How blue whales became Earth’s largest creature—ever
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Think of the largest elephant you can. Now multiply that by 30. That’s the size of a blue whale, the largest animal to exist, ever. The ocean-going mammals weigh up to 330,000 pounds and can stretch over 100 feet, the length of a Boeing 737. Even the biggest dinosaur only weighed something like 75 tons, less than half the weight of a blue whale.
But what caused blue whales to grow to such an extraordinary size? It all comes down to living in water and feeding on tiny shrimp-like creatures called krill.
The constraints of gravity
On land, the maximum size mammals can reach is limited by gravity. Large land mammals have huge bones, massive blood vessels, and strong legs to support their weight.
As animals get bigger, their weight increases much faster than the strength of their bones. At a certain point, an increase in size would lead an especially large animal’s legs to collapse under the weight of gravity.
In water, gravity doesn’t have the same effect. Instead, the buoyancy of water helps support the weight of aquatic mammals: a key factor in how blue whales reached their enormous size.
The energy sweet spot
But there is more to the story of how blue whales got so dang big.
Craig McClain, a professor of biodiversity at the University of Louisiana at Lafayette, compared almost 7,000 living animals and fossils to analyze factors that affect size. He discovered that water doesn’t just reduce the force of gravity, it pushes warm-blooded mammals to grow.
Water conducts heat away from the body much faster than air so mammals need to be larger otherwise it’s impossible to stay warm. At the same time, as bodies increase in size, the energy needed to fuel them rises.
“What we found is that aquatic mammals are drawn toward an optimal body size, because of energetic tradeoffs,” he says. Their ideal size is a careful balance between how much energy they use weighed against how fast they feed. It’s not simply becoming as big as possible. “The ‘sweet spot’ is where [energy] income comfortably exceeds costs.” An aquatic mammal can only become huge if it feeds on enough food to offset the demands of a larger body.
Eating dense swarms of krill has led blue whales to become larger
Blue whales are able to grow much larger than other aquatic mammals because they feed on krill.
Krill are only about 2 inches long and swim slowly in tightly packed swarms. To take advantage of this, blue whales have developed huge mouths and a highly expandable throat pouch beneath their skin, so they can lunge forward and swallow enormous amounts of krill in one gulp. They’re capable of eating up to 794 pounds of krill in one mouthful. That’s the equivalent to roughly 16 vending machines.
The pouch is formed by deep folds that stretch from the bottom of their jaw to their navel, creating a stretchy extension of their mouth that balloons out to accommodate such a large intake.
But lunging for food takes a lot of energy. “This feeding strategy only becomes energetically worthwhile at large body sizes,” says Elliott Hazen, an ecologist at NOAA Fisheries. “Blue whales survive on tiny prey because they have developed one of the most efficient bulk feeding systems in the natural world.”
To test how efficiently they can feed, Jeremy Goldbogen, a professor of oceans at Stanford, used motion recording sensors to measure the swimming speed, dive patterns, and acceleration of 256 blue whales as they lunged toward their prey.
When Goldbogen modeled the energy cost of each lunge against the energy gained from krill, he found that a single mouthful of krill can give blue whales more than 200 times as much energy as they use. That’s a really good payoff.
Blue whales are built for migration
But the whales’ size doesn’t just help as they feed, it also allows them to travel long distances. Krill can only be found in large numbers in isolated areas of the ocean so blue whales journey thousands of miles between their feeding and breeding grounds.
Their massive bodies means they can store huge amounts of energy as fat even when they’re not feeding. So even though blue whales end up using more energy than smaller animals, they burn it more slowly, at least relative to their size. This gives them even greater endurance.
A streamlined shape also reduces drag so they can move through the oceans with minimal energy.
Whales weren’t always giants
Blue whales haven’t always been so big. Fossil records show their ancestors, the Pakicetus, were wolf-sized land mammals that were only three to six feet long. This ancient ancestor actually lived on land and hunted in shallow water.
When the first whales appeared in the ocean around 40 million years ago, they were also a relatively modest size. For instance, one of the earliest known whales, Mystacodon selenensis, was only around 13 feet long, roughly the same size as a bottlenose dolphin.
Then, around 3 million years ago, the oceans changed. As massive ice sheets spread across the earth, temperatures dropped and wind patterns increased. This led to an increase in upwelling, a process where strong winds cause nutrient-rich water to rise up from the deep ocean. The extra nutrients led krill to cluster in dense swarms along coastlines favoring animals that could feed in bulk and travel long distances.
“Blue whales did not just get big because they could,” says McClain. “They got big because the ocean started serving food in a way that rewarded giants.”
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Could whales get even bigger?
Blue whales are now close to the limit of how large they can grow. As animals get bigger, their heart works harder to pump blood to the extremities, oxygen delivery becomes more difficult, and reproduction slows down.
In a 2019 study, researchers measured the heart rate of a blue whale using a small heart monitor. Whales can’t breathe as they dive so they slow their heart to reduce the oxygen they use and increase the time they can stay underwater. During deep dives the whale’s heart rate slowed to as low as two beats per minute, the minimum needed to keep vital organs alive.
Bigger blue whales would also demand more krill and at some point the energy cost would become unsustainable. They wouldn’t be able to consume enough krill to get the energy they needed.
“Although the ocean allows whales to become larger than any land animal in Earth’s history, biology and physics likely place an upper limit on how large they can evolve to be,” says Hazen.
So the next time you spot something moving through the waves, consider how the blue whale became a giant: an ocean changed by ice, a body shaped by water, and a feeding strategy built around swallowing tiny krill. Together those factors produced an animal whose size surpasses all other creatures on earth.
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