Why are eggs shapes widely different? Scientists solve the puzzle!

Owl’s eggs are almost perfectly round, while hummingbird eggs look like teeny tiny watermelons. Fowl’s eggs are spherical and shorebirds’ are pointy.  And everything in between. So why are eggs shape so widely different?


A flippant answer could be something in the line of “That’s should be a no-brainer. Birds are not of one species, nor family for that matter so it’s expected that their eggs should be widely different as well.”


The reality is something much more practical as evolutionary scientists led by Dr Mary Caswell Stoddard of Princeton University have found out.


 The shape of an egg is related to flight ability, and the egg membrane may play a critical role in determining the shape.


In case you’re wondering what the fuss is about,  the topic has fascinated enthusiasts for millennia going back to the time of Aristotle. “Eggs are not symmetrically shaped at both ends: in other words, one end is comparatively sharp, and the other end is comparatively blunt; and it is the latter end that protrudes first at the time of laying. 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.


A number of theories had been advanced since then to explain the phenomenon, which usually is based on the advantages of having a particular egg shape over another. For instance, spherical eggs are uniformly strong and would be robust to incidental damage in the nest; conical eggs may protect the blunt end from debris contamination/increase resistance to impacts. Some theories have also focused on flight as having an influence on egg shape indirectly through the morphology of the pelvis, abdomen, or oviduct.


With regard to the present findings, Dr Stoddard 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.”


Mary Casswell Stoddard in the bird collection at Princeton University. (Denise Applewhite)


In the study, the team studied an impressive 50,000 eggs from 1400 bird 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. The team also used high-throughput digital image analysis to analyse the data from the museum egg collection, which enable the mapping of 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.


Finally, the researchers used an evolutionary framework to test hypotheses about egg shape. Using a recently constructed phylogeny (family tree) of birds, they compared egg shapes across different bird lineages.


Click here to read the published article.




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