No, chocolate milk still does not come from brown cows. However, birds that are really good at flying come from eggs with a more pointy, elliptical shape, according to a surprising new study.

Researchers investigating how and why bird eggs acquire their remarkably various shapes examined images of more than 49,000 eggs representing about 1,400 species and used a computer to quantify them in terms of shape.

They discovered the shape of an egg for any given bird species appears to be driven in part by physiological features related to its capability for flight. Those with the higher hand-wing index – the best, most efficient fliers – had the most asymmetric, elliptical eggs.

The egg’s membrane, rather than its outer shell, may play a critical role in determining the many different shapes of eggs, said the study’s lead author, Mary Caswell Stoddard, assistant professor in the department of ecology and evolutionary biology at Princeton University.

“In contrast to classic hypotheses, we discovered that flight may influence egg shape,” Stoddard said. “Birds that are good fliers tend to lay asymmetric or elliptical eggs. In addition, we propose that the stretchy egg membrane, not the hard shell, is responsible for generating the diversity of egg shapes we see in nature.”

Egg shapes appear to have evolved as birds did, the scientists suggest. Birds with strong flight capability may have developed aerodynamic body shapes that in turn shape the configuration of their internal organs, including the reproductive organs.

Stoddard and her colleagues developed a biophysical model to explain how processes in the bird’s oviduct might generate different egg shapes. They suggest that as an egg moves through a bird’s oviduct, the egg’s flexible membrane determines the egg’s final shape. The egg is shaped by the membrane and also pressures in the isthmus, the portion of the egg canal just before the shell gland, where the membrane is coated with the layer of calcium carbonate that hardens to form the shell.

As birds’ bodies became adapted for powered flight, their body size and abdominal cavities were reduced, the researchers believe. Thus, an egg with a more asymmetric or elliptical shape might pass more easily through the oviduct of a bird with an aerodynamic, streamlined body.

Stoddard and her team used software to sort through digital images of the egg collection of the Museum of Vertebrate Zoology at the University of California, one of the largest in North America. Once egg shapes were organized and analyzed in an evolutionary context, the dataset provided strong evidence of a correlation between an egg’s asymmetry and ellipticity and a bird’s hand-wing index, a measure of a species’ flight capability.

“Variation across species in the size and shape of their eggs is not simply random but is instead related to differences in ecology, particularly the extent to which each species is designed for strong and streamlined flight,” says co-author Dr. Joseph Tobias of Imperial College London.

The study, Avian egg shape: Form, function and evolution,” was published in the journal Science .

Stoddard said the new study will lead researchers to explore how eggs changed shape during the dinosaur-to-bird transition.

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