Sex and Age Differences in Mass, Morphology, and Annual Cycle in European Ground Squirrels, Spermophilus citellus

Abstract

We collected 6 years of data (1990-1995) on survival, hibernation, body-mass changes, and morphological parameters for a suburban population of European ground squirrels (Spermophilus citellus) using censusing and capture-recapture techniques. We attempted to determine differences in sex and age in these parameters. In the focal population, sex ratios of adults and yearlings were female-biased, but the sex ratio of juveniles was balanced. Overwinter mortality was low, but losses during the active season were high in all sex and age groups. Male and female juveniles disappeared from their natal area in roughly equal proportions. Among older individuals, more males disappeared than females. Hibernation patterns were similar to other Spermophilus species, except that females began hibernation before males. Therefore, females had significantly longer hibernation periods and higher proportional overwinter losses of mass than males. All adult (greater than or equal to 2 years) but only a few yearling males were reproductively active. Yearling males emerged from hibernation with a lower mass than adults but were indistinguishable from them before subsequent immergence. Adult and yearling females reproduced and lost mass during late lactation, a pattern more pronounced in older than yearling females. Juvenile males were heavier than juvenile females before hibernation, although no difference was found at weaning. Skull size increased in the first two active seasons but did not change thereafter. Differences in sex and age resembled those of most hibernating,ground squirrels in many aspects but also showed some unusual patterns that probably have evolved due to specific environmental constraints in this species.