Knowing how much energy an individual requires is fundamental to resolving a number of questions about marine mammals such as how do they successfully exploit their environment, and whether or not they compete with fisheries for prey?

Determining energy expenditure of free-swimming individuals is difficult. One promising technique may be as simple as measuring the number of heart beats per minute.

In theory, heart rate increases as energy expenditure (metabolism) increases. This has been shown for a number of species such as horses, steers, dogs, goats, calves, rats, penguins and seals. However, the relationship appears to be unique for each species.

Jan McPhee and colleagues tested whether a relationship exists between heart rate and metabolism of four captive Steller sea lions at the Vancouver Aquarium Marine Science Centre. Data were collected while each animal rested on land, or while swimming, diving or resting in water. The goal was to determine whether the heart rate technique could be used in Alaska to estimate the energy that Steller sea lions are expending.