Heart Rate Supports Accelerometry as a Reliable Predictor of Energy Expenditure in Bats

Measuring the energetic cost of behavior is essential for understanding decision-making in wild animals. This is particularly true for bats where life near their energetic ceiling has resulted in a diversity of behaviors to compensate for social and environmental unpredictability. While GPS and tri-axial accelerometry has improved movement-based estimates of energy expenditure in the field, complex flight kinematics in bats may violate key assumptions accelerometry-based estimates rely on. To test whether energy expenditure is predicted by acceleration-based movement in bats, we deployed heart rate loggers alongside GPS with tri-axial accelerometers on free-ranging greater spear-nosed bats (Phyllostomus hastatus) in Panamá. We hypothesized that heart rate would positively reflect increases in Vectorial Dynamic Body Acceleration (VeDBA), a commonly used proxy for energy expenditure, and that including information on behavior would enhance this relationship. We also hypothesized that heart rate and VeDBA would reflect theoretical speed-power curves of flight. We found VeDBA had a strong positive relationship with heart rate, and model accuracy improved when behavioral states of flying and not flying were defined. Additionally, incorporating behavioral duration and sequence number improved predictions, showing that temporal context and activity patterns further refine estimates of energy expenditure. The relationship between VeDBA and airspeed resembled the mechanical power curve previously estimated for P. hastatus, supporting the relevance of aerodynamic theory in interpreting bat energetics. Overall, acceleration-based estimates reliably predict energy expenditure in free-ranging P. hastatus and incorporating behavior improves model accuracy and ecological relevance, providing a powerful tool for understanding energy allocation in wild bats.