Why are blue whales so enormous? - Asha de Vos

Transcriber: Andrea McDonough
Reviewer: Bedirhan Cinar

Blue whales are the largest animals that have ever roamed the planet. They’re at least two times as big as the biggest dinosaurs—“that’s big!”—the length of a basketball court, and as heavy as 40 African elephants. If that’s not enough to make you marvel, here’s something that will. They’ve grown to this enormous size by feeding exclusively on tiny shrimp-like creatures called krill—that are no bigger than your little finger.

In many ways, the sheer size of krill seems to have driven the evolution of the blue whale. See, krill are so small but are found in dense patches. For increased efficiency, blue whales have evolved to use a feeding strategy called lunge feeding. Basically, the whale accelerates towards a prey patch and opens its mouth wide. To increase the capacity, its mouth expands. The special, accordion-like blubber layer that extends from its snout to its belly button enables the whale to engulf large quantities of prey-laden water.

With each giant gulp, the whale takes in 125% of its body weight in water and krill. The whale must then expel the water while retaining the yummy krill. To do this, it uses its baleen, the comb-like structure made of the same stuff our nails and hair are made of, and its tongue. It’s pretty crazy that the blue whale’s heart is as big as a small car; a child could crawl through its arteries. Its tongue weighs as much as an elephant, but its esophagus is so small, the whale could choke on a loaf of bread. These whales are really not designed to feed on anything larger than krill.

It’s estimated that blue whales eat four tons of krill per day. Because of their incredible design, each dive provides the blue whale with 90 times as much energy as is used. Every mouthful of krill provides almost 480,000 calories, the same amount you get from eating 1,900 hamburgers. But, why are blue whales so big?

Blue whales are considerably larger than the largest living land animal, the elephant. The heavier an animal is, the greater its relative surface area. As weight increases, there’s a point at which the legs of that animal would simply collapse. That explains why elephants don’t stand on the delicate legs of a horse. They need legs shaped like stout pedestals to hold their bodies up against gravity. In water, the situation is quite different. Buoyancy counteracts the gravitational pull on the body, and their great bulk is therefore partially supported by the water. So, the ocean is a great place for species that want to grow bigger.

The other secret to their size is their diet. By evolving such a huge mouth, the whales have specialized to catch enormous quantities of highly abundant and nutritious prey, which provides the energy needed to grow so big. But, now maybe you’re wondering why blue whales aren’t any bigger? After all, the ocean sounds like nirvana for any growing beast.

Well, while lunge feeding may have allowed blue whales to become the biggest animal to have ever roamed the planet, by enabling them to feed efficiently in dense prey patches, it isn't cost-free. Scientists compared all the costs involved with lunge feeding to the energy gained from the krill they eat. What they found is that when the whale’s body increases in size, the energy that body demands rises faster than the extra energy they get from their food. Feeding whales need 15 times the energy required to remain still and 5 times more energy than used when swimming.

Calculations show that the largest a lunge feeder can grow is 33 meters, pretty much blue whale size. Turns out blue whales have a lot to thank krill for: neat, evolutionary adaptations that would not have been possible if krill were not so small. It’s incredible that these tiny creatures have allowed blue whales to really push the limits of size on our planet. Makes you wonder if that old adage, “you are what you eat,” really does apply in blue whale world.