Birds’ Flying Ability, Shape Of Their Eggs Could Be Linked, Study Suggests

It is easy to tell a bird’s egg when you see one, even though eggs from different birds can look quite different from each other. And while we have known these differences exist, perhaps since we have known of eggs, we are still not sure why and how these different shapes evolved. In a study published Friday, a small international team of researchers suggests an answer to the long-standing question: it is related to the birds’ flight capabilities.

Eggs have a hard outer shell and a thin, film-like flexible membrane, and the researchers proposed in the study that it is not the shell but the membrane that actually shapes the egg. After the egg leaves the ovary, it moves into a tube-like organ called the oviduct. The end-portion of the oviduct is called the isthmus, and after that, the egg goes through the shell gland where the thin membrane gets a coating of calcium carbonate that hardens to become the shell.

Senior author of the study, L. Mahadevan of Harvard University, said in a statement: “By adjusting two basic properties — changes in the thickness of the egg membrane as a function of location, and a pressure jump across the membrane — we show that our model can produce a wide variety of egg shapes, encompassing the entire range of observations. This mechanistic approach to shape has a long history in biology, and our work suggests how tinkering with just two functional forms could allow evolution to move through the two-dimensional morphospace of egg shapes.”

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Mary Caswell Stoddard of Princeton University led the team —seven researchers from four continents — in studying the shapes of almost 50,000 eggs from over 1,400 bird species, which represent 14 percent, or one in seven, of all avian species that exist on Earth. And their analyses led them to suggest “that birds with a strong flight capability have developed aerodynamic body shapes that have influenced the configuration of these birds’ internal organs, including the reproductive system,” according to a statement Thursday on the Princeton website.

The varying internal configurations among different bird species would affect the shapes and pressure conditions of the oviduct and the isthmus, and also properties of the membrane, the study’s authors say. So, for instance, a bird with an aerodynamically streamlined body could have an egg more elliptical than ovoid, which could pass easier through a narrower oviduct while still containing enough nutrients for the embryo within.

“The question of how eggs get their shapes is really wide open. Now that we’ve analyzed an enormous dataset of eggs, we can ask the question of whether there is a mechanistic model that makes sense, that can generate all the diversity in shape that we have observed. And the answer is yes. … Just by adjusting the membrane’s properties and the oviduct’s pressure, we can derive the entire world of egg shapes. And we think it’s biologically realistic, what we’ve proposed,” Stoddard said in the statement.

Birds, who are the only surviving dinosaurs, have the ability to lay asymmetric and pointy eggs that are seen rarely among vertebrates since the time of their extinct relatives.

Catherine Sheard from the University of Bristol, a co-author on the study, said in a statement: “Our work opens up several exciting avenues for future research, such as the coevolution of egg shape with the transition to flight during the time of the dinosaurs.”

The researchers studied digitized versions of the eggs that make up the collection of the Museum of Vertebrate Zoology at the University of California, Berkeley. The assortment of eggs was collected from across the world between the years 1850 and 1950.

The study, which appeared online in the journal Science, is titled “Avian egg shape: Form, function, and evolution.”