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Plasma glucocorticoid concentrations and body mass in ground squirrels: seasonal variation and circannual organization.

Nunes S, Pelz KM, Muecke EM, Holekamp KE, Zucker I

Department of Biology, University of San Francisco, San Francisco, CA 94117, USA. nunes@usfca.edu

We examined variation in plasma glucocorticoid concentrations of free-living Belding's ground squirrels (Spermophilus beldingi) and captive golden-mantled ground squirrels (Spermophilus lateralis) housed in constant environmental conditions. Plasma corticosterone concentrations were not significantly correlated with time held captive in traps prior to collection of blood samples in males or non-breeding females, but these variables were significantly correlated in breeding females during the pre-mating, lactation, and post-lactation periods. Among male S. beldingi, plasma corticosterone concentrations increased over the course of the active season, and were significantly higher in non-breeding than breeding individuals. Corticosterone concentrations also increased in non-breeding females throughout the active period. In breeding females, baseline (non-stress) corticosterone concentrations, determined from blood samples collected within 3 min of capture, increased during gestation and declined during lactation, whereas stress values of corticosterone, determined from blood samples collected within 4-11 min of capture, increased during gestation and then again during the period after young emerged from the natal burrow. Changes in plasma corticosterone concentrations of S. beldingi paralleled changes in body mass. Among S. lateralis, plasma concentrations of cortisol were elevated when males and females were in reproductive condition and lower in reproductively quiescent squirrels. The annual peak in plasma cortisol concentrations occurred just prior to increases in body mass associated with pre-hibernation fattening. Collectively, these results suggest that concentrations of circulating glucocorticoids fluctuate with circannual rhythmicity in conjunction with annual cycles of change in body mass and activity; within these circannual cycles glucocorticoid concentrations are influenced by stress and breeding status.

Published 13 March 2006 in Gen Comp Endocrinol, 146(2): 136-43.
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