Endothermy, the maintenance of a high and relatively stable body temperature via internal heat production, comes at a high energetic price, so some mammals use torpor – a controlled decrease in metabolic rate and body temperature – to decrease metabolic costs during energetically challenging times. For a long time, torpor use was believed to be the sole preserve of a small number of Northern Hemisphere small mammals that resorted to torpor during the lean winter months or cold nights. However, more recently, species from the tropics and subtropics have been added to the list of animals that conserve energy by becoming torpid when food and other resources – such as water – become scarce. Fritz Geiser's research group at the University of New England, Australia, has recently expanded our understanding of how mammals employ torpor to survive in highly unpredictable environments, with two recent studies which investigated how some animals respond to natural disasters that suddenly restrict foraging opportunities.

In the first study, led by Claire Stawski, the team observed the effects of fire, an ever increasing threat to terrestrial fauna, on a small, nocturnal marsupial insectivore, the brown antechinus. They implanted body temperature data loggers into the animals in a section of Guy Fawkes River National Park that was scheduled for a controlled burn. As the fire was managed, it burnt the undergrowth, where the antechinus forage, while leaving the large trees, where most of the animals nested, untouched. This meant that while the majority of the animals survived the fire, their foraging was restricted immediately after the fire. Interestingly, rather than move to a nearby unburnt section of the forest following the fire, the animals remained in their original, burnt, habitat. During this period they significantly decreased their nocturnal activity, and increased their use of daytime torpor. The authors suggested that this increase in torpor may allow the animals to save energy in addition to affording them greater protection from an influx of invasive predators to the site after the fire.

The second study, led by Julia Nowack, used a similar approach and followed sugar gliders, small tree-dwelling marsupials. In comparison with the antechinus, the gliders used torpor less frequently, although they tended to use it more often on windier nights when their ability to glide between trees to forage was impeded. During the study, a cyclone with wind speeds in excess of 45 km h−1 hit the study site, bringing heavy rainfall (95 mm on the first night). On that occasion, the majority of the sugar gliders reduced their body temperature and entered torpor early in the evening when they would usually be foraging. However, most rewarmed and resumed normal activity the following evening. The authors estimated that the torpid animals reduced their energy use by up to 67%, which made up for the lack of foraging opportunities when grounded by the high winds.

Both of these studies demonstrate that species that use torpor opportunistically have an advantage when faced with unexpected natural disasters. In both cases, the use of torpor resulted in energy savings that compensated for the animals’ inability to forage. The authors suggest that the ability to reduce body temperature and reduce energy consumption would give these species a competitive advantage over strictly homeothermic species that maintain a costly stable body temperature. They also suspect that this is one possible reason why heterothermic species with lower cost lifestyles have fared better than metabolically costly homeotherms since the European colonization of Australia.

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