Brown and white adipose tissue in relation to age and sex in juvenile Richardson's ground squirrels

Deposits of brown and white adipose tissue were monitored from birth to hibernation in laboratory-born and field-caught juvenile Richardson's ground squirrels (Spermophilus richardsonii). The weight-specific mass of brown adipose tissue was low at birth and, except for a brief increase at 3 days, declined postnatally. Total mass of brown adipose tissue and its lipid content were also low at birth, but increased postnatally, up to hibernation. Brown adipose tissue probably plays a greater role in hibernation thermogenesis than in neonatal thermoregulation. Both total and weight-specific mass of white adipose tissue increased postnatally, with maximum values prior to hibernation. Laboratory-born squirrels were fatter than wild squirrels and acquired more fat for a given increase in body mass. Females, both laboratory-born and wild, were fatter than males, and likewise acquired more fat for a given increase in body mass. Although at entry into hibernation wild juvenile females weighed 20% less than juvenile males, we calculated that females had twice as much total body lipid (91.1 versus 45.8 g).