Personality and gonadal development as sources of individual variation in response to GnRH challenge in female great tits

Seasonal timing of reproduction is a key life-history trait, but we know little about the mechanisms underlying individual variation in female endocrine profiles associated with reproduction. In birds, 17β-estradiol is a key reproductive hormone that links brain neuroendocrine mechanisms, involved in information processing and decision making, to downstream mechanisms in the liver, where egg-yolk is produced. Here we test, using a simulated induction of the reproductive system through a GnRH-challenge, whether the ovary of pre-breeding female great tits responds to a brain stimulation by increasing estradiol. We also assess how this response is modified by individual-specific traits like age, ovarian follicle size and personality, using females from lines artificially selected for divergent levels of exploratory behaviour. We show that a GnRH injection leads to a rapid increase in circulating concentrations of estradiol but responses varied among individuals. Females with more developed...