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 () 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 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 , breathing rate () and body mass (), we measured simultaneous and in five cetacean species in human care. We found that a higher was associated with a higher mean instantaneous (i) and minimum i of the RSA. By contrast, scaled inversely with such that larger animals had lower mean and minimum is of the RSA. There was a significant allometric relationship between maximum i of the RSA and , but not , which may indicate that this parameter is set by physical laws and not adjusted dynamically with physiological needs. RSA was significantly affected by and was greatly reduced with small increases in . Ultimately, these data show that surface s of cetaceans are complex and the patterns we observed are controlled by several factors. We suggest the importance of considering RSA when interpreting measurements and particularly how may drive changes that are important for efficient gas exchange. This article is part of the theme issue 'Measuring physiology in free-living animals (Part I)'.