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Whales getting bigger and bigger

Study looks at which creatures heed evolution rule

The Columbian
Published: March 6, 2015, 12:00am

A new study says sea creatures follow a hypothesis called Cope’s rule, which states that in a lineage, animals will get heftier as time goes on. Published last month in Science, the study finds that the average size of marine mammals has increased by 150 times since the Cambrian period 542 million years ago.

So why are all the whales getting chubby? They’re just adding weight to a theory posited back in the 1800s: Edward Drinker Cope and his colleagues had noticed that many ancient fossils showed much smaller animals than their modern descendants. They decided this might be an evolutionary trend, not a coincidence.

It makes sense: Larger animals would be better at avoiding predators, and at catching some prey of their own; they’d be more equipped to store body fat and survive cold temperatures and famine; and they’d be able to develop and support larger, more complex brains.

Of course, Cope’s rule didn’t turn out to be a universal truth. If you’ve ever seen a fossil of a giant sloth, for example, you know that these massive sluggards went extinct in favor of much, much smaller cousins. And dinosaurs are another great example: Scientists now believe that birds, the surviving descendants of a select lucky few dinosaur species, are around because they shrank faster than their compatriots, allowing them to seek out new resources and habitats.

But in the sea, it seems, Cope’s rule rules supreme. From the BBC:

It appears that the explosion of different life forms near the start of that time window eventually skewed decisively towards bulkier animals. Today’s tiniest sea critter is less than 10 times smaller than its Cambrian counterpart, measured in terms of volume; both are minuscule crustaceans. But at the other end of the scale, the mighty blue whale is more than 100,000 times the size of the largest animal the Cambrian could offer: another crustacean with a clam-like, hinged shell.

To see if the theory held up in the murky deep, Stanford University research Noel Heim analyzed body size data from every available source.

“We’ve had at least 50 high school students working on it over the last five years,” Heim told the BBC.

While not every single kind of animal got bigger over time (arthopods, such as crustaceans, have actually gotten smaller), those that were larger had many more evolutionary offspring. In other words, more modern species descended from common ancestors who were larger in size.

To confirm that evolution really did favor size, Heim and his team ran computer simulations to see which species would survive, flourish and reproduce given different parameters. Some of the simulations allowed for random size variation, and some gave larger animals an advantage over their smaller neighbors. But the simulation that made size an advantage matched the fossil record the best, which indicates that the size change wasn’t random.

Not to worry: It’s not likely that whales will keep growing and growing forever.

“In the short term, there’s certainly evidence (the animals) will continue to get bigger,” Heim told Live Science. But over time, it would get more difficult for the marine creatures to feed their gargantuan bodies — so this evolutionary advantage definitely has its limits.

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