BOONSTRA, R.; University of Toronto at Scarborough: Stress responses of mammals and birds to severe environments.

Making babies and raising them successfully is difficult even in the best environments. When birds and mammals are confronted by this imperative in the arctic, the difficulties are magnified enormously. The summer days are long, but the growing season is short and climatic unpredictability and severity is potentially great. When faced with these challenges, these animals may suspend physiological adaptations which are the norm in more moderate environments or at other times in their life cycle in order to permit reproduction. The hypothalamic-pituitary-adrenal axis is key to coordinating the stress response to environmental challenges. Usually activation of this axis causes an inhibition other hormonal axes, such as the reproductive axis. However this inhibition may not occur in arctic animals Detailed physiological studies of migratory birds in the arctic indicate that they modulate the sensitivity of the stress response. Their stress axis fails to activate when challenged during the breeding season. Male arctic ground squirrels have evolved a different strategy. During the period of intense intrasexual conflict (2-3 weeks), their stress axis is strongly activated, resulting in inhibition of immune function, but not of reproductive function. The benefits are obvious, but many trade off survival for reproduction. Bird species may adapt more readily to warming of the arctic then mammals, as there appears to be pronounce population variation in the stress response and thus there is within species plasticity to permit change . In mammals, we do not know the degree of plasticity in these hormonal axes and thus their capacity for change. In addition, changes in other environmental conditions (snow cover in winter, food quality in summer, etc.) may have unknown consequences.