Scaling of heart rate with breathing frequency and body mass in cetaceans

Plasticity in the cardiac function of a marine mammal facilitates rapid adjustments to the contrasting metabolic demands of breathing at the surface and diving during an extended apnea. By matching their heart rate (f_H) to their immediate physiological needs, a marine mammal can improve its metabolic efficiency and maximize the proportion of time spent underwater. Respiratory sinus arrhythmia (RSA) is a known modulation of f_H that is driven by respiration and has been suggested to increase cardiorespiratory efficiency. To investigate the presence of RSA in cetaceans and the relationship between f_H, breathing rate (f_R) and body mass (M_b), we measured simultaneous f_H and f_R in five cetacean species in human care. We found that a higher f_R was associated with a higher mean instantaneous f_H (if_H) and minimum if_H of the RSA. By contrast, f_H scaled inversely with M_b such that larger animals had lower mean and minimum if_Hs of the RSA. There was a significant allometric relationship between maximum if_H of the RSA and M_b, but not f_R, which may indicate that this parameter is set by physical laws and not adjusted dynamically with physiological needs. RSA was significantly affected by f_R and was greatly reduced with small increases in f_R. Ultimately, these data show that surface f_Hs of cetaceans are complex and the f_H patterns we observed are controlled by several factors. We suggest the importance of considering RSA when interpreting f_H measurements and particularly how f_R may drive f_H changes that are important for efficient gas exchange. This article is part of the theme issue 'Measuring physiology in free-living animals (Part I)'.