Hormonal effects of maltreatment in Nazca booby nestlings: implications for the "cycle of violence"

Chronic stress increases adaptive immune response over six weeks in the house sparrow, Passer domesticus

The vertebrate stress response enables an organism to shift energy towards activities that promote immediate survival when facing a threat to homeostasis, but it can also have detrimental effects on organismal health. Acute and chronic stressors generally have contrasting effects on immune responses, but the timeline of this transition between acute and chronic stressors and their effects on immune responses remains unclear. In this study, we investigate changes in immune markers in captive house sparrows (Passer domesticus) after exposure to normal laboratory conditions, an acute stressor, and chronic stressors for 42 days. Specifically, we examined changes in baseline and stress-induced corticosterone concentrations, body condition, heterophil/lymphocyte (H:L) ratio, hemolysis-hemagglutination, and wound healing. We found that individuals exposed to a single acute stressor had significantly higher stress-induced corticosterone concentrations 24 h after stressor exposure, however this effect was reversed after 48 h. Chronic stressor exposure resulted in generally stronger adaptive immune responses, demonstrated by higher baseline and stress-induced lysis, higher baseline hemagglutination, and slower wound healing. Within-trait correlations also increased with chronic stressor exposure, suggesting limitations on phenotypic plasticity. Most of the effects of chronic stressor exposure on immune markers strengthened over the 42 days of the experiment and differences between captivity-only and treatment groups were not apparent until approximately 20 days of chronic stressor exposure. These results highlight the importance of stressor duration in understanding the effects of chronic stressor exposure on immune responses.

Early-life maltreatment predicts adult stress response in a long-lived wild bird

Persistent phenotypic changes due to early-life stressors are widely acknowledged, but their relevance for wild, free-living animals is poorly understood. We evaluated effects of two natural stressors experienced when young (maltreatment by adults and nutritional stress) on stress physiology in wild Nazca boobies (Sula granti) 6-8 years later, an exceptionally long interval for such studies. Maltreatment as a nestling, but not nutritional stress, was associated years later with depressed baseline corticosterone in females and elevated stress-induced corticosterone concentration [CORT] in males. These results provide rare evidence of long-term hormonal effects of natural early-life stress, which may be adaptive mechanisms for dealing with future stressors.

Yawning, acute stressors, and arousal reduction in Nazca booby adults and nestlings

Yawning is a familiar and phylogenetically widespread phenomenon, but no consensus exists regarding its functional significance. We tested the hypothesis that yawning communicates to others a transition from a state of physiological and/or psychological arousal (for example, due to action of a stressor) to a more relaxed state. This arousal reduction hypothesis predicts little yawning during arousal and more yawning (above baseline) during and after down-regulation of arousal. Experimental capture-restraint tests with wild adult Nazca boobies (Sula granti), a seabird, increased yawning frequency after release from restraint, but yawning was almost absent during tests. Natural maltreatment by non-parental adults also increased yawning by nestlings, but only after the maltreatment ended and the adult left. CORT (corticosterone) was a logical a priori element of the stress response affecting the stressor-yawning relationship under the arousal reduction hypothesis, and cannot be excluded as such for adults in capture-restraint tests but is apparently unimportant for nestlings being maltreated by adults. The arousal reduction hypothesis unites formerly disparate results on yawning: its socially contagious nature in some taxa, its clear pharmacological connection to the stress response, and its temporal linkage to transitions in arousal between consciousness and sleep.

Corticosterone stress response shows long-term repeatability and links to personality in free-living Nazca boobies

The concept of "coping styles", or consistently different responses to stressors, is of broad interest in behavioral ecology and biomedicine. Two critical predictions of this concept are individual consistency of neurophysiological and behavioral responses (relative to population variability) and a negative relationship between aggression/proactivity and hypothalamic-pituitary-adrenal axis reactivity. Recent studies failed to provide strong support for these predictions, especially outside of strictly controlled conditions, and long-term measures to test the first prediction are rare. Here, we demonstrate individual repeatability across 2-3years of maximum circulating corticosterone concentration [CORT] and area under the [CORT] response curve (AUCI) during a standard capture-restraint test in wild, free-living adult Nazca boobies (Sula granti). We also show that the stress response predicts the personality traits aggression and anxiety in these birds (measured in the wild); however, the strength of these results was weak. Maximum [CORT] and AUCI showed higher repeatability between years than baseline [CORT]. After controlling breeding status, sex, mass, date sampled, and their interactions, baseline [CORT] was most closely related to personality traits, followed by AUCI, and then maximum [CORT]. The direction of these relationships depended on whether the testing context was social or non-social. [CORT] parameters had little to no relationship with cross-context plasticity in personality traits. Our results generally affirm two critical predictions of coping styles, but match the emerging trend that these relationships are weak in the wild, and may depend on testing context.