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

Abstract

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 (<i>f</i>_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 <i>f</i>_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 <i>f</i>_H, breathing rate (<i>f</i>_R) and body mass (<i>M</i>_b), we measured simultaneous <i>f</i>_H and <i>f</i>_R in five cetacean species in human care. We found that a higher <i>f</i>_R was associated with a higher mean instantaneous <i>f</i>_H (i<i>f</i>_H) and minimum i<i>f</i>_H of the RSA. By contrast, <i>f</i>_H scaled inversely with <i>M</i>_b such that larger animals had lower mean and minimum i<i>f</i>_Hs of the RSA. There was a significant allometric relationship between maximum i<i>f</i>_H of the RSA and <i>M</i>_b, but not <i>f</i>_R, which may indicate that this parameter is set by physical laws and not adjusted dynamically with physiological needs. RSA was significantly affected by <i>f</i>_R and was greatly reduced with small increases in <i>f</i>_R. Ultimately, these data show that surface <i>f</i>_Hs of cetaceans are complex and the <i>f</i>_H patterns we observed are controlled by several factors. We suggest the importance of considering RSA when interpreting <i>f</i>_H measurements and particularly how <i>f</i>_R may drive <i>f</i>_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)'.