A total of 126 trials were analyzed, with 42 trials conducted at each depth (0.5 m, 5 m, and 10 m).
Diving heart rate profile
The figures below illustrate representative traces of temporal changes in instantaneous heart rate and ODBA throughout 120 s trials at 0.5 (A), 5 (B), and 10 m (C), performed on the same day by the same dolphin.
During the pre-dive and post-dive periods, respiratory tachycardia was evident, being characterized by an increase in instantaneous heart rate during inspiration, followed by a return to baseline levels after expiration. Regardless of the dive depth, the instantaneous heart rate dropped immediately at the onset of apnea and quickly stabilized with minor fluctuations for the remainder of the apnea period. Likewise, ODBA remained consistently low during apnea at all three depths, with minimal movement due to adjustments required to maintain position.
Pre-diving physiological parameters
None of the pre-diving physiological parameters (heart rate, breathing frequency, RMSSD, and CVRR) varied with depth.
Diving physiological parameters
Diving heart rate and CVRR decreased with dive depth (see figures below). However, diving ODBA and RMSSD showed no significant changes across depths.
Influence of diving behaviour on diving heart rate
- Diving heart rate decreased with increasing dive depth, being significantly lower at 10 m compared to 0.5 m and 5 m. However, no significant difference in heart rate was observed between 0.5 m and 5 m;
- Higher breathing frequency before diving was associated with higher heart rate during the dive;
- Heart rate decreased with dive duration;
- ODBA and body mass did not significantly influence heart rate during apnea.
Influence of diving behaviour on diving HRV
- Diving CVRR decreased with increasing dive depth, being significantly lower at 5 m compared to 0.5 m and at 10 m compared to 0.5 m. However, no significant difference was found in diving CVRR between 5 m and 10 m;
- Diving CVRR decreased with increasing dive duration;
- Neither diving ODBA, pre-dive breathing frequency, nor body mass significantly influenced CVRR during the apnea;
- None of the physiological parameters included in these models significantly affected RMSSD during apnea.