Energy Expenditure Optimization in the Echolocation of Rhinolophus nippon: Evidence from Heart Rate Stability

Acoustic behavior, essential for communication and perception, is metabolically demanding. Studying the energy costs of echolocation helps us to understand animal energy allocation and provides key insights into the evolutionary constraints of acoustic signals. We examined the constant-frequency bat Rhinolophus nippon using a miniature electrocardiogram system and a custom servomotor that moved prey toward stationary bats. This setup allowed for synchronous recording of high-resolution electrocardiogram and echolocation calls from the search phase to the approach phase. During the search phase, bats emitted isolated echolocation pulses characterized by long pulse durations and inter-pulse intervals (IPIs), together with higher root mean square (RMS) amplitude, pulse energy, and peak amplitude. In the approach phase, call rate increased significantly (3.15-fold), and bats predominantly produced sonar sound groups. Meanwhile, pulse duration, IPIs, RMS amplitude, and pulse energy decreased to 65.23%, 25.82%, 78.50%, and 86.32% of the corresponding search-phase values, whereas peak amplitude increased to 110.99%, indicating that R. nippon can flexibly adjust the structure of its echolocation calls. However, despite the increased call rate (p < 0.05), neither heart rate nor metabolic rate differed between phases. This study provides direct physiological evidence for understanding energy expenditure in bat echolocation and offers a methodological reference for future research.