Behavioural and hormonal stress responses during chick rearing do not predict brood desertion by female in a small Arctic seabird

Eye Region Surface Temperature and Corticosterone Response to Acute Stress in a High-Arctic Seabird, the Little Auk

Measuring changes in surface body temperature (specifically in eye-region) in vertebrates using infrared thermography is increasingly applied for detection of the stress reaction. Here we investigated the relationship between the eye-region temperature (TEYE; measured with infrared thermography), the corticosterone level in blood (CORT; stress indicator in birds), and some covariates (ambient temperature, humidity, and sex/body size) in a High-Arctic seabird, the Little Auk Alle alle. The birds responded to the capture-restrain protocol (blood sampling at the moment of capturing, and after 30 min of restrain) by a significant TEYE and CORT increase. However, the strength of the TEYE and CORT response to acute stress were not correlated. It confirms the results of a recent study on other species and all together indicates that infrared thermography is a useful, non-invasive measure of hypothalamic-pituitary-adrenal (HPA) axis reactivity under acute activation, but it might not be a suitable proxy for natural variation of circulating glucocorticoid levels.

Baseline and stress-induced prolactin and corticosterone concentrations in a species with female offspring desertion - The case of Whiskered Tern Chlidonias hybrida

To understand the proximate mechanisms regulating brood desertion, we studied hormonal and behavioural stress responses during the chick-rearing period in adult Whiskered Terns (Chlidonias hybrida), a socially monogamous, semi-precocial species with prolonged post-fledging parental care. In contrast to males, almost all females of this species desert during the chick-rearing and post-fledging periods. Because of the expected link between corticosterone, prolactin and parental investment, we hypothesized that males and females should differ in circulating prolactin and corticosterone concentrations. Baseline hormone concentrations did not differ between males and females. In both sexes, prolactin and corticosterone concentrations decreased and increased in response to acute stress (30 min after capture), respectively. Baseline and stress-induced prolactin concentrations decreased significantly in both sexes with advancing brood age. As expected, males had significantly higher stress-induced prolactin concentrations than females. None of the nine males released after being held in captivity for 24 h deserted, whereas four (29%) of the 14 females kept in captivity for 24 h did so. Altogether, these results suggest that higher prolactin concentrations may be involved in the maintenance of parental care under stress. However, there was no statistically significant difference in stress-induced hormone levels between males, females that deserted and those that returned to the nest after prolonged stress (24 h). Our data indicate that males are probably more resistant to stress as regards the continuation of parental care. The pattern of male and female behavioural and hormonal responses to stress partially predicts their behaviour in terms of natural desertion.

The Little Auk Alle alle: an ecological indicator of a changing Arctic and a model organism

The Little Auk Alle alle is a small planktivorous auk breeding colonially in the High Arctic. Owing to its large population size and bi-environmental lifestyle, resulting in the large-scale transport of matter from sea to land, the Little Auk is one of the most important components of the marine and terrestrial ecosystems in the Arctic. As a result of globalization, which facilitates access to remote areas of the Earth, a growing number of studies is being dedicated to this endemic Arctic seabird. Research has focussed primarily on the importance of the Little Auk as an ecological indicator reacting to the climatic and oceanological changes that are particularly evident in the Arctic as a result of Arctic amplification (warming is more rapid in the Arctic than in any other region on Earth). Importantly, the species is also used as a model to investigate matter and energy flow through the ecosystem, mate choice, parental care and biological rhythms. Here, we review the natural history of the Little Auk, highlighting studies with the potential to provide answers to universal questions regarding the response of seabirds to climate variability and avian reproductive behaviour, e.g. threshold of foraging flexibility in response to environmental variability, carry-over effects between the breeding and non-breeding periods, the reasons for the transition from bi- to uni-parental care, parental coordination mechanisms.

