How Eggs Get Their Shapes

Mahadevan is also a Core Faculty Member of the Wyss Institute for Biologically Inspired Engineering at Harvard University.

The researchers began by plotting the shape — as defined by the pole-to-pole asymmetry and the ellipticity — of some 50,000 eggs, representing 14 percent of species in 35 orders, including two extinct orders.

 Image courtesy of L. Mahadevan/Museum of Vertebrate Zoology, Berkeley

The researchers found that egg shape was a continuum — with many species overlapping. The shapes ranged from almost perfectly spherical eggs to conical-shaped eggs.

So, how is this diverse spectrum of shapes formed?

Researchers have long known that egg membranes play an important role in egg shape — after all, if an egg shell is dissolved in a mild acid, like vinegar, the egg actually maintains its shape. But how do the properties of the membrane contribute to shape?

Think of a balloon. If a balloon is uniformly thick and made of one material, it will be spherical when inflated. But if it is not uniform, all manner of shapes can be obtained.

“Guided by observations that show that the membrane thickness varies from pole to pole, we constructed a mathematical model that considers the egg to be a pressurized elastic shell that grows and showed that we can capture the entire range of egg shapes observed in nature,” said Mahadevan.

The variations of shape come from the variation in the membrane’s thickness and material properties and the ratio of the differential pressure to the stretchiness of the membrane.

The next question is, how are these forms related to the function of the bird?

The researchers looked at correlations between egg shape and traits associated with the species of bird, including nest type and location, clutch size (the number of eggs laid at a time), diet and flight ability.

“We discovered that flight may influence egg shape,” said lead author Mary Caswell Stoddard, Assistant Professor of Ecology and Evolutionary Biology at Princeton University and former Junior Fellow in the Harvard Society of Fellows. “To maintain sleek and streamlined bodies for flight, birds appear to lay eggs that are more asymmetric or elliptical. With these egg shapes, birds can maximize egg volume without increasing the egg’s width – this is an advantage in narrow oviducts.”

So an albatross and a hummingbird, while two very different birds, may have evolved similarly shaped eggs because both are high-powered fliers.

Broad-tailed hummingbird egg

Credit: Museum of Comparative Zoology and Harvard University

“It’s clear from our study that variation in the size and shape of bird eggs is not simply random but is instead related to differences in ecology, including the amount of calcium in the diet, and particularly the extent to which each species is designed for powerful flight,” says coauthor Dr. Joseph Tobias from Imperial College, UK.

Next, the researchers hope to observe the egg laying process in real time, to compare it to and refine their model.

This paper was coauthored by E. H. Yong, D. Akkaynak and C. Sheard. The major funding sources for this work include Princeton University, the L’Oréal USA For Women in Science Fellowship, the Harvard Society of Fellows, the Milton Fund, Nanyang Technological University, the Oxford Clarendon Fund, the Fulbright Commission, the Natural Environment Research Council, the MacArthur Foundation and the Radcliffe Institution.