RESEARCH: The wing of a bird may determine the shape of her eggs

Eggs can be big or small, round or pointy.

There are many explanations for the variety of birds’ egg shapes.

Mary Caswell Stoddard and her researchers of Princeton University analysed the shape almost 50,000 eggs from around 1,400 species of birds in museum collections. She thinks the shape of a female bird’s wing determines the shape of the eggs.

The researchers quantified their shapes according to two measurements: 1) the ellipticity, or length relative to width and 2) the asymmetry, whether one end was pointier and the other rounder.

While elliptical eggs can be symmetrical or asymmetrical, spherical asymmetric eggs – like the shape of a hot air balloon – don’t seem to exist in nature.

The researchers compared egg shape with lots of data about each bird species, but found no correlation with clutch size, environmental factors, or nest characteristics. One measurement however did correlate with egg shape: the hand-wing index, a measure of the shape of the wing.

Birds with a large hand-wing index tend to be more adept fliers, and also tend to have elliptical or asymmetric eggs. This might be because the demands of flying favour light, streamlined bodies. To maximise the volume of an egg that has to fit through a narrow pelvis and oviduct, eggs have to get longer.

white-breasted cormorant eggs

eggs of a white-breasted cormorant

African spoonbill eggs

eggs of an African spoonbill

Flightless birds, like ostriches, are more likely to have spherical eggs. Penguins are an exception, perhaps because their need to swim favours a streamlined body in a similar way to flight.

ostrich egg

an ostrich egg

But this idea has yet to be proved.

“At best this is correlational, not causative,” says co-author L. Mahadevan at Harvard University. “We don’t have direct mechanisms that tell us how one might lead to another.”

But why is a symmetrical egg advantageous for birds that don’t have demands for high-intensity flight? One possibility is that it minimises the shell material needed for the same volume.

Another part of the research looked at the biophysical mechanisms that create the egg’s shape. This isn’t determined by the shell: if you dissolve an egg shell in vinegar, the egg retains its shape due to its outer membrane. An asymmetric shape can be created through variations in the thickness or stretchiness of the membrane, or the arrangement of molecular constituents like collagen.

The research team didn’t have enough anatomical data to look for a correlation between pelvis width and egg shape, but that would be an the next test in their research. They also plan to study how the egg develops as it moves through the oviduct to look at the mechanisms that directly shape the egg.


New Scientist, 22 June 2017

Journal reference: Science, DOI: 10.1126/science.aaj1945

Photographer: Martina Nicolls





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