The results of this study indicate that heart rate fluctuations follow a clear day night cycle, with variations in heart rate being strongly influenced by the turtles’ activity levels (acceleration, ODBA), which were also driven by the time of day. Besides, the algorithm for predicting breathing frequency highlighted heart rate as one of the most important outcomes influenced by breathing frequency.
Activity
The significant difference in activity levels between day and night suggests that the turtles followed a diel activity pattern influenced by natural light exposure. These findings align well with previous research on sea turtles, where activity levels typically decrease at night (Parrish, 1958; Shoop & Dodd, 1989; Wyneken and Salmon, 1992).
Heart rate
Although variability in heart rate and activity was observed within turtles, the trend of increasing heart rate and ODBA during the day and then declining towards nighttime, further supports the role of external environmental factors. Since ODBA was influenced by the time of day, heart rate can be considered indirectly influenced by the time of day through the animal’s activity.
From this study, a non-linear relationship between heart rate and activity was observed. The results showed that there was a positive association between ODBA and heart rate during both day and night, suggesting that when movement increases this leads to elevated cardiac response when turtles are active.
Breathing frequency
These results demonstrated that breathing frequency played a key role in shaping average heart rate dynamics. Average heart rate showed the strongest association with breathing patterns, reinforcing the physiological link between cardiac and respiratory function (Kinoshita et al., 2022) and suggesting that fluctuations in respiration drive corresponding changes in heart rate. Acceleration also contributed substantially to the algorithm, indicating that physical activity influences both breathing and heart rate, possibly through increased oxygen demand during movement. Temperature and time of day had a lower effect on the model but still contributed to predicting breathing frequency, indicating potential thermal and diel influences on breathing patterns.
Conclusion
These findings underscore the importance of individual variability and environmental factors (day vs night) in shaping heart rate dynamics contributing to a deeper understanding of how turtles adapt to their surroundings, and could potentially have implications for conservation and rehabilitation strategies. Future studies could explore how additional variables, such as depth, diving patterns, temperature changes, tidal cycles, or moon phases, contribute to these patterns. Examining the influence of these factors and comparing the physiology and behavior of captive and free-ranging sea turtles could provide deeper insights into the broader regulatory mechanisms governing their physiology.
Acknowledgments
Huge thanks to my supervisors and the incredible turtle team in Japan – couldn’t have done it without you!
