Energetic Significance of Torpor and Other Energy-Conserving Mechanisms in Free-Living Sminthopsis-Crassicaudata (Marsupialia, Dasyuridae)

Abstract

Factors influencing the use of energy-conserving mechanisms including torpor, as well as their energetic significance, were studied in free-living Sminthopsis crassicaudata at Werribee (Victoria) during winter 1981 and 1982. Possible correlations between behavioural or physiological condition of captured animals and climatic variables or food availability were investigated. Daily energy expenditure was calculated by combining time-budget analysis in the field (based on radio-tracking) with respirometric measurements of metabolic rates in captivity. The energy-conserving mechanisms used were torpor, reduced activity, basking, huddling in groups, use of nests, choice of a thermally favourable resting site and slightly lowered resting body temperature. Torpor and reduced activity only occurred after cold (< 6-degrees-C) and dry nights, when surface activity of prey was very low, leaving S. crassicaudata in a negative energy balance. Rainy nights increased the availability of prey (particularly slugs and earthworms), and the animals did not enter torpor. During the non-breeding season (April-June), the energy savings [compared with a reference budget (E(r)) where no energy-conserving mechanism is used] reached 20-25% of E(r) after rainy or mild and dry nights, the major contributors being huddling and use of a nest. After cold dry nights, the savings may reach 40-50% of E(r), primarily due to torpor and reduced activity. Various energy-conserving mechanisms were used, even in the absence of short-term energetic problems, resulting in spontaneous energy savings and a reduced depletion of food. More prey was then available at the onset of breeding (mid-July), when energy requirements increase noticeably, because of smaller group size, smaller nests and energy channelled into offspring. The adpative value of spontaneous energy savings is discussed and the overwintering energetics of S. crassicaudata are compared with those of some European shrews.