Development of thermoregulation in Spermophilus richardsonii was investigated by determining the ability of neonates to maintain a normal body temperature when exposed to 30 and 25 °C, to maintain an elevated oxygen consumption (cubic centimetre oxygen per gram per hour) at 21 °C compared with that at 35 °C, and to move toward a warm object and assume curling postures when exposed to the cold. Newborn animals were essentially poikilothermic but by day 5 showed strong thermotaxis. By 30 days, the age of emergence from natal burrows, homeothermy had developed to the point at which normal body temperature could be maintained for at least 2 h at an ambient temperature of 25 °C. This process was correlated with improvements in heat production and heat retention. Subsequent growth was marked by a decreased metabolic response to cold (21 °C) owing to decreasing thermal conductance.
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).
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