Energy is the currency of life and knowing how much organisms use as they go about their daily activities is essential for ecologists wishing to understand the complex interactions underpinning ecosystems. However, estimating energy consumption is far from straightforward, and most of the current methods have drawbacks. Beth Young and her colleagues from the University of British Columbia and Vancouver Aquarium, Canada, explain that many scientists convert heart rate measurements collected from active animals into energy expenditure. However, the calculation is based on measurements taken from animals in laboratory settings and the team says, ‘The artificial modes of locomotion and environments employed in these studies raise questions of applicability to animals that spend considerable time diving to depth.’ Curious to find out how reliable these estimates are for diving animals in their natural surroundings, Young and her colleagues measured the heart and oxygen consumption rates of Stellar sea lions foraging in open water and tested whether heart rate could predict an animal's diving and average metabolic rates (p. 2267).
Training three sea lions to make a sequence of dives for fish to depths of 10 and 40 m, the team found that heart rate was a good indicator of average metabolic rate over a single dive and a series of dives, including the time spent at the surface catching their breath. However, heart rate was not a good indicator of the sea lion's metabolic rate during the dive.
Also, the team found that the equations that they derived to predict oxygen consumption from heart rate were different for multiple dives and single dives. For single dives the equation was essentially the same as that for resting sea lions. However, the animals that dived repeatedly consumed more oxygen per heartbeat than the single divers, presumably because they accumulated a greater oxygen debt.
So, heart rate measurements can be used to estimate average metabolic rate in diving sea lions, but only over complete dive cycles where the recovery period is also included. The team admits, ‘Logistically, it is not always possible to distinguish single recovery dive cycles from dive bout cycles in free-ranging animals,’ and recommends that physiologists calculate the average metabolic rate over a dive bout to provide the most accurate metabolic rate estimates.