Environment and feeding change the ability of heart rate to predict metabolism in resting Steller sea lions (Eumetopias jubatus)

Abstract

The ability to use heart rate (fh) to predict oxygen consumption rates ($\dot{V}_{{{\text{O}}_{2} }}$) in Steller sea lions and other pinnipeds has been investigated in fasting animals. However, it is unknown whether established fh:$\dot{V}_{{{\text{O}}_{2} }}$ relationships hold under more complex physiological situations, such as when animals are feeding or digesting. We assessed whether fh could accurately predict $\dot{V}_{{{\text{O}}_{2} }}$ in trained Steller sea lions while fasting and after being fed. Using linear mixed-effects models, we derived unique equations to describe the fh:$\dot{V}_{{{\text{O}}_{2} }}$ relationship for fasted sea lions resting on land and in water. Feeding did not significantly change the fh:$\dot{V}_{{{\text{O}}_{2} }}$ relationship on land. However, Steller sea lions in water displayed a different fh:$\dot{V}_{{{\text{O}}_{2} }}$ relationship after consuming a 4-kg meal compared with the fasting condition. Incorporating comparable published fh:$\dot{V}_{{{\text{O}}_{2} }}$ data from Steller sea lions showed a distinct effect of feeding after a 6-kg meal. Ultimately, our study illustrated that both feeding and physical environment are statistically relevant when deriving $\dot{V}_{{{\text{O}}_{2} }}$ from telemetered fh, but that only environment affects the practical ability to predict metabolism from fh. Updating current bioenergetic models with data gathered using these predictive fh:$\dot{V}_{{{\text{O}}_{2} }}$ equations will yield more accurate estimates of metabolic rates of free-ranging Steller sea lions under a variety of physiological, behavioral, and environmental states.