Why are whales so big?


Have you ever wondered why whales are so big? It's an obvious question to ask, but we got to the answer only as recently as 2018, and it's to do with food, and temperature.

Whales are, of course, mammals, just like bears, horses, and us humans. We all have a fixed body temperature, which is maintained through process called thermoregulation. When a mammal's body temperature drops below that optimum level, they lose their ability to function properly, and mammalian enzymes usually perform well over only a narrow temperature range. So, mammals have to consume food, to make energy, to keep warm. But, whales are marine mammals, and underwater it's a bit more complicated.

Physics tells us that water conducts or shifts heat a lot better than air does. So while you can walk around comfortably all day in air that is at 15 degrees Celsius, you would start shivering after less than half-an-hour if you were in water at 15 degrees Celsius. Most mammals just can't generate enough heat to make up for what they lose in the water.

And geometry explains why bigger is better for maintaining body heat — the bigger you are, the less surface area you have, relative to your volume. Why is this important? Well, you lose heat from your surface, and you generate heat from your mass (and hence your volume). So for a mammal to survive in water, you need lots of volume, but not much surface area, and you get this by growing bigger.

In fact, the biggest whales are close to their potential maximum size. When they feed, they can suck in more than their own body weight in water! In just six seconds! A 60 tonne whale will suck in 70 tonnes of water. That's amazing!

But hang on, why are marine mammals so much bigger than fish? How come fish of any size are fine living in water?

The physiology for fish is easy. Their body temperature is the same as that of the water around them. Their physiology runs perfectly well, even if they're a little colder or warmer than normal. But a small mammal in water can't stop its body temperature from dropping — and eventually their systems will shut down and they'll die.

So that's temperature, what about food?

Well, the reason whales don't keep evolving to be kilometres long is because they can't eat enough food to be that big!

The sperm whale is about as big as a marine mammal can get if it uses "teeth" to get its food. Some whales evolved to use filter feeding instead. The filters are inside the whale's mouth, and are called "baleen". Baleen hangs from the rim of the whale's upper jaw and looks like a kind of feathery, pale moustache. It's made of keratin (as in fingernails and hair), and acts like a net to catch food.

With its mouth wide open to almost a right angle, a baleen whale swims through their lunch — schools of tiny sea creatures called krill. Krill are small crustaceans, usually only a centimetre or so in size. It's weird to think that the largest creature that ever lived (the blue whale), feeds on such tiny ocean critters. But because krill hang around in tightly packed schools, they tend to behave as a single unit — or target.

As the whale engulfs the krill, its smooth and streamlined shape transforms into something like a huge parachute. The whale slows down abruptly. Sometimes it will come to almost a complete halt — even though its tail is flipping up and down vigorously. In just six seconds, a large baleen whale can go from being a 60 tonne whale to a 130 tonne whale - just by swallowing 70 tons of water! Now that's a gulp!

Where does this 70 tonnes of water go? Well, you might've noticed strange longitudinal grooves running along the belly of some of the large baleen whales. They can expand enormously — just like the folds of an accordion — when the whale swallows water. The whale then pushes that huge mass of water out, by closing its mouth. The water rushes out, the filter comes into play, and the krill get stuck in the baleen. But how much krill would a whale catch from swallowing 70 tonnes of water? Only a lousy 11 kg.

The fin whale, which is almost as big as a blue whale, needs about 900 kg of food each day to keep its 60 tonne body alive. That's about 83 gulps, swallowing up three hours of their day. It's hard work being a marine mammal.

So as with many things in nature, we must look to goldilocks for some wisdom. Any smaller, and whales would freeze to death. Any bigger, and they would starve. They've really got their size just right.