Résumé
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 ovarian follicles
showed stronger responses and females from lines selected for fast
exploratory behaviour showed stronger increases compared to females from
the slow line, indicating a heritable component. This study shows that the
response of the ovary to a reproductive stimulation from the brain greatly
varies among individuals and that this variation can be attributed to
several commonly measured individual traits, which sheds light on the
mechanisms shaping heritable endocrine phenotypes.