Modulation of the prolactin and the corticosterone stress responses: do they tell the same story in a long-lived bird, the Cape petrel?

Over the last decades, the corticosterone stress response has been suggested as a major physiological tool to understand what strategy an individual might adopt in response to environmental perturbations. More recently, another hormone related to parental care--prolactin--has been suggested as a complementary tool to investigate this question. Indeed, both of these hormones are affected by stressors and are involved in parental decisions, such as deserting the nest. Because of these similarities, it remains unclear what the functional distinction between the prolactin and corticosterone stress responses is. Here, we investigated whether natural variations of the corticosterone and prolactin stress responses are functionally linked in free-living Cape petrel (Daption capense) parents. If prolactin and corticosterone mediate the same functional response to a stressor and are the proxies of the same response, we predict that corticosterone and prolactin stress responses (1) will be modulated according to the same factors; (2) will affect reproductive performances in the same way; and, (3) of course, will be correlated. Contrary to these predictions, we found that the corticosterone and prolactin stress responses were respectively modulated according to body condition and breeding status. Moreover, prolactin levels, but not corticosterone levels, were related to hatching success in this species. Finally, we did not find any significant correlation between these two stress responses under any circumstances (failed breeders, incubating or chick rearing birds) and this result was overall supported by a review of the existing literature. Therefore, these two stress responses do not seem to be tightly linked and we believe that they may provide complementary pieces of information on parental investment in birds.