UPThèses

Breeding at the right time is a key-component of fitness, and requires flexible responses to environmental conditions and physiological state. However, the proximate mechanisms underlying this pattern remain poorly understood. This doctoral research aims to explore the hormonal mechanisms that mediate the onset of egg-laying in two polar seabirds, the blacklegged kittiwake and the snow petrel. First of all, a food-related stress during the pre-laying period allowed us to associate a late-breeding year with high levels of corticosterone (stress hormone) and with low levels of LH (luteinizing hormone that triggers reproduction). Moreover, an experimentally delayed breeding promoted an accentuated stress response, a decline of parental effort, and a high reproductive failure. Descriptive and experimental approaches supported the role of corticosterone in the individual adjustment of egg-laying dates and in the non-breeding decision, through the inhibition of LH release. However, this functional action of corticosterone differed between sex and between species. At last, we investigated the effects of age (7-45 years old) and senescence on the dynamics of corticosterone and LH, since very young and very old birds did not breed or bred late in the season. We discussed the possible hormonal orchestration of reproductive henology, as an interaction between environmental stress and individual state, in the theoretical framework of evolutionary strategies.