Annual cycle of body composition and hibernation in free-living arctic ground squirrels

We monitored a natural population of arctic ground squirrels (Spermophilus parryii kennicottii) on the North Slope of Alaska for seasonal changes in body mass and composition and dates of immergence into and emergence from hibernation. Yearlings and adult females were at the lowest body mass of their active season at emergence in spring. Their mean body mass did not increase for 1 month after emergence and peaked in July (adult females) and August (yearlings). Body mass of adult males was near the highest of the active season when they emerged from hibernation and decreased by 21% over the subsequent 10-day mating season. Juveniles gained body mass during their active season, except for significant losses associated with dispersal. During hibernation, females lost >30% of their body mass, but adult males emerged in spring without significant decreases in body mass, fat, or lean. Yearling and nonreproductive males were significantly lower in fat but not lean mass at emergence than immergence, and females were significantly lower in fat and lean mass. Arctic ground squirrels entered hibernation over a >1-month interval beginning in early August; females entered before males, and adults of each sex immerged before juveniles. Reproductive males emerged before females, and fatter females emerged significantly earlier than leaner females. Vaginal estrus was maximal at 3 days post-emergence. Nonreproductive males emerged last from hibernation. Mean +/- SE days in hibernation was 240.1 +/- 12.1 for adult females (69% of the year), 235.8 +/- 10.3 for juvenile females, 230.3 +/- 4.2 for nonreproductive males, 220.3 +/- 12.5 for adult males, and 214.7 +/- 6.5 for juvenile males. Timing of immergence into and emergence from hibernation for arctic ground squirrels did not differ significantly from sciurid populations in temperate latitudes.