Testing different forms of regulation of yolk thyroid hormone transfer in pied flycatchers

Hormones transferred from mothers to their offspring are considered a maternal tool to prepare progeny for expected environmental conditions, increasing maternal fitness. To flexibly influence offspring, mothers should be able to transmit the hormonal signals independent of their own hormonal status. However, the ability to regulate hormone transfer to the next generation is under debate. We studied the transfer of thyroid hormones (THs) to eggs in a bird model. We elevated thyroxine (T 4, the prohormone for the biological active triiodothyronine, T 3 ) during egg-laying using T 4 implants on females of a wild population of pied flycatchers ( Ficedula hypoleuca ), and measured plasma and yolk T 4 and T 3 as a response. To our knowledge, studies that manipulated a prohormone and measured the change in its active metabolites have rarely been conducted. We found an increase in plasma and yolk T 4 and no change in T 3 concentrations leading to a similar decrease in yolk T 3 /T 4 ratio in response to the T 4 treatment in plasma and yolk. This suggests that mothers are able to regulate the conversion of T 4 in T 3 to avoid potential costs of elevated exposure to the active hormone to herself and to her progeny. Finally, contrary to our predictions, we found no evidence of regulatory mechanisms at the follicle level, which is essential for independent regulation of yolk hormone transfer. Summary statement Thyroid hormones have been overlooked in the context of hormone-mediated maternal effects. We found that mothers may regulate yolk thyroid hormone transfer by regulating the conversion of the active form of the hormone.