Development of thermoregulation in Richardson's ground squirrel, Spermophilus richardsonii

Abstract

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.

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

“…temperatures are held near adult levels (Dawkins and Hull 1964;Briick and Wiinnenberg 1965 b). In contrast, newborn S. richardsonii cannot maintain high body temperature at or below 30°C ambient temperature (Dolman 1980), and on evidence presented here we conclude that BAT has no substantial thermogenic role in the cold-stressed newborn of this species. Hull and Segall (1965) have described a "threshold effect, wherein newborn rabbits having BAT fat content below "threshold" (24-38% of wet mass) cannot increase heat production in response to cold or to noradrenaline, whereas those with fat content above "threshold respond maximally.…”

“…Hull and Segall (1965) have described a "threshold effect, wherein newborn rabbits having BAT fat content below "threshold" (24-38% of wet mass) cannot increase heat production in response to cold or to noradrenaline, whereas those with fat content above "threshold respond maximally. Between 1 and 2 weeks of age, Richardson's ground squirrels show a limited ability to maintain body temperature above ambient (Dolman 1980), and by 1 week BAT intracellular lipid droplets have enlarged so that tissue lipid has increased to 25%. Thus, 25% might represent a "threshold" level for this species, at which point BAT could begin to contribute to the maintenance of high body temperature, with further limits to thermogenesis then being set by total BAT mass.…”

“…By 2 weeks of age, total BAT mass is three to six times that at birth, but still is only 0.7% of body mass. Moreover, other thermoregulatory mechanisms are developing by this age, such as fur insulation, shivering, and behavioural postures (Dolman 1980), so that the relative contribution of BAT to thermoregulation throughout the early postnatal period is likely very small. In both laboratory-reared and wild squirrels the highest BAT lipid levels were found in the oldest age classes.…”

“…5-8) suggests that both types of tissue are under a common control mechanism. The brief preweaning pause in fattening correlates with a lag in general body growth rate (Dolman 1980). Limitations to maternal milk production might impose this restriction.…”

“…They and their litters were held in captivity as described by Dolman (1980). Wild juveniles In many hibernating rodents, body lipid plays a were live-trapped between the time of their emergence from critical role in overwintering survival; white adipose natal burrows (April or M~~) until their immergence into tissue (WAT) is a major source of stored energy (Davis burrows for winter hibernation (August or September).…”