RESUMO
Little is known about how animals from tropical and subtropical climates adjust their energy expenditure to cope with seasonal changes of climate and food availability. To provide such information, we studied the thermal physiology, torpor patterns and energetics of the nocturnal blossom-bat (Syconycteris australis 18 g) from a subtropical habitat in both summer and winter. In both seasons, S. australis frequently entered daily torpor at ambient temperatures between 12 and 25°C when food and water were withheld. Unlike patterns observed in temperate animals, mean minimum metabolic rates during torpor were lower in summer (0.47 ± 0.07 ml O2 g-1 h-1) than in winter (0.75 ± 0.11 ml O2 g-1 h-1). Body temperatures during torpor were regulated at 19.3 ± 1.0°C in summer and at 23.4 ± 2.0°C in winter. Torpor bout duration was significantly longer in summer (7.3 ± 0.6 h) than in winter (5.5 ± 0.3 h), but in both seasons, bout duration was not affected by ambient temperature. Consequently, average daily metabolic rates were also significantly lower in summer than in winter. Body temperatures and metabolic rates in normothermic bats did not change with season. Our findings on seasonal changes of torpor in this bat from the subtropics are opposite to those made for many species from cold climates which generally show deeper and longer torpor in winter and are often entirely homeothermic in summer. More pronounced torpor in subtropical S. australis in summer may be due to low or unpredictable nectar availability, short nights which limit the time available for foraging, and long days without access to food. Thus, the reversed seasonal response of this subtropical bat in comparison to temperate species may be an appropriate response to ecological constraints.
RESUMO
Hair is considered to be a basic mammalian feature that provides protection and insulation, promoting energy conservation and survival. To quantify the functional significance of mammalian pelage, we tested the short-term experimental effects of fur removal in a natural population of the California vole, Microtus californicus, in winter. The daily energy expenditure (DEE) of seven voles was directly measured in the field using stable isotopes, first with the animals in their natural condition and then again after experimental removal of pelage by shaving. The initial mean±SD DEE of 96.0±23.1 kJ/day increased by only about 10%, to 106.3±21.4 kJ/day, following shaving. The voles showed a body mass loss of 5%, about half of which was due to fur removal. Comparing slightly larger samples of all animals whose local survival could be documented, we found 100% survival over the 5 initial days of the experimental manipulation in 16 control animals and nine shaved animals; over the following 3 weeks the survival of shaved mice did not differ significantly from that of controls. We were surprised that the average increase in energy expenditure of voles without fur was so modest, though the range of individual values was great, and likewise we were surprised that shaved voles survived as well as they did. M. californicus survives naturally in winter under conditions of social aggregation that include huddling together of individuals in nests; this situation probably provided our experimentally shaved voles an opportunity to minimize the energetic disadvantages of pelage loss. They may also have employed a variety of compensatory physiological and behavioral responses, including reduction in activity time and food intake, and perhaps a related small decline in body mass. Our limited sample sizes made it difficult to detect subtle differences that may have been biologically significant in the system we studied. Nonetheless, we can reaffirm that fur has an insulative value that promotes energy economy and survival. However, we also conclude that mammalian physiology and behavior are sufficiently complex and flexible that a variety of responses can be deployed to promote survival under unusual circumstances such as those of our experimental test.