How Do Eggs Get Their Shapes? Scientists Think They've Cracked It : The Two-Way Eggs evolved over 300 million years ago and now come in all kinds of shapes, from Tic Tacs to teardrops to pingpong balls. After studying some 50,000 eggs, a team of researchers thinks it knows why.
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How Do Eggs Get Their Shapes? Scientists Think They've Cracked It

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How Do Eggs Get Their Shapes? Scientists Think They've Cracked It

How Do Eggs Get Their Shapes? Scientists Think They've Cracked It

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KELLY MCEVERS, HOST:

One of nature's most efficient life support systems is the egg. And over millions of years, eggs have taken on lots of different shapes, especially among birds. Biologists have been trying to figure out for a long time why that is. What is it that determines the shape of a bird egg? NPR's Christopher Joyce reports on a mystery solved.

CHRISTOPHER JOYCE, BYLINE: Chicken eggs are boring. Hummingbirds have eggs like Tic Tacs. Birds called murres produce eggs shaped like big teardrops. Some eggs are more like ping-pong balls. In the 19th century, collectors almost wiped out many species; so enraptured were they with bird eggs.

Biologist Mary Caswell Stoddard at Princeton University was in rapture, too. She'd heard the shape theories. Cone-Shaped eggs don't roll away. They roll in a tight circle. Maybe that's good for birds that nest on cliffs. Or elliptical eggs like slightly flattened spheres - maybe they incubate better in nests. Stoddard looked at nearly 50,000 eggs and cross-checked them with 1,400 bird species. And she says, nope, eggs are shaped the way they are because...

MARY CASWELL STODDARD: Surprisingly, egg shape appears to be related to flight ability in birds.

JOYCE: Take chickens - lousy flyers and more oval eggs. But long-distance migrating birds...

STODDARD: We find that good flyers, birds that seem to be adapted for strong, powerful flight tend to lay more asymmetric or more elliptical eggs. And this came as quite a shock to us.

JOYCE: But it made sense. Birds that evolved for powerful or long-distance flight needed a body to match - sleeker, more streamlined. That meant less internal cargo space.

STODDARD: We think that the bird's abdominal cavity becoming smaller, the internal organs becoming compressed - this has an effect on the egg-shaping process.

JOYCE: Think of a banana compared to a grapefruit - similar volume, sleeker shape. Even penguins, who are marathon flyers through the water, have elongated eggs. Soddard describes how she cracked the egg mystery - her words, not mine - in the journal Science. Christopher Joyce, NPR News.

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