Stroke effort and relative lung volume influence heart rate in diving sea lions

The dive response, bradycardia (decreased heart rate) and peripheral vasoconstriction, is the key mechanism allowing breath-hold divers to perform long-duration dives while actively swimming and hunting prey. This response is variable and modulated by factors such as dive duration, depth, exercise and cognitive control. This study assessed the potential role of exercise and relative lung volume in the regulation of heart rate (f_H) during dives of adult female California sea lions instrumented with electrocardiogram (ECG), depth and tri-axial acceleration data loggers. A positive relationship between activity (minimum specific acceleration) and f_H throughout dives suggested increased muscle perfusion associated with exercise. However, apart from late ascent, f_H during dives was still less than or equal to resting f_H (on land). In addition, the activity-f_H relationship was weaker in long, deep dives consistent with prioritization of blood oxygen conservation over blood oxygen delivery to muscle in those dives. Pulmonary stretch receptor reflexes may also contribute to f_H regulation as f_H profiles generally paralleled changes in relative lung volume, especially in shallower dives and during early descent and late ascent of deeper dives. Overall, these findings support the concept that both exercise and pulmonary stretch receptor reflexes may influence the dive response in sea lions.