It's a mystery that goes back to the days of Aristotle, flummoxing biologists and mathematicians for centuries: Why do birds' eggs come in so many different shapes and sizes?
Why are owl eggs almost perfectly round, while hummingbird eggs look like tiny watermelons? And why are still other eggs pointy, but only on one end?
Now researchers have made a serious attempt to answer this deceptively simple question. Their conclusion: Egg shape is related to how much time a bird spends in flight, according to a report in Friday's edition of the journal Science.
Scientists have long hypothesized about the reasons for eggs to have the shapes they do. Even Aristotle had a theory: "Long and pointed eggs are female; those that are round, or more rounded at the narrow end, are male," the Greek philosopher wrote in "The History of Animals" in the fourth century B.C.
More recently, scientists have proposed that there were practical reasons for eggs to have certain shapes. For instance, perhaps eggs that are pointy on one side offer an evolutionary advantage to birds that nest in tall cliffs because such eggs will roll in a tight circle, preventing catastrophic tumbles over the cliff edge.
But these studies have mostly been speculative, or used small sample sizes that focused on limited types of birds.
The new study, led by evolutionary biologist Mary Stoddard from Princeton University, considered vast amounts of data to understand what, exactly, determines the shape of an egg.
Stoddard and her colleagues started with a database of egg images from the Museum of Vertebrate Zoology at UC Berkeley. The pictures were taken in the late 19th and early 20th centuries, in places all over the world.
The research team developed software capable of scanning these images and extracting detailed information on each egg's size and shape. The software analyzed a total of 13,049 pictures containing 49,175 individual bird eggs. These eggs represented 1,400 different species, including members of every bird family.
Next, Stoddard's team mapped out the full range of egg shapes, graphing them according to their asymmetry (how "pointy" they were on one side) and their ellipticity (how elongated they were).
Then they consulted the map to see if characteristics like body weight, nest type and wing shape were correlated with an egg's location on the graph. Two of those factors stood out.
The first was total egg size. This made sense to the researchers because the larger an egg, the more it has to be compressed to fit through its mother's narrow oviduct.
The second, to their great surprise, was the bird's hand-wing index. This is a measure of how pointed and elongated a bird's wings are, and biologists use it as a proxy for flight strength, which is higher in birds that fly frequently or over large distances.
For instance, birds that fly a lot, like barn swallows, tend to have eggs that are more elongated or pointy. Birds that fly less often or more weakly, like the screech owl, have eggs that are more round and symmetric.
This connection between egg shape and flight strength prompted the researchers to explore an idea that was first proposed in 1991 but had been slow to gain traction: As birds evolved to become stronger fliers, their bodies may have become more streamlined. This, in turn, could have affected the shape of their eggs.
To understand how this could happen, researchers created a biophysical model to test the forces an egg might experience while it's being shaped in the oviduct. The model showed that, given the right conditions, those forces could produce all of the egg shapes included on the graph.
As the work became increasingly complex, so did the makeup of the research team. By the end, Stoddard had assembled a diverse team with expertise in evolutionary biology, computer science, mathematics and physics.
Despite the overwhelming number of eggs analyzed in the study, it doesn't prove that adaptations for flight were responsible for the differences in egg shape — the correlation could be simply a coincidence.
Charles Deeming, an ecologist who studies bird reproduction at the University of Lincoln in England and who was not involved in the study, said that pelvis shape, in particular, could be critical in determining egg shape. With further research, he said, scientists may be able to narrow down a more specific link between bird anatomy and egg shape.
The study authors were careful to point out that they've just gotten started. Indeed, their work raises more questions than it answers.
Stoddard acknowledged that a lot of experimentation would be needed to see whether their models accurately describe what's happening in real life. She already has some ideas as to how to tackle this next question.
Though there is a lot of work left to be done, University of Cambridge behavioral ecologist Claire Spottiswoode said the results represent the beginnings of a general theory to explain trends in egg shape across all types and species of birds.
"It's tremendously exciting," said Spottiswoode, who was not involved in the study. "It's the first time that we've got a really unified theory for both how and why egg shape variation has evolved."