Abstract
In this paper we studied the energetics and thermoregulation of the Chilean mouse-opossum Thylamys elegans (Dielphidae) a nocturnal small marsupial, endemic of southern South America. We studied their standard energetic and determined whether they exhibit shallow daily torpor or deep prolonged torpor as a function of ambient temperature and food availability. Thylamys elegans partially supports the hypothesis that Neotropical marsupials have somewhat a higher basal metabolic rate (BMR) and thermal conductance (Cm) than Australian ones. In fact, BMR was higher but Cm was lower than expected for their body mass. The higher mass-independent BMR of the Chilean mouse-opossum may be explained by its insectivorous food habits and its low Cm by its temperate habitats. Euthermic Chilean mouse-opossum showed daily fluctuations in body temperature being significantly higher during night time. In addition T. elegans entered in daily torpor and aroused spontaneously only was food was absent. That is, this species display a facultative type of daily torpor because propensity to enter in torpor was dependent of the combination of food absence and low ambient temperature. No torpor was observed when food was available. During torpor ambient temperature was slightly above ambient temperature between 0.3 to 0.5 °C. Torpor in this species as well as in marsupials in general, appears to be a flexible and an opportunistic response to unpredictable environmental conditions.
Highlights
Torpor in small endotherms is a well-studied example of energy budget regulation
Bozinovic et al (2004) demonstrated that the South American living fossil Dromiciops gliroides does not support the energetic standards for Neotropical marsupials
These authors hypothesized that this is because Dromiciops is a well-differentiated lineage from South American opossums within the Australidelphian radiation of metatherians (Palma 2003)
Summary
Torpor in small endotherms is a well-studied example of energy budget regulation. Torpor is characterized by a decrease in body temperature and a decline in metabolism of c.a. 90 %, in comparison to euthermy (Geiser 1994, Ortmann & Heldmaier 1997). This relationship between torpor, ambient temperature and food availability has been well documented for many years. Hainsworth et al (1977) suggested that daily torpor is used by hummingbirds exclusively when food is scarce. Extensive use of torpor by a population should be advantageous when food is scarce. The mean duration of individual episodes of torpor increases for kangaroo mice when food supply is reduced (Brown & Bartholomew 1969). Ambient temperature largely explains the reduction of metabolism of daily heterotherms and hibernators (Geiser 1986, 1988, 1994, López-Calleja et al 1997)
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