Heart rate reveals torpor at high body temperatures in lowland tropical free-tailed bats.

M Teague O'Mara,Dina K N Dechmann,Sebastian Rikker,Andries Ter Maat,Martin Wikelski,Henry S Pollock

doi:10.1098/rsos.171359

Abstract

Reduction in metabolic rate and body temperature is a common strategy for small endotherms to save energy. The daily reduction in metabolic rate and heterothermy, or torpor, is particularly pronounced in regions with a large variation in daily ambient temperature. This applies most strongly in temperate bat species (order Chiroptera), but it is less clear how tropical bats save energy if ambient temperatures remain high. However, many subtropical and tropical species use some daily heterothermy on cool days. We recorded the heart rate and the body temperature of free-ranging Pallas' mastiff bats (Molossus molossus) in Gamboa, Panamá, and showed that these individuals have low field metabolic rates across a wide range of body temperatures that conform to high ambient temperature. Importantly, low metabolic rates in controlled respirometry trials were best predicted by heart rate, and not body temperature. Molossus molossus enter torpor-like states characterized by low metabolic rate and heart rates at body temperatures of 32°C, and thermoconform across a range of temperatures. Flexible metabolic strategies may be far more common in tropical endotherms than currently known.

Highlights

Maintaining body temperature (Tb) is energetically costly, when ambient temperatures (Ta) are cold and food resources scarce [1]
After physiological arousal generated by observers tapping on chamber walls, fit of heart rate (f H) dropped rapidly into a low metabolic state, and Tb eventually followed at a slower rate
When we further examined the stable MR at our two Ta, we found that metabolic rates were lower at 28°C (0.131 ± 0.014 s.e. kJ h−1) than at 32°C (0.160 ± 0.001 s.e. kJ h−1; χ12 = 445.77, p < 0.001; figure 2)

Summary

Introduction

Maintaining body temperature (Tb) is energetically costly, when ambient temperatures (Ta) are cold and food resources scarce [1]. Often described as daily heterothermy or torpor [1,12,13], this phenomenon is widespread among mammals and birds, and is typically perceived as a response to mitigate low food availability and low Ta [13]. It is pronounced in the large and diverse order of bats (Chiroptera), which need effective energy-saving strategies due to their small size, high metabolic requirements, and loss of large amounts of heat and water through large, naked wing membranes [14]. In much of the lowland tropics, Ta remains high throughout the year despite large variation in rainfall and subsequent food availability, and this low variation in Ta could make measuring low energy states via Tb a challenge

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