Abstract
<jats:p>Given its sensitivity to environmental variation and its central role in physiological signalling, the endocrine system has strong potential to mediate the responses of wild animals to ongoing climate change. Evidence has accumulated, showing that breeding females can transfer maternal hormones to their offspring, thereby modifying offspring phenotype, constituting a potential avenue of transgenerational plasticity to adaptively shape offspring phenotype according to the anticipated environment. This hypothesis requires rigorous testing of several important prerequisites, including whether maternal hormone transfer is itself environmentally responsive. In this study, using a widely distributed bird species – the great tit (Parus major) – as our model system, we conducted a large-scale study across 12 European populations to examine if the amount of maternally transferred yolk thyroid hormones (THs) covaries with environmental variables such as latitude, environmental iodine, and ambient temperature, as well as laying dates as an integrative measure of early spring conditions. Across the 12 sampled populations, we found very little among-population variation in the amount of maternally transferred yolk THs. Within populations, however, yolk triiodothyronine (T3), the active form of THs, showed significant negative relationships with laying date and the average temperature during the period of egg formation, whereas thyroxine (T4), the other main TH, directly produced by the thyroid gland, only showed a less pronounced relationship with average temperature. Given the well-established role of T3 in thermoregulation, our results suggested that the temperature-dependent pattern in maternal yolk T3 reflects the thermoregulatory requirement in the maternal system during egg formation, a hypothesis that warrants further experimental validation. Whether this pattern facilitates or constrains offspring phenotypic plasticity in response to climate change represents an important avenue for further research.</jats:p>