Why Did Whales Get So Big?

Which do you think would be bigger?

“Sue,” one of the largest T. rex specimens ever found, is 42 feet (12.8 meters) long and weighed an estimated 12,600 to 14,200 pounds (around 5,700 - 6,400 kilograms). Blue whales on the other hand, can reach up to 110 feet (33.5 meters) long and weigh 333,000 pounds (around 150,000 kilograms)! That means that blue whales can be as heavy as about 25 Tyrannosaurus rex and more than twice as long!

Blue whales are can be as heavy as 25 Tyrannosaurus rex and more than twice as long!

While the blue whale is the largest animal to have ever lived on Earth, baleen whales were not always so big. Baleen whales are a group of whales that use bristles called baleen to filter feed and include the blue whale and humpback whale.

The chart below shows how baleen whales have skyrocketed in size since they first appeared 34 million years ago.

You can see that whale’s increase in size happened gradually over a period of around 20 million years ago, and then very dramatically in the recent geological past.

But why did whales get so big?

Large animals need a large amount of food to live, and whales are no different. Blue whales, for example, can eat up to 4 tons of krill in a day. In order for these whales to survive, there has to be a lot of krill.

When there are more nutrients in the ocean, more krill can exist. More nutrients were available during periods when cold, deep ocean water would bring nutrients to coastal waters in a process called upwelling.

Baleen whales' growth spurt occurred over periods of increased upwelling when nutrient rich ocean waters could support larger populations of krill.

As baleen whales grew in size their need for large amounts of food also increased. Since whales are bigger than ever, they rely on a massive supply of food to meet their equally large need for energy.

How do whales find enough food to meet their huge requirements for energy?

Baleen whales travel long distances, sometimes thousands of miles a year, to find food sources and breed. As the world's oceans warm and krill populations are expected to decrease, many baleen whales will need to travel even longer distances to find enough food.

Longer migratory paths mean that whales would need to expend more energy to consume the same amount of krill. The advantage of being so large is being able to swim longer distances, but at a cost. Larger whales also have to consume more food to support their larger bodies.

As a warming climate threatens to disrupt the complex relationship between ocean currents, nutrients in coastal waters and krill populations, the future of baleen whales is uncertain.