Heritability of gonad size varies across season in a wild songbird

Many organisms advance their seasonal reproduction in response to global warming. In birds, which regress their gonads to a non-functional state each winter, these shifts are ultimately constrained by the time required for gonadal development in spring. Gonadal development is photoperiodically-controlled and shows limited phenotypic plasticity in relation to environmental factors, such as e.g. temperature. Heritable variation in the time required for full gonadal maturation to be completed, based on both onset and speed of development, is thus a crucial prerequisite for an adaptive advancement of seasonal reproduction in response to changing temperatures. We measured gonadal seasonal development in climate-controlled aviaries for 144 great tit (Parus major) pairs, which consisted of siblings obtained as whole broods from the wild. We show that the extent of ovarian follicle development (follicle size) in early spring is highly heritable (h2=0.73) in females, but found no heritability of the extent of testis development in males. The heritability in females decreased as spring advanced, caused by increasing environmental variance and a decrease in additive genetic variation. Heritable variation in a physiological mechanism underlying reproductive timing may enable genetic adaptation to climate change, a key insight as this great tit population is currently under directional selection for advanced egg laying.