Tropical Bats Take it Slow

Finding enough food to sustain the energy costs of foraging is a problem faced by all mammals. However, it is made more difficult for mammals that feed on unpredictable and limited prey. Such is the case in certain neotropical bats. Molossus molossus (Figure 1), for example, is an aerial insectivore that forages in groups over large areas. Not only must they expend large amounts of energy to fuel these foraging flights, but they only forage for approximately one hour each night. How can they maintain a positive energy balance with such high energetic costs and apparently low energy gains?

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Figure 1. A common free tailed bat (Molossus molossus). (From Burton Lim, ROM blog)

Previous research shows that flying bats expend roughly 15 times the energy they do at rest. Furthermore, small bats have higher resting metabolic rates and lose more heat because of their relatively high surface area to volume ratio. Thus, small aerial feeding bats in temperate zones often enter torpor each day as a way to conserve precious energy. How do tropical bats cope with energy loss?

German researchers (
Dechmann et al. 2011) may have found the answer. They attached tiny transmitters to molossid bats that were designed to measure heart rates. These transmitters sent back data for 48 hours before falling off the bats. The data was used to estimate energy budgets over a two-day period. The researchers hypothesized that the daily energy expenditure (DEE) would be high in these neotropical bats because they have very brief foraging periods and do not undergo daily torpor.

The researchers were surprised to find that these bats exhibited low mean heart rates. This corresponded to a DEE of only half what a similarly sized bat would experience. In addition, field metabolic rates (FMRs), predicted from doubly labeled water were 10 times higher than the DEE predicted from heart rate telemetry for a 10g bat. How were the molossid bats in this study able to conserve so much energy?

One possibility is that molossid bats lower their energy consumption during roosting in a manner similar to daily torpor (Figure 2). The heart rate data reveal that molossid bats do reduce their heart rates while roosting, but not so much that it would be called “torpor.” The authors suggest that “lowered metabolism” in tropical bats may be much more common that previously believed.

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Figure 2. Heart rate measurements from seven bats tracked for 48 hours. The two days were averaged and plotted over a 24 hour period. Note the drop in heart rates during the day. (From Dechmann et al. 2011)

References

Dechmann, D., Ehret, S., Gaub, A., Kranstauber, B., & Wikelski, M. (2011). Low metabolism in a tropical bat from lowland Panama measured using heart rate telemetry: an unexpected life in the slow lane Journal of Experimental Biology, 214 (21), 3605-3612 DOI: 10.1242/jeb.056010