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A surprisingly simple explanation for the shape of bird eggs

A sandpiper’s egg is shaped like a teardrop, an owl’s like a golf ball, and a hummingbird’s like a jelly bean. Now, for the first time, scientists have a convincing explanation for this stunning diversity: The shape of a bird’s egg depends on how much its species flies.

“It’s nice to see a complete story of egg shape,” says Mark Hauber, a behavioral ecologist at Hunter College in New York City who was not involved in the work. “[It’s] an instant classic article.”

Princeton University evolutionary biologist Mary Stoddard has long been fascinated that eggs are so diverse, even though they all basically do one thing—nourish and protect the developing chick. Fortunately, over the past century, the Museum of Vertebrate Zoology in Berkeley, California, has amassed thousands of egg shells from 1400 species and put digital photos of them online.

Stoddard and her colleagues wrote a computer program, Eggxtractor, that picks out the egg in any image and measures its length, width, and shape. The team used those measurements to determine how far from perfectly spherical each of nearly 50,000 eggs was—that is, how pointy or elongated it was. Some eggs are both pointy and elongated, some are one but not the other, and some are neither. But no eggs are short and pointy—approximately the shape of a hot air balloon.

Knowing that an egg’s shape is determined not by the shell itself but by the membrane inside, Stoddard worked with Harvard University physicist L. Mahadevan and his student Ee Hou Yong to come up with a mathematical representation based on the membrane’s properties and how much pressure it received—from the developing chick on the inside. They then used their model to create scores of egg shapes by altering the membrane’s stiffness and changing the pressure. “Adjusting these [features], allows us to generate the entire diversity of egg shapes that we observe in nature,” Stoddard says. The only one a real bird couldn’t easily generate: an egg shaped like a hot air balloon.

This alone impresses experts in the field. “What’s cool is you have the [overall] formula for egg shape,” says Martin Sander, a paleontologist at Bonn University in Germany.

When Stoddard and her colleagues made a family tree of 1000 bird species, they realized that each group of birds tended to have a characteristic egg shape. But there seemed to be little correlation between that shape and nest type, nest location, or the number of young in a clutch—all previous proposed explanations for the shape of eggs.

As part of the work, the team also evaluated whether a proxy for flying ability—the ratio of a bird’s wing length to its width—had an effect. “There was an obscure hypothesis that egg shape could be related to flight ability that no one had paid any attention to,” Stoddard says. To her team’s surprise, flying ability matters, they report today in Science. Good flyers like sandpipers and murres tend to lay eggs that are more elongated and more asymmetrical—a bit like the shape of a Zeppelin—likely because lots of time in the air requires lightweight, compact bodies. Meanwhile, birds that spend little or no time in the air, like tropical pittas and trogons, have more spherical eggs.

The reason, Stoddard speculates, is that round eggs require a wider pelvis than ones that are more elongated. Thus, just as birds that spend most of their time airborne have evolved more streamlined bodies and lighter, small skeletons, they have also evolved streamlined egg shapes to fit through the pelvis, she says.

“I’m surprised, but I’m also convinced,” Sander says. “Based on this data set, they are making a very good case.” He and Hauber are pleased that now they can guess how good a flyer a species is just by the shape of its eggs. “You can take this study and look at the egg and vey immediately get some general information,” Sander says.

The work is significant on two levels, Stoddard says. For one, understanding egg shape and the role the membrane plays “could be of value to the egg industry,” she says, perhaps by helping it create more durable eggs. But for her, just solving the puzzle of egg diversity is reward in itself. “Eggs aren’t just a favorite breakfast food,” she explains. A specialized egg, like that of modern birds, made it possible for developing young to survive on land, she notes, and thus allowed our land vertebrate ancestors to leave the seas about 360 million years ago. “They kick-started a revolution.”

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doi:10.1126/science.aan7006

Elizabeth Pennisi

Liz is a senior correspondent covering many aspects of biology for Science.

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