Why do blue whales need to be so big when they only eat plankton?
• Blue whales need to be so big precisely because they eat tiny food, specifically krill, that feed on plankton.
Krill defend themselves against smaller predators by forming very dense shoals that confuse attackers. Blue whales get around this by swimming very fast at the shoal with their mouth wide open, often from below, and engulfing as much as they can. Their pleated throat can expand enormously to take in as much of a shoal as possible. The whales then strain out the krill by forcing the water out through the baleen plates in their mouth.
Advertisement
Because each attack uses up a lot of energy, this feeding strategy – called lunge feeding – only works if the animal can take in enough krill to more than repay the cost, so the whale needs to be huge to use it effectively.
Peter Wright, Polegate, East Sussex, UK
• One struggle for mammals that live in a cold ocean is to maintain their body temperature. Besides having a thick layer of blubber as insulation, another strategy is to be as large as possible, because this minimises their surface area-to-mass ratio. In other words, there is less surface area through which to lose heat per unit of body weight. Young whale calves don’t have this benefit, which is one reason why many whale species migrate to warmer waters to give birth. The calves then have the chance to fatten up before they venture to colder waters.
The upper limit on body size is determined by the skeletal structure needed to support the body. For land mammals this is a more serious limit because they don’t have the buoyancy of the water to help support their weight. That is why land mammals can’t grow as large as whales.
Simon Iveson, University of Newcastle, Callaghan, New South Wales, Australia