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

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.