The amount of oxygen that an animal consumes as it goes about its daily activities can teach us how that creature integrates into its environment, how much food it must consume and whether its lifestyle is sustainable in increasingly disturbed ecosystems. However, our understanding of the energetics of some of the most enigmatic inhabitants of the oceans remains sparse. ‘Measuring metabolic rates of the free-ranging cetaceans directly is unfeasible’, says Marjoleine Roos from the University of St Andrews, who explains that scientists had previously used the respiration rate of surfacing whales and dolphins as a proxy for elusive metabolic rate measurements. ‘However, doing so assumes that every respiration reflects a fixed amount of oxygen take up, whereas oxygen uptake has been shown to fluctuate per respiration’, says Roos. In addition, metabolic estimates based on this assumption have failed to identify the optimal swimming speeds for ocean-going mammals. Inspired by these potential failings, Roos and Patrick Miller decided to test the accuracy of a new oxygen consumption model in a bid to better understand the energetics of free-ranging killer whales.
Attaching digital acoustic recording tags (DTAGs) to 10 killer whales over a 4 year period, Miller recorded the animals’ movements and the sound of water rushing past as they swam and dived, which the team then analysed to learn more about the animals’ activities. According to Roos, killer whales are single-breath divers, continually diving between visits to the surface, rather than resting at the surface for extending periods to recover like other species. Roos and Miller had assumed that the animals would increase their respiration rate as they speeded up and used more energy, if they were inhaling the same amount of oxygen each time they surfaced. However, when the duo and Gi-Mick Wu examined over 50 h of recordings and calculated the animals’ respiration rates, they were puzzled to see that they did not increase. ‘We considered that there must be some breath-to-breath variation in oxygen uptake that wasn't accounted for by respiration rate alone’, says Roos.
The team recalculated the inhalation values based on respiration measurements taken from captive killer whales and adjusted the amount of oxygen that they took on board in response to the amount of oxygen that they retained after the previous dive, and the rate at which it was replenished. This time, they were impressed to see that their simulations produced realistic predictions for the amount of oxygen carried by the diving animals, and when they calculated the amount of oxygen consumed as the animals swam at different speeds, they found that the cost of transport was lowest at speeds ranging from 1.7 to 2.4 m s−1. ‘The predicted cost of transport was substantially lower than previous studies have predicted for killer whales. This could be vitally important as it might indicate that killer whale feeding requirements are much lower than previously thought’, says Roos. These new estimations could have significant implications for our understanding of their ecology.
Having found the killer whales’ optimal swimming speed, Roos warns that estimates of the amount of energy used by cetaceans based on their respiration rate alone could be unreliable and she recommends that scientists begin considering when cetaceans inhale air relative to the amount of oxygen that they are carrying to build a better understanding of the ecology of some of the ocean's top predators.
