VOA慢速英语2017--鲸鱼什么时候变得如此之大?

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Three million years may seem like a long time. But when you consider that planet Earth is more than 4.5 billion years old, it is a pretty short period of time.

And it is over that shorter time period that the whales in our oceans today got to be so large.

The blue whale is the largest animal that has ever lived. They weigh more than even the biggest dinosaurs 1. They can grow to 30 meters long and weigh up to 130,000 kilograms.

Blue whales are part of the baleen 2 whale family. Baleen is a kind of food-filtering structure in the mouths of the whales. The whales developed this system about 30 million years ago.

Other whales in the baleen family include fin 3 whales, bowhead whales, and right whales. Today, these whales can weigh well over 70,000 kilograms.

But for millions of years, they were not nearly that large, scientists say.

“All of a sudden – ‘boom’ – we see them get very big, like blue whales,” scientist Nick Pyenson told the New York Times. He and other researchers recently published an article on whale size in the British research journal Proceedings 4 of the Royal Society B.

“It’s like going from whales the size of minivans to longer than two school buses,” Pyenson added.

The researchers -- Pyenson, Jeremy Goldbogen and Graham Slater -- believe that a change in the Earth’s climate caused whales to get so large only about 3 million years ago.

This change led to food becoming more easily available.

An oncoming ice age made the ocean water cold. High winds mixed the water in the ocean faster than before. Rain and snow made more nutrients 5 run off from the land into the water. The nutrients resulted in algae 6. That attracted small fish and shrimp 7 called krill.

The mixture of nutrients resulted in more food for large whales to eat.

Whales that were able to quickly eat a lot of food survived. And they got larger. Whales that had teeth instead of baleen, however, did not survive.

The largest baleen whales then produced other large whales.

Over the next three or four million years, the baleen whales grew 10 times bigger than they had been, the researchers say.

It is possible that the largest whales are gone, the researchers said. But, as they told the BBC, blue whales could grow even bigger -- or they could slowly die – depending on Earth’s changing climate.

Words in This Story

baleen – n. cartilage that filters food and water from the mouths of large whales

cartilage – n. a strong but flexible material found in some parts of the body (such as the nose, the outer ear, and some joints)

filter – n. a device that is used to remove something unwanted from a liquid or gas that passes through it

algae – n. simple plants that have no leaves or stems 8 and that grow in or near water

minivan – n. a small van designed to carry people and families