The heart responds to pressure via the lung: Dive depth alters cardiac control in bottlenose dolphins

Marine mammals face the dual challenge of foraging underwater while relying on limited oxygen stores, making efficient oxygen management essential for survival. The dive response, which includes bradycardia and peripheral vasoconstriction, helps conserve oxygen for critical organs, such as the heart and brain. While the heart rate (HR) is known to decline with dive duration, the independent effect of dive depth, often confounded by longer dives, remains unclear. To isolate this effect, we measured HR and heart rate variability (HRV) in seven bottlenose dolphins (Tursiops truncatus) during voluntary dives to 0.5 m, 5 m and 10 m, with controlled dive durations ranging from 60, 90 and 120 s. We found that both HR and HRV decreased with increasing dive depth and duration, and that pre-dive breathing rate was positively correlated with HR before and during diving. Additionally, minimal activity to maintain depth did not influence cardiac metrics. These findings support the hypothesis that hydrostatic pressure influences cardiac function during diving, likely through its effects on lung volume. This study underscores the power of comparative physiology to reveal how air-breathing marine animals finely regulate cardiovascular function in response to environmental changes. Understanding these mechanisms is not only fundamental to evolutionary biology but also critical for predicting how diving species may be impacted by climate-driven changes in ocean temperature and hydrostatic pressure. KEY POINTS: Dive depth significantly influences cardiac function in bottlenose dolphins, even when dive duration and activity are controlled. Pre-dive respiratory rate influences diving heart rate, highlighting cardiorespiratory regulation. Cardiac responses followed a triphasic pattern during dives, mirroring patterns observed in other marine mammals. Findings provide mechanistic insight into oxygen management strategies critical for survival and foraging in diving marine mammals and offer predictive value for how air-breathing marine predators may physiologically adapt to climate-driven shifts in their environment.