Heart rate as a predictor of metabolic rate in heterothermic bats

Abstract

Summary While heart rate (fH) has been used as an indicator of energy expenditure, quantitative data showing the relationship between these variables are only available for normothermic animals. To determine whether fH also predicts oxygen consumption (VO2) during torpor we simultaneously measured VO2, fH and subcutaneous body temperature (Tsub) of a hibernator, Gould's long-eared bats (Nyctophilus gouldi, 9 g, n=18), at ambient temperatures (Ta) between 0 and 25°C. At rest, fH of normothermic resting bats was negatively correlated with Ta, with maximum fH of 803 bpm (Ta=5°C). During torpor the relationship between fH and Ta was curvilinear, and at low Tsub (~6°C) fH fell to a minimum average of 8 bpm. The minimum average values for both VO2 and fH in torpor reported here were among the lowest recorded for bats. The relationship between fH and VO2 was significant for both resting (r2=0.64, p<0.001) and torpid bats (r2=0.84, p<0.001), with no overlap between the two states. These variables were also significantly correlated (r2=0.44, p<0.001) for entire torpor bouts. Moreover, estimates of VO2 from fH did not differ significantly from measured values during the different physiological states. Our study is the first to investigate the accuracy of fH as a predictor of VO2 during torpor and indicates the reliability of this method as a potential measure of energy expenditure in the field. Nevertheless, fH should only be used to predict VO2 within the range of activities for which robust correlations have been established.