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(H)s 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(H)s 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)'.

Automated summary

The plasticity of cardiac function in marine mammals allows for rapid adjustments to meet the contrasting metabolic demands of surface breathing and extended diving. Respiratory sinus arrhythmia (RSA) serves to increase cardiorespiratory efficiency by modulating heart rate in response to respiration. While higher breathing rates are associated with higher heart rates, larger animals have lower RSA values. The study suggests that RSA is affected by breathing rate and implies the importance of considering RSA in interpreting heart rate measurements in cetaceans.