Blue whale feeding methods are ultra-efficient

Blue whales are the biggest and perhaps most efficient animals alive. Their method of filter-feeding takes in 90 times more energy than it uses.

The enormous mammals dive up to 500 metres beneath the surface, then lunge into the swarms of tiny krill above them at several metres per second. As they strike, their massive mouths fill with huge volumes of water, including plenty of krill. The water is pushed out through the filters, or baleen, in each whale's mouth, trapping the krill.

This feeding technique takes a lot of effort due to the energy needed for the lunges. "We wondered how they coped," says Robert Shadwick of the University of British Columbia in Vancouver, Canada.

Shadwick's colleague Jeremy Goldbogen of the University of California, San Diego, led a team who set out to track blue whales as they fed. In small boats they zoomed up alongside surfacing whales and attached tracking devices to them using suction caps.
Energy efficient

In total the team tracked 265 blue whales as they carried out 200 foraging dives and 654 lunges. From the speeds the whales reached while lunging, they calculated that each lunge used about 3200 kilojoules of energy.

That may seem high, but it was dwarfed by the amount of energy the whales got from their food. Based on known krill densities in the whales' feeding grounds, each lunge netted between 34,000 and 1,912,000 kJ – up to 237 times the energy used. Even when the energy costs of diving are included, the whales still gained 90 times the energy they used.

Shadwick says the results could explain how blue whales survive their migratory lifestyles. They feed in Antarctic waters in the summer, then head north to their tropical breeding grounds where little food is available. Even so, the females must still produce enormous volumes of milk for their calves. "This explains how they can cope with seasonal starvation," Shadwick says.

Foraging whales must have high densities of krill for their feeding methods to be effective, says Alejandro Acevedo-Gutiérrez of Western Washington University in Bellingham. Lunge feeders "have to get more bang for the time underwater, so to speak", he says.