Heart rate as a predictor of metabolic rate in heterothermic bats

While heart rate ('f'H) 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 'f'H also predicts oxygen consumption (V₀₂) during torpor, we simultaneously measured V₀₂, 'f'H and subcutaneous body temperature ('T'sub) of a hibernator, Gould's long-eared bats ('Nyctophilus gouldi', 9 g, N=18), at ambient temperatures ('T'ₐ)mbetween 0 and 25°C. At rest, fH of normothermic resting bats was negatively correlated with 'T'a, with maximum 'f'H of 803 beats min-¹ ('T'a=5°C). During torpor, the relationship between 'f'H and 'T'a was curvilinear, and at low 'T'sub (~6°C), 'f'H fell to a minimum average of 8 beats min−¹. The minimum average values for both V₀₂ and 'f'H in torpor reported here were among the lowest recorded for bats. The relationship between 'f'H and V₀₂ was significant for both resting ('r'²=0.64, 'P'<0.001) and torpid bats ('r'²=0.84, P<0.001), with no overlap between the two states. These variables were also significantly correlated ('r'²=0.44, P<0.001) for entire torpor bouts. Moreover, estimates of V₀₂ from 'f'H did not differ significantly from measured values during the different physiological states. Our study is the first to investigate the accuracy of 'f'H as a predictor of V₀₂ during torpor and indicates the reliability of this method as a potential measure of energy expenditure in the field. Nevertheless, 'f'H should only be used to predict V₀₂ within the range of activities for which robust correlations have been established.