Efficiency is a priority for any animal that dives for its dinner. ‘It is crucial [for divers] to adopt strategies that minimize their oxygen consumption and therefore maximize their foraging time at depth’, says Kagari Aoki, from the University of St Andrews, UK, and the University of Tokyo, Japan. However, Aoki and her colleagues had a hunch that pilot whales may buck this trend. While other toothed whales opt for a leisurely low-cost descent and ascent in order to conserve oxygen supplies for use while foraging at depth, short-finned pilot whales have been reported to prefer a fast-dash approach. Intrigued by the seemingly profligate strategy of the short-finned pilot whales, Aoki and Patrick Miller attached depth and motion tags to 18 long-finned pilot whales off the coast of Norway to learn more about the animals’ antics beneath the waves.
Recording over 150 h of data and analysing 140 dives exceeding 250 m, Aoki and colleagues found that, on average, the whales reached depths of 444 m in dives lasting approximately 9 min, with one whale descending 617 m and the longest dive lasting over 13.5 min. And when the team calculated the average speed of the whales’ ascent, Aoki and her colleagues were surprised that it was significantly higher (2.7 m s−1) than the expected speed (1.2–1.5 m s−1 based on the animals’ estimated masses). Wondering whether the whales were taking advantage of a slick skin to accelerate their rapid ascent or were swimming particularly hard, the team calculated the whales’ drag coefficient and found that it was no different from that of other species: the whales must be swimming harder and consuming more energy during their ascent. In addition, the team noticed that the whales’ ascent slowed as they became increasingly buoyant near the end of their return to the surface, slightly reducing their most efficient swimming speed.
‘Our results indicate that long-finned pilot whales maintain high diving metabolic rates during deep foraging dives’, says Aoki, who suggests that animals might be using a ‘spend more, gain more’ hunting strategy, where they expend more energy swimming faster in order to consume more calories than other cetacean species.