Prolactin mediates behavioural rejection responses to avian brood parasitism

Adaptations resulting from co-evolutionary interactions between avian brood parasites and their hosts have been extensively studied, yet the physiological mechanisms underlying antiparasitic host defences remain little known. Prolactin, one of the main hormones involved in the regulation of avian parental behaviour, might play a key role in the orchestration of the host responses to avian brood parasitism. Given the positive association between prolactin and parental behaviour during incubation, decreasing prolactin levels are expected to facilitate egg-rejection decisions. We tested this prediction by implanting Eurasian blackbird (Turdus merula) females with an inhibitor of prolactin secretion, bromocriptine mesylate, to experimentally decrease their plasma prolactin levels. Bromocriptine mesylate-implanted individuals ejected mimetic model eggs at higher rates, and showed shorter latency to egg ejection, than placebo-treated birds. To our knowledge, this is the first experimental evidence that behavioural host defences against avian brood parasitism are mediated by prolactin.

Sex- and breeding stage-specific hormonal stress response of seabird parents

Changes in corticosterone (CORT) and prolactin (PRL) levels are thought to provide complementary information on parental decisions in birds in the context of stressful situations. However, these endocrine mechanisms have yet to be fully elucidated, appearing to vary among avian species without any clear pattern. Here, we examined CORT and PRL stress responses in a small Arctic seabird, the little auk (Alle alle). We analysed the levels of these hormones (baseline, and stress response, i.e. the change in the baseline in response to stress) with respect to the breeding phase (mid incubation and mid chick rearing) and the sex of the birds. Baseline CORT concentrations were similar during both breeding phases but baseline PRL levels were higher during incubation than chick rearing. The CORT and PRL stress responses were stronger during incubation than chick rearing (although with respect to CORT the effect was only marginally significant). There were also some sex-specific baseline levels and stress responses for both hormones (during the incubation period males compared to females exhibited higher CORT stress response and lower baseline PRL; during the chick rearing period males exhibited higher PRL stress response). Our results suggest that in the case of the little auk, both the incubation and the chick rearing periods may represent similar levels of physiological stress. However, the birds may be more sensitive to stress during incubation than during chick rearing, possibly because of inter-phase differences in predation pressure. The sex differences suggest differential exposure of males and females to stressors.

Does prolactin mediate parental and life-history decisions in response to environmental conditions in birds? A review

This article is part of a Special Issue "Parental Care". In vertebrates, adjustments of physiology and behavior to environmental changes are often mediated by central physiological mechanisms, and more specifically by hormonal mechanisms. As a consequence, these mechanisms are thought to orchestrate life-history decisions in wild vertebrates. For instance, investigating the hormonal regulation of parental behavior is relevant to evaluate how parents modulate their effort according to specific environmental conditions. Surprisingly and despite being classically known as the 'parental hormone', prolactin has been overlooked in birds relative to this context. Our aim is to review evidence that changes in prolactin levels can mediate, at least to some extent, the response of breeding birds to environmental conditions. To do so, we first examine current evidence and limits for the role of prolactin in mediating parental behavior in birds. Second, we emphasize the influence of environmental conditions and stressors on circulating prolactin levels. In addition, we review to what extent prolactin levels are a reliable predictor of breeding success in wild birds. By linking environmental conditions, prolactin regulation, parental behavior, and breeding success, we highlight the potential role of this hormone in mediating parental decisions in birds. Finally, we also review the potential role of prolactin in mediating other life history decisions such as clutch size, re-nesting, and the timing of molt. By evaluating the influence of stressors on circulating prolactin levels during these other life-history decisions, we also raise new hypotheses regarding the potential of the prolactin stress response to regulate the orchestration of the annual cycle when environmental changes occur. To sum up, we show in this review that prolactin regulation has a strong potential to allow ecological physiologists to better understand how individuals adjust their life-history decisions (clutch size, parental behavior, re-nesting, and onset of molt) according to the environmental conditions they encounter and we encourage further research on that topic.