Seasonal changes in plasma glucocorticosteroids of free-living female yellow-pine chipmunks: effects of reproduction and capture and handling.

We measured plasma levels of cortisol and corticosterone in female yellow-pine chipmunks (Tamias amoenus) while observing seasonal reproductive and life-history events by live-trapping a natural population during the active (nonhibernating) season. Both glucocorticosteroids (GCs) varied significantly from March through September, starting with minimal values at the time of mating (cortisol approximately 900 ng/ml, corticosterone approximately 50 ng/ml), rising to a peak by late lactation (cortisol approximately 1600 ng/ml, corticosterone approximately 175 ng/ml), and then declining prior to hibernation. Following their emergence from natal burrows, young of the year had GC levels indistinguishable from those of adults. Body mass also varied significantly over the season, increasing after mating and again after parturition to a peak in lactation, after which it declined steadily until hibernation. In addition to the use of standard trapping to describe seasonal hormonal patterns, we also trapped chipmunks using a special protocol to examine the effects of capture and handling on GCs; we obtained an initial (basal) blood sample immediately, within 1-3 min of observing a capture, and then a second sample 30 min after holding the animal in the trap. Chipmunks consistently increased GCs above the initial (basal) level during the 30 min after capture and initial handling; these significant increases in GCs ranged approximately 70-130% for cortisol and 50-190% for corticosterone, depending on season and reproductive state. GC levels at 30 min after capture and handling were similar to those obtained from samples drawn from our standard trapping and blood sampling. We conclude that although capture and handling increase the absolute level of plasma GC hormones, that effect does not obscure natural patterns of seasonal variation in GCs. Overall, our observations suggest an important role of adrenocortical activity in the energy balance of these free-living rodents in two different contexts: (1) the seasonal regulation of physiological state, including body mass, energy reserves, and reproductive function, and (2) an acute response to stimulatory events, encompassing physiological stress, as represented here by capture and handling. Copyright 2000 Academic Press.