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Predators and Prey

What’s the biggest animal you can think of? An elephant? A tyrannosaurus rex? How about a blue whale? The blue whale isn’t just the biggest whale: it’s the biggest animal ever to have lived on the planet. It can weigh as much as 200 tonnes (as heavy as 30 African elephants) and is about the length of three double-decker buses!

It’s not just the blue whale that’s huge – lots of whale species are really big. But why they’re quite so gigantic has always been a bit of a mystery. To try and find out, scientists in the United States recently measured the lengths of more than 100 fossilised whales from different time periods. The scientists discovered that, 30 million years ago, the ancestors of blue whales were much smaller – no bigger than a single African elephant. They seem to have stayed about that size until around 4.5 million years ago, when they mysteriously began to get a lot bigger! So why did the whales start growing?

To understand why, first we need to know that Earth’s climate naturally changes. It cools over thousands of years, and then it warms up over several thousand more years. The cooler periods are called ice ages, and our planet has had a few of them! Around 4.5 million years ago, much of the northern hemisphere was covered in huge sheets of ice. During the warmer summer months, the ice sheets melted and flowed into the oceans. The meltwater contained lots of nutrients, and millions upon millions of krill (shrimp-like creatures about 5cm long) came to feed on it. They gathered in large groups that were thousands of metres wide.

Despite their size, most whales feed on tiny animals – in enormous quantities! Even back then, whales could eat millions and millions of krill in a day. But such huge numbers of krill weren’t available all year round. They only really appeared in summertime, when meltwater flowed into the oceans. The krill patches were often far apart, too, so whales were forced to travel massive distances between meals.

This is where the feature – or characteristic – of being big became helpful. Bigger whales had an advantage over smaller whales: they could eat more food and therefore store more energy. So when it came to swimming long distances to their next meals, bigger whales didn’t tire out as quickly. They found it easier to survive than smaller ones, which started to die out.

When big whales mated with other big whales, they tended to produce big babies too. Again, the biggest whales had an advantage: they could eat even more krill and swim even longer distances, helping them stay healthy and live longer. Being big was obviously a big advantage!

The big whales kept giving birth to big babies – and the bigger the baby, the more likely it was to survive and reproduce. The process by which a favourable characteristic such as size becomes more common over time is called natural selection. So that's why whales are so huge today: size was an advantage that helped them find and eat food, so over millions of years whales just kept getting bigger!

Watch Predators and Prey to find out more about the different ways animals have evolved to capture their food.