MODULATION OF THE ADRENOCORTICAL RESPONSES TO ACUTE STRESS IN ARCTIC BIRDS - A POSSIBLE ECOLOGICAL BASIS

There is considerable evidence that the increases in circulating corti costerone levels following acute stress, such as sudden storms, can tr igger facultative behavioral patterns designed to maximize survival. D uring the breeding season, adrenocortical responses to aseasonal storm s may trigger facultative behavioral patterns resulting in temporary d isruption of nesting. A renesting cycle often follows when conditions become favorable again. However, in arctic ecosystems the brief breedi ng season limits the capacity of most avian populations to renest, and yet spring weather may be extreme. This led to the hypothesis that ar ctic birds may down-regulate their sensitivity to acute stress (such a s severe storms) so that breeding can begin and be completed before th e 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 t hat capture, handling and restraint constitutes a more-or-less equal s tress among all vertebrate species, and that corticosterone concentrat ions in small blood samples collected during the first hour post-captu re indicate sensitivity of the hypothalamo-adenohypophysial-adrenal ax is to acute stresses in general. Comparisons of the increases in plasm a levels of corticosterone following capture in several taxa of arctic birds indicated that suppression of the adrenocortical response to ac ute stress was not ubiquitous. Although some species did show low ampl itude responses of the circulating corticosterone increase during the stress series protocol, others did not, and some (especially males) sh owed an increase in sensitivity to acute stress. Additional hypotheses were suggested as follows: 1) species with greater body mass have lar ger relative energy reserves and would be more able to resist acute st resses than smaller species; 2) short-lived birds with an expectancy o f one or two breeding seasons should be more resistant to acute stress than long-lived birds that may have many attempts at successful breed ing; 3) resistance of the adrenocortical response to stress is a funct ion of the degree of parental care provided by the individual (also ta kes into account sex differences in parental investment). Correlations of the maximum corticosterone level and the ratio of maximum to minim um corticosterone levels generated during the stress series protocol w ith body mass and longevity were not significant. However, maximum cor ticosterone level was significantly lower in birds providing most pare ntal care and almost significant for the ratio of maximum to minimum c orticosterone levels. These comparative data from free-living arctic b irds suggest an ecological basis for modulation of the adrenocortical responses to stress. In at least one species, up-regulation of the res ponse appears to involve a change in sensitivity to glucocorticosteroi d feedback. Further investigations will explore neuroendocrine mechani sms further in the light of these ecological bases.