Seasonal changes in energetics and torpor patterns in the subtropical blossom-bat Syconycteris australis (Megachiroptera)

Abstract

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 O g h) than in winter (0.75 ± 0.11 ml O g h). 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.

Activity patterns and roosting of the eastern blossom-bat (Syconycteris australis)

“…Daily energy expenditure of S. australis in winter is 76.9 kJ day -1 (Geiser and Coburn 1999) and of this~57 kJ is required for flight as predicted for a 18-g bat (Speakman and Racey 1991;Winter and von Helversen 1998;Geiser and Coburn 1999). A 18.5-min commute flight, as observed on average for bat #35, would require~2.6 kJ, which is less than the energy saved by an average 5.5-h torpor bout observed in S. australis in winter (Coburn and Geiser 1998;Geiser and Coburn 1999). So purely from an energetic point of view, the bats should be able to compensate for the loss of energy intake.…”

“…In its southern distribution range this bat is known to have one the highest field metabolic rates reported for endotherms (Geiser and Coburn 1999). On the other extreme, like other small subtropical bats (Geiser and Stawski 2011), S. australis employs torpor to minimise energy expenditure even during summer (Geiser et al 1996;Coburn and Geiser 1998), and it therefore appears that the species must carefully balance energy uptake and requirements by employing optimal or at least appropriate foraging strategies.…”

Effect of diet quality and ambient temperature on the use of torpor by two species of neotropical nectar-feeding bats

“…In tropical environments, where T a is usually high (i.e. higher than 20°C; McKnight and Hess, 2000), our study species use torpor mainly in response to variation in nectar quality and/or availability (Coburn and Geiser, 1998;Kelm and von Helversen, 2007). In subtropical and high elevation regions, the use of this energy-saving strategy may help neotropical bats to survive when low T a increases their energetic demands (Cruz-Neto and Abe, 1997; Bartels et al, 1998).…”

Daily Torpor In A Pregnant Common Blossom-Bat (Syconycteris Australis: Megachiroptera)

“…However, as in tenrecs (Insectivora: Stephenson 1993, Stephenson andRacey 1993), the proclivity towards torpor in the pregnant bats seemed to be reduced, as one individual did not enter torpor and the torpor bout observed in the second pregnant female was shorter than in the other, apparently non-reproductive, bats. Nevertheless, the minimum metabolic rate of the torpid, pregnant bat was reduced to about 46% of the basal metabolic rate of S. australis in winter (Coburn and Geiser 1998). The low minimum metabolic rate of the torpid, pregnant bat was similar to that of the non-reproductive bats, suggesting that its body temperature during torpor fell to about 22 °C (Coburn and Geiser 1998).…”

“…Nevertheless, the minimum metabolic rate of the torpid, pregnant bat was reduced to about 46% of the basal metabolic rate of S. australis in winter (Coburn and Geiser 1998). The low minimum metabolic rate of the torpid, pregnant bat was similar to that of the non-reproductive bats, suggesting that its body temperature during torpor fell to about 22 °C (Coburn and Geiser 1998).…”