THs on egg hatchability or nestling survival. Yolk THs increased nestling growth during the second week post hatching, but this potentially beneficial effect did not incur any costs in terms of oxidative stress. The results should stimulate more research on thyroid hormone mediated maternal effects, further studies into the underlying mechanistic pathways for these effects and how they translate into adulthood and fitness.Summary statementThyroid hormones have been overlooked in the context of hormone-mediated maternal effects. We found that yolk thyroid hormones in a wild bird species increase growth without incurring oxidative stress.