Contrasting stress response of male arctic ground squirrels and red squirrels

A hormonal-challenge protocol was used to compare the stress response of ma les of Arctic ground squirrels and red squirrels during the breeding season (May). These squirrels live in the same boreal forest of the Yukon, but ha ve very different Life histories and utilize the forest in markedly differe nt ways. Red squirrels had levels of total cortisol, maximum corticosteroid -binding capacity, and free cortisol that were 5, 7, and 2 times, respectiv ely, those of Arctic ground squirrels. Red squirrels were resistant to supp ression by an artificial glucocorticoid, dexamethasone (DEX); Arctic ground squirrels were not. Cortisol levels in red squirrels responded slowly but continuously to the ACTH injection; Arctic ground squirrels responded rapid ly and then stabilized. Testosterone levels in red squirrels were extremely sensitive to the challenge, being suppressed by both DEX and ACTH; levels in Arctic ground squirrels were resistant to the challenge, being modestly suppressed by DEX and stimulated by ACTH. Energy mobilization, as measured by glucose and free fatty acid responses, was not affected. Red squirrels h ad four times the levels of white blood cells and higher proportions of lym phocytes and lower proportions of eosinophils than Arctic ground squirrels, indicating that the latter mere in worse condition immunologically. Our ev idence suggests that the functions associated with the hypothalamic-pituita ry-adrenal axis are compromised in breeding male Arctic ground squirrels, b ut not in red squirrels. We propose that in male red squirrels this axis ha s evolved in the context of a stable social system based on long-lived anim als with individual territories which are needed to deal with unpredictable winter food supplies. In contrast, Arctic ground squirrels escape the rigo rs of winter by hibernation and this hormonal axis has evolved in short-liv ed males in the context of intense intra-sexual competition in a social sys tem based on female kin groups and regular male dispersal to avoid inbreedi ng. (C) 2000 Wiley-Liss, Inc.