Have you noticed that the avian eggs that we find in nature have different shapes? From spherical to elliptical or asymmetric (pointed). For hundreds of years, scientists have wondered why, and now a team of researchers led by Princeton University (United States) believes they have found the answer.
The shape of the egg would be linked to the flight capacity of birds – those that are good at this activity tend to lay asymmetric or elliptical eggs. Likewise, the egg membrane – not the shell – would play a fundamental role in determining this diversity in the design of the eggs that we can find.
For a long time theories suggested that, in most cases, the shape of the egg was related to the life history of the bird or the environment in which it lived . However, the Princeton study, published by the journal Science , suggests that the shape of the eggs would actually depend on how each bird has adapted to the act of flying.
The team of scientists, led by Mary Caswell Stoddard, a researcher in the Department of Ecology and Evolutionary Biology at Princeton University , studied the photographs of 49,175 bird eggs from around the planet and collected over a century, representing some 1,400 species –which represents 14% of the 10,000 that exist–. With the help of a customized computer program, they obtained the measurements of the eggs and observed two main characteristics in them: the asymmetry – or how pointed the eggs were – and their ellipticity – how much they deviate from the perfect sphere shape .
The different shapes of eggs
With the results, they made a map that collected the different egg shapes. Among them there were ellipticals –like those of the maleo talégalo–; others, asymmetrical – such as the giblets -; with both characteristics –those of the common murre-; and finally those that do not have any of them – those of the owl, which form a perfect sphere .
Then they compared these different shapes taking into account factors such as, for example, what each species of bird eats, where they build their nests, if they are large or if they are good fliers. Of these characteristics, what seems to influence the design of the egg, according to their conclusions, is that the birds are better adapted to flight.
According to the researchers, as the birds evolved and their bodies adapted to fly, changes in their morphology occurred, such as a smaller body size. This entailed a smaller abdominal cavity, so that elliptical or pointed eggs allow them to increase their volume without the need to increase the maximum width of the egg. And those birds that were better fliers evolved in that direction, since they spend more time of their lives in the air.
References:
Mary Caswell Stoddard, Ee Hou Yong, Derya Akkaynak, Catherine Sheard, Joseph A. Tobias, L. Mahadevan. (2017). Avian egg shape: Form, function, and evolution. Science, 2017 DOI: 10.1126 / science.aaj1945
