Modulation of the Adrenocortical Responses to Acute Stress in Arctic Birds: A Possible Ecological Basis

Abstract

There is considerable evidence that the increases in circulating corticosterone levels following acute stress, such as sudden storms, can trigger facultative behavioral patterns designed to maximize survival. During the breeding season, adrenocortical responses to aseasonal storms may trigger facultative behavioral patterns resulting in temporary disruption of nesting. A renesting cycle often follows when conditions become favorable again. However, in arctic ecosystems the brief breeding season limits the capacity of most avian populations to renest, and yet spring weather may be extreme. This led to the hypothesis that arctic birds may down-regulate their sensitivity to acute stress (such as severe storms) so that breeding can begin and be completed before the first storms of autumn (only 6–8 weeks later). To test this we have used the “stress-series protocol” that takes advantage of the fact that capture, handling and restraint constitutes a more-or-less equal stress among all vertebrate species, and that corticosterone concentrations in small blood samples collected during the first hour post-capture indicate sensitivity of the hypothalamo-adenohypophysial- adrenal axis to acute stresses in general. Comparisons of the increases in plasma levels of corticosterone following capture in several taxa of arctic birds indicated that suppression of the adrenocortical response to acute stress was not ubiquitous. Although some species did show low amplitude responses of the circulating corticosterone increase during the stress series protocol, others did not, and some (especially males) showed an increase in sensitivity to acute stress. Additional hypotheses were suggested as follows: 1) species with greater body mass have larger relative energy reserves and would be more able to resist acute stresses than smaller energy reserves and would be more able to resist acute stresses than smaller species; 2) short-lived birds with an expectancy of one or two breeding seasons should be more resistant to acute stress than long-lived birds that may have many attempts at successful breeding; 3) resistance of the adrenocortical response to stress is a function of the degree of parental care provided by the individual (also takes into account sex differences in parental investment). Correlations of the maximum corticosterone level and the ratio of maximum to minimum corticosterone levels generated during the stress series protocol with body mass and longevity were not significant. However, maximum corticosterone level was significantly lower in birds providing most parental care and almost significant for the ratio of maximum to minimum corticosterone levels. These comparative data from free-living arctic birds suggest an ecological basis for modulation of the adrenocortical responses to stress. In at least one species, up-regulation of the response appears to involve a change in sensitivity to glucocorticosteroid feedback. Further investigations will explore neuroendocrine mechanisms further in the light of these ecological bases.