Androgen in free-living arctic ground squirrels: seasonal changes and influence of staged male-male aggressive encounters

We tested the responsiveness of plasma androgen in free-living male arctic ground squirrels (Spermophilus parryii) to challenges from conspecific males by staging aggressive encounters between males and then comparing androgen concentrations in manipulated and unmanipulated animals. The effect of staged encounters on androgen depended on time of year. In spring, androgen levels significantly increased after staged encounters, except during the peak of mating activity when levels were maximal in all reproductive males. In late summer, staged encounters had no significant effect on androgen levels. In spring but not summer, androgen was positively correlated to the relative intensity of encounters. These results suggest that circulating levels of androgen in male arctic ground squirrels result from a combination of seasonal and social factors. Responsiveness of androgen to encounters during spring but not late summer support the challenge hypothesis for this polygynous mammal. (C) 2003 Elsevier Science (USA). All rights reserved.