Why do sea turtles, softshell turtles and snappers lay round eggs whereas pond sliders and many other turtles have elliptical eggs shaped more like those of a chicken? This question came up recently in a discussion I had with another turtle biologist, Jeff Lovich of the U.S. Geological Survey.
A similar question has been asked about birds, most of which also have round or elliptical eggs, depending on the species. A few lay cone-shaped eggs. Seeking answers to such questions in order to explain why the world around us is the way it is is what makes ecology intriguing.
Turtle biologists have not fully solved the mystery of egg shape, but ornithologists have come up with some answers. Cassie Stoddard (Princeton University) and colleagues addressed the question in the journal Science by examining photographs of nearly 50,000 bird eggs that have been deposited in museums. Of the approximately 10,000 species of birds in the world, the researchers examined the eggs of 1,400 species, representing each of the 35 major bird groups, called orders. Songbirds, waterfowl, penguins, owls, pelicans, parrots and woodpeckers are well-known representatives of bird orders familiar to most people.
The researchers set about their task of comparing egg shapes to the types of birds that laid them, taking thousands of measurements of eggs and relating them to the size, shape and wings of birds that carried them. Which shape is better for a particular species could depend on a variety of factors, such as the type of nest or the number of eggs. But one relationship seemed to be consistent within the various bird orders. In the simplest terms, they found that round eggs are more likely to be found in birds that are not strong fliers. In those that are strong fliers, the eggs tend to be more elongated, which provides more volume, and therefore more room for yolk and albumen, for the width of the egg. As with many scientific studies, exceptions exist and must be explained in other ways.
Nonetheless, the conclusions from the global comparison of bird species with egg shapes led to a general conclusion that a bird’s ability to fly was the basic feature of importance. However, a handful of other hypotheses have been proposed by other scientists in the past. Perhaps the most intriguing one was an idea based on the physical fact that a round egg would roll down a slope (even a turtle biologist would know that). Hence, a spherical or elliptical egg laid on the edge of cliff would not be an ideal plan. So the hypothesis was developed that a more adaptive approach for birds nesting on cliff faces would be to lay cone-shaped eggs.
Support for this hypothesis, although not a universal explanation for what other birds do, has been presented for cliff-nesting birds known as murres. Murres are in the same bird family as puffins. Without question, murre eggs look different from most other bird eggs in being roughly the shape of a pear but even more pointed. Try rolling an apple and a pear off of a counter top and see which one is more likely to fall to the floor. Likewise, like a pear on a counter, a murre egg that gets bumped out of a nest would most likely roll in a circle and not plummet down to the rocks below.
A fascinating by-product of the Science article was the development of an illustrated website that shows the process and conclusions of the study. It’s definitely worth a look. Interestingly, the website ends the presentation with the age-old question, what came first, the chicken or the egg? That part of the puzzle is already solved. Eggs were on earth millions of years before chickens. Questions regarding turtle eggs have not made as much progress, although Jeff and I have ruled out any aspect of flying as an explanation.