As with many other hibernating ground-dwelling squirrels, Richardson's ground squirrels follow a general pattern of gaining weight during the active season and losing it during hibernation. However, this is not a simple two-part cycle, and a number of factors determine the details of the weight trajectory throughout the year.
Variation in over-winter weight loss corresponds with variation in duration of hibernation, which varies with age and sex. (see Hibernation) In general, adult ground squirrels lose more mass over winter than juveniles, and juvenile females lose more mass than juvenile males.
Body weight patterns also vary during
the active time of year, corresponding to the differential pressures of
mating, rearing young, and growth for different age and sex categories.
data
Adult Males
When adult male Richardson's ground squirrels emerge from hibernation in southern Alberta, they weigh around 350-400g. (see Hibernation) Adult males maintain or slightly increase their mass in the interval before females emerge from hibernation. Males then lose mass during the peak mating season when they compete for access to estrous females. Males drop to their minimum annual body mass several days after the peak mating period, typically 3-5 weeks after having resumed above-ground activity in spring. data
Males begin regaining mass slowly
after the mating season, then rapidly increase in mass several weeks before
immergence into hibernation. Adult male Richardson's ground squirrels
begin fattening earlier than do adult females (see Annual
Activity Cycle) and enter hibernation in late June weighing around
500-600g.
Adult Females
Adult female Richardson's ground squirrels are at their minimum body weight when they emerge from hibernation. (see Hibernation) At emergence, females weigh around 220-270g, then they begin increasing in mass immediately. Except for a brief interruption in weight gain on the day of mating, weight gain continues throughout gestation, slows during lactation due to the energy demands of producing milk for the litter, and increases again after the pups are weaned. data
Yearling females weigh less than
older females on emergence from hibernation, but they rapidly increase
in mass during pregnancy. They finish growing to adult size during
pregnancy, so by early lactation yearlings do not differ significantly
in weight from older females. All females start fattening several
weeks after the litter is weaned, and at immergence in July all adult females
weigh around 400-450g.
Juveniles
Newborn pups weigh about 6.5g, and the entire litter (usually of 6-8 infants) weighs about 50 g at birth. During lactation litter mass increases about 10 fold to become greater than the mass of mother herself. data During this time, maternal mass remains almost constant. Litters of yearling and older females are of similar size and weight. (see Litter Size)
At the age of four weeks, Richardson's
ground squirrel pups emerge from their natal burrows weighing 65-85g each.
Juvenile males remain active as late as October in order to grow to their
final adult size before hibernation. In contrast, juvenile females
only grow to 80% their adult size before hibernation; they finish their
growth during pregnancy the next spring. data