Over-Winter Weight Loss by Richardson's Ground Squirrels in Relation to Sexual Differences in Mating Effort

Abstract

Duration of hibernation and over-winter changes in mass and body composition were studied in Richardson's ground squirrels ( Spermophilus richardsonii ) hibernating under natural conditions in southern Alberta, Canada. Adult males stored more fat, hibernated for slightly fewer days, and lost more mass over winter than adult females. Juvenile males grew larger, fattened later, immerged later, spent 2–3 months less time in hibernation, and lost less mass over winter than juvenile females. Despite differences in prehibernatory body mass and composition, immergence dates, duration of hibernation, and magnitude of over-winter weight loss, yearling and older males emerged from hibernation at the same time and had similar posthibernatory body mass and body composition. Males emerged before females and with twice as much residual fat as females. Sexual differences in proportion of adult size achieved before hibernation by juveniles and in size of posthibernatory fat stores are explained in terms of sexual differences in the energetic costs of acquiring mates in spring.