Sex and age differences in mass, morphology, and annual cycle in European ground squirrels, Spermophilus citellus

We collected 6 years of data (1990-1995) on survival, hibernation, body-mas s changes, and morphological parameters for a suburban population of Europe an ground squirrels (Spermophilus citellus) using censusing and capture-rec apture techniques. We attempted to determine differences in sex and age in these parameters. In the focal population, sex ratios of adults and yearlin gs were female-biased, but the sex ratio of juveniles was balanced. Overwin ter 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 dis appeared than females. Hibernation patterns were similar to other Spermophi lus species, except that females began hibernation before males. Therefore, females had significantly longer hibernation periods and higher proportion al overwinter losses of mass than males. All adult (greater than or equal t o 2 years) but only a few yearling males were reproductively active. Yearli ng males emerged from hibernation with a lower mass than adults but were in distinguishable from them before subsequent immergence. Adult and yearling females reproduced and lost mass during late lactation, a pattern more pron ounced in older than yearling females. Juvenile males were heavier than juv enile females before hibernation, although no difference was found at weani ng. 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 tha t probably have evolved due to specific environmental constraints in this s pecies.