[ti:When Did Whales Get So Large?] [by:www.51voa.com] [00:00.00]更多听力请访问51VOA.COM [00:00.04]This is What's Trending Today... [00:04.16]Three million years may seem like a long time. [00:09.12]But when you consider that planet Earth is more than 4.5 billion years old, [00:16.20]it is a pretty short period of time. [00:20.44]And it is over that shorter time period [00:23.80]that the whales in our oceans today got to be so large. [00:30.68]The blue whale is the largest animal that has ever lived. [00:36.32]They weigh more than even the biggest dinosaurs. [00:40.79]They can grow to 30 meters long and weigh up to 130,000 kilograms. [00:50.00]Blue whales are part of the baleen whale family. [00:54.32]Baleen is a kind of food-filtering structure in the mouths of the whales. [01:01.72]The whales developed this system about 30 million years ago. [01:08.00]Other whales in the baleen family include fin whales, [01:12.69]bowhead whales, and right whales. [01:16.23]Today, these whales can weigh well over 70,000 kilograms. [01:23.36]But for millions of years, they were not nearly that large, scientists say. [01:31.76]"All of a sudden – ‘boom' – we see them get very big, [01:36.20]like blue whales," scientist Nick Pyenson told the New York Times. [01:42.36]He and other researchers recently published an article on whale size [01:48.32]in the British research journal Proceedings of the Royal Society B. [01:54.60]"It's like going from whales the size of minivans [01:59.68]to longer than two school buses," Pyenson added. [02:05.44]The researchers -- Pyenson, Jeremy Goldbogen and Graham Slater [02:12.00]-- believe that a change in the Earth's climate [02:15.08]caused whales to get so large only about 3 million years ago. [02:21.44]This change led to food becoming more easily available. [02:26.84]An oncoming ice age made the ocean water cold. [02:31.56]High winds mixed the water in the ocean faster than before. [02:37.36]Rain and snow made more nutrients run off from the land into the water. [02:44.56]The nutrients resulted in algae. [02:48.12]That attracted small fish and shrimp called krill. [02:53.54]The mixture of nutrients resulted in more food for large whales to eat. [03:00.12]Whales that were able to quickly eat a lot of food survived. [03:04.76]And they got larger. [03:07.32]Whales that had teeth instead of baleen, however, did not survive. [03:14.04]The largest baleen whales then produced other large whales. [03:19.68]Over the next three or four million years, [03:23.48]the baleen whales grew 10 times bigger than they had been, the researchers say. [03:30.44]It is possible that the largest whales are gone, the researchers said. [03:36.72]But, as they told the BBC, blue whales could grow even bigger [03:42.52]-- or they could slowly die – depending on Earth's changing climate. [03:49.20]And that's What's Trending Today. [03:53.12]I'm Dan Friedell. 更多听力请访问51VOA.COM