BLUE WHALE-SIZED MOUTHFULS MAKE FORAGING SUPER EFFICIENT

When a blue whale dives to the krill fields where it feeds, it can remain submerged for anything up to 15 min. However, Bob Shadwick from the University of British Columbia, Canada, explains that blue whales may be able to dive for longer, because of the colossal oxygen supplies they could carry in their blood and muscles, so why don't they? ‘The theory was that what they are doing under water must use a lot of energy,’ says Shadwick. Explaining that the whales feed by lunging repeatedly through deep shoals of krill, engulfing their own body weight in water before filtering out the nutritious crustaceans, Shadwick says, ‘It was thought that the huge drag effect when they feed and reaccelerate this gigantic body must be the cost’. However, measuring the energetics of blue whale lunges at depth seemed almost impossible until Shadwick and his student Jeremy Goldbogen got chatting to John Hildebrand, John Calambokidis, Erin Oleson and Greg Schorr (p. 131). Hoping to record feeding blue whale conversations, Calambokidis and his colleagues had skilfully attached hydrophones, pressure sensors and two-axis accelerometers to the elusive animals and tracked them to recover the valuable data. Shadwick and Goldbogen realised that they could use Calambokidis's measurements to calculate the energetic cost of blue whale lunges.Analysing the behaviour of each whale, Goldbogen saw that dives lasted between 3.1 and 15.2 min and a whale could lunge as many as 6 times during a single dive. Having found previously that he could correlate the acoustic noise of the water swishing past the hydrophone with the speed at which a whale was moving, Goldbogen calculated the blue whales' speeds as they lunged repeatedly during each dive. Next the team had to calculate the forces exerted on the whales as they accelerated their colossal mouthful of water. Noticing that the whales' mouths inflated almost like a parachute as they engulfed the krill, Goldbogen tracked down parachute aerodynamics expert Jean Potvin to help them build a mathematical model to calculate the forces acting on the whales as they lunged. With Potvin on the team, they were able to calculate that the whales used between 3226–8071 kJ of energy during each lunge. But how did this compare with the amount of energy that the whales could extract from each gigantic mouthful of krill?Goldbogen estimated the volume of the whales' mouths by searching the whaling literature for morphological data and teamed up with paleontologist Nick Pyenson to measure the size of blue whale jaw bones in several natural history museums. He also obtained krill density values from the literature – which are probably on the low side. Then he calculated the volume of water and amount of krill that a whale could engulf and found that the whales could consume anything from 34,776 kJ up to an unprecedented 1,912,680 kJ from a single mouthful of krill, providing as much as 240 times as much energy as the animals used in a single lunge. And when the team calculated the amount of energy that a whale could take on board during a dive, they found that each foraging dive could provide 90 times as much energy as they used.Shadwick admits that he was initially surprised that the whales' foraging dives were so efficient. ‘We went over the numbers a lot,’ he remembers, but then he and Goldbogen realised that the whales' immense efficiency makes sense. ‘The key to this is the size factor because they can engulf such a large volume with so much food in it that it really pays off,’ says Shadwick.

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