Resting Heart Rate of Trained Pantropical Spotted Dolphins Stenella attenuata: Sinus Arrhythmia Associated with Respiration

Measuring the resting heart rate of diving mammals at the water surface is crucial for evaluating the influence of various factors such as exercise and diving depth and duration. However, it has only been studied in a few cetacean species. Here, we present the first report on the resting heart rate of adult pantropical spotted dolphins (Stenella attenuata) (body mass, 71 kg, 76 kg, n = 2 individuals) during surfacing. Measurements were taken exclusively during the morning fasting period. The instantaneous heart rate (ifH) rapidly increased to approximately 1.5 times the pre-breath value immediately following respiration (maximum ifH, mean ± SD, 131 ± 10 beats per minute [bpm], n = 79 breaths). Then, ifH gradually declined, returning to the pre-breath value after 15–20 s, remaining relatively stable until the next breath (minimum ifH 80 ± 18 bpm, breathing frequency 2.4 ± 1.1 breath min–1). These findings support the association of ifH with respiration. Additionally, average and minimum ifH increased with the respiration rate, highlighting the importance of considering respiration rate when examining resting heart rate. Our study describes the magnitude of variation in the resting heart rate during surfacing and provides crucial insights into cardiovascular function in small delphinids. Published online 17 June, 2025; Print publication 31 October, 2025