Egg Evolution

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Egg-scuse me?

Thanks as always to Ed Yong:

Why Are Bird Eggs Egg-Shaped? An Eggsplainer

A new study points to a surprising reason for the varied shape of bird eggs—and shows that most eggs aren’t actually egg-shaped.

When Mary Caswell Stoddard started measuring bird eggs from hundreds of species, she wasn’t expecting to learn that most eggs are not egg-shaped.Think about an egg and you’ll probably conjure up an ellipse that’s slightly fatter at one end—the classic chicken egg. But chickens are outliers. Hummingbirds lay eggs that look like Tic Tacs, owls lay nigh-perfect spheres, and sandpipers lay almost conical eggs that end in a rounded point. After analyzing hundreds of species, Stoddard showed that the most common shape—exemplified by an unremarkable songbird called the graceful prinia—is more pointed than a chicken’s.“We mapped egg shapes like astronomers map stars,” Stoddard says. “And our concept of an egg is on the periphery of egg shapes.”

Beyond displacing chickens as the Platonic ideal of egg-dom, Stoddard’s data also helped her to solve a mystery that scientists have debated for centuries: Why exactly are eggs shaped the way they are?

Researchers have argued that pointy eggs are common to cliff-nesting birds because they roll in a circle and are less likely to tumble off an edge. Or that asymmetric eggs pack together more easily and would allow females with large clutches to incubate their broods efficiently. Or that spherical eggs are stronger and less prone to breaking, or use the least amount of shell for a given volume, which would be useful for birds that can’t get enough calcium in their diet.

“There are a lot of hypotheses, but no conclusive explanation or theory,” says Stoddard, who’s an evolutionary biologist based at Princeton University. “It was a good puzzle.”

To solve it, Stoddard teamed up with L. Mahadevan, a biophysicist at Harvard University who has studied “how leaves ripple, how tendrils coil, and how the brain folds, among other things.” He realized that all eggs could be described according to two simple characteristics—how asymmetric they are, and how elliptical they are…

Read thee whole article here.

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