Effect of apnea and food reward on the autonomic modulation of heart rate in bottlenose dolphins ( Tursiops truncatus ) and belugas ( Delphinapterus leucas )

1. Abstract Background Heart rate (HR) and HR variability (HRV) reflect the autonomic modulation of cardiac activity and are key indicators of physiological state and stress in vertebrates. However, measuring these parameters in cetaceans has been historically challenging due to the difficulty of recording electrocardiograms (ECGs) underwater. Methods We developed an all-in-one ECG suction-cup to record HR during apnea, respiration and food rewards in bottlenose dolphins ( Tursiops truncatus , N = 8) and belugas ( Delphinapterus leucas , N = 2). Moreover, we obtain comparative measurements of ECG waveform and HR oscillation in one killer whale ( Orcinus orca ). Results Species-specific ECG patterns were observed, including biphasic T waves in all three species and bifid P waves unique to belugas. All three species exhibited oscillations in resting HR and pronounced bradycardia during short apnea (∼1 min), reflecting respiratory-linked autonomic modulation. Moreover, HR oscillations persisted throughout apnea, possibly reflecting a mechanism to cyclically improve body oxygenation during diving. Short-term food deprivation had no significant effects on bottlenose dolphins and belugas’ HR, while subsequent feeding ad libitum reduced HR (by ∼20% in dolphins, ∼40% in belugas) and increased HRV, suggesting the activation of vagal modulation. Conclusions This study establishes baseline HR and HRV parameters during breathing, apnea and food reward in three cetacean species, highlighting autonomic modulation across different behaviors and conditions and validate a non-invasive tool for studying marine mammal physiology.