The Evolution of the Stress Axis in Ground Squirrels

The hypothalamic-pituitary-adrenal (HPA) axis, or stress axis, is a key physiological system that mediates the relationship of the organism with its environment. Because activation of the HPA axis mobilizes energy stores for immediate use, but sustained activation can have deleterious effects on survival, the HPA axis has been implicated in the tradeoff between reproduction and survival. In this thesis, I investigate whether there is an association between one life history trait, reproductive lifespan, and the functioning of the HPA axis as predicted by the “adaptive stress hypothesis”. The adaptive stress hypothesis predicts that species adopting life history strategies characterized by short lifespans and early reproduction should maximize the energy available for reproduction through high levels of circulating glucocorticoids caused by the dysregulation of the HPA axis in the breeding season, whereas those characterized by long lifespans and extended reproduction should maintain a functioning HPA axis with low levels of glucocorticoids throughout life. To test this hypothesis, I studied five species of ground squirrels that vary dramatically in male reproductive lifespan: arctic, Richardson’s, Columbian, thirteen-lined, and Franklin’s ground squirrels (Urocitellus parryii, U. richardsonii, U. columbianus, Ictidomys tridecemlineatus, and Poliocitellus franklinii). I used a stress profile to characterize the HPA axis of male ground squirrels immediately before and immediately after the breeding season. The stress profile included measures of plasma glucocorticoid concentrations, determinants of plasma glucocorticoid concentrations (corticosteroid binding globulin levels, adrenal sensitivity/capacity, negative feedback, and intrinsic restraint), and markers of the biological effects of glucocorticoids (energy mobilization, health, and immune function). Contrary to the adaptive stress hypothesis, I found no relationship between reproductive lifespan and postbreeding glucocorticoid levels. Species also varied significantly and unexpectedly in how determinants of glucocorticoid levels changed over the breeding season, and in how glucocorticoids levels translated into biological effects. I also observed unexpected patterns of individual variation within species. Thus, life history alone did not predict HPA axis functioning. My results suggest that the HPA axis is so flexible in its functioning, that we will need to adopt a much more detailed model of the HPA axis in order to fully understand the relationship between the HPA axis and life history variation.Ph