Energetics of reproduction and overwintering in some insectivorous mammals (Mammalia; insectivora)
Sorex araneus L., the common shrew, has a basal metabolic rate higher than predicted by allometry. This study investigates how such a small insectivore is able to respond to the extra energy expenditures associated with both reproduction and winter survival. A comparison is also made with the reproductive costs in a tropical insectivore with a low BMR, Echinops telfairi Martin. The daily energy expenditure of S. araneus, estimated using both indirect calorimetry (IC) and doubly-labelled water (DLW), increased during pregnancy and lactation. The energy expenditures estimated using IC were however lower than the estimates made using DLW. During gestation this was explained by the physical confinement and the higher ambient temperatures within the respirometry chambers. During lactation however 30% of the maternal energy expenditure could not be explained by such factors and was probably due to the entry of unlabelled CO2 into the female. The increase in energy expenditure during reproduction was associated with an increase in food consumption. Both the resting and the daily energy expenditure of S. araneus increased in response to falls in ambient temperature, reflecting the cost of thermoregulation during winter. Homeothermy was always maintained at low temperatures. Prey availability did not decline during the winter months however, so that the increase in thermo-regulatory costs were not compounded by a decrease in the potential intake of energy. In winter, thermoregulatory costs were reduced by a drop in body mass and size, a consequence of which was a decrease in lower critical temperature. Further adaptations included a reduction in activity in cold temperatures, involving long periods spent within the nest. Habitat selection was also important during winter, since shrews remaining within sheltered microclimates would conserve up to 15% of the total daily energy budget. The increase in body mass in Echinops during gestation was attributed to both the growth of the foetuses and the deposition of fat. The lipid stores were carried over into lactation. Both daily and resting energy expenditure increased throughout pregnancy, but there was no further increase in expenditure throughout lactation. Whilst the body temperature (Tb) of adult males and juveniles was dependent upon ambient temperature (Ta), body temperature in pregnant and lactating females was independent of Ta. The females maintained homeothermy during reproduction. Despite the increase in energy expenditure there was no increase in the temperature differential (Tb-Ta) prior to parturition in these females, and this was attributed to an increase in conductance, a consequence of behavioural thermoregulation ('spread-eagling') during late gestation. These behavioural changes included a reduction in activity and hence compensation in the respiratory energy budget, leading to a decrease in food consumption from mid to late gestation. During early to mid gestation and also throughout lactation the increased energy demands were associated with an increase in food consumption.