The eggs laid by birds come in an astonishing variety of shapes: ellipses in hummingbirds, spheres in owls, pointy ovoids in shorebirds and almost everything in between. Avian egg shape has fascinated humans for millennia - even Aristotle wrote about it - but we still lack the answer to this simple question: Why did different egg shapes evolve, and how?
The answer - that egg shape is related to flight ability, and that the egg membrane may play a critical role in determining shape - is the surprising finding of a new study in Science.
The study's lead author, Dr. Mary Caswell Stoddard of Princeton University, explains: "In contrast to classic hypotheses, we discovered that flight may influence egg shape. 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."
To unravel the mystery of egg shape, an international team of researchers used a multi-step, multidisciplinary process, applying tools from computer science, comparative biology, mathematics and biophysics. First, the team used photographs to analyze the shapes of nearly 50,000 eggs representing 1400 species. The eggs, from the online database of The Museum of Vertebrate Zoology at Berkeley, came from across the globe and were largely collected by naturalists in the late 19th and early 20th centuries. Using computer code, the researchers quantified each egg's asymmetry and ellipticity.
By combining the power of high-throughput digital image analysis with the wealth of data in the museum egg collection, the team was able to map the world of egg shapes. The most asymmetric egg? The Least Sandpiper's. The most elliptical? The watermelon-shaped egg of the Maleo, an Indonesian species that incubates its eggs by burying them in sun-heated sand or volcanic soils.
The team then developed a biophysical model to explain how processes in the bird's oviduct might generate different egg shapes. They started with the simple observation that an egg's shape is not determined by the shell: dissolve away the calcified shell and the shell-less, membrane-encased egg still retains its shape.
"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," says senior author Dr. L. Mahadevan of Harvard University.
Finally, the team used an evolutionary framework to test hypotheses about egg shape. Using a recently constructed phylogeny, or family tree, of birds, the researchers compared egg shapes across different bird lineages. In this analysis, they included details about nest type and location, clutch size, diet and flight ability.
"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 analysis revealed that birds tend to lay eggs that are more asymmetric and more elliptical if they are better fliers. The researchers suggest that as birds' bodies became adapted for powered flight, this resulted in morphological changes like reduced body size and a reduced abdominal cavity.
Birds may then have had a packaging problem, needing to fit a large volume into an egg (for healthy chick development) while still maintaining a sleek body for flight. They likely solved this by laying eggs that were pointier and more elliptical, which increases egg volume without increasing the egg's maximum width. The discovery that morphological constraints associated with flight may contribute to egg shape challenges the conventional wisdom that egg shape is largely influenced by clutch size or nest location.
The study opens new lines of inquiry. For example, birds are special in that they can lay pointy, asymmetric eggs. Such eggs appear almost nowhere else in vertebrates, except in some theropod dinosaurs. "The idea that powered flight and pointy eggs evolved around the same time is particularly intriguing. Moving forward, we are excited to explore how eggs changed shape during the dinosaur-to-bird transition," says Stoddard.
The paper, "Avian egg shape: Form, function and evolution," is published in Science. Other members of the scientific team include Dr. Ee Hou Yong (Nanyang Technological University), Dr. Derya Akkaynak (University of Haifa and Interuniversity Institute of Marine Sciences in Eilat) and Dr. Catherine Sheard (University of Bristol).