Egg-shape and climate

Daisy’s second published paper from her PhD research is published today in Scientific Reports and is Open Access and can be downloaded here.

The media release for this story is below and explains the main findings and their implications.

EggShape
All of these species lay their eggs in open-cupped nests. The three species at the top breed in moist habitats, whereas the three at the bottom breed in the dry and hot desert regions, and their eggs are noticeably rounder.

New scientific study examines why eggs are shaped as they are

  • The shape of a species’ egg is determined by the climate in which they typically breed, and the extent to which their nest protects the egg from the sun.
  • The classically-shaped eggs with which we are so familiar have a greater surface area to volume ratio than a spherical egg, allowing for a greater level of gas exchange between the embryo and the environment.
  • As Easter approaches and the shops are full of chocolate eggs, it is a good time to be wondering why bird eggs are egg shaped and not nice round balls.

A new study published today in Scientific Reports by scientists from Macquarie University helps us understand why the classic egg shape evolved in birds and explains some of the species variation in egg shape.

The study examined egg shape in over 300 species of Australian birds, that live across some of the most extreme climates on earth, from the wet and humid tropics, to the dry and hot inland deserts.

The study found that the shape of a species’ egg is determined by the climate in which they typically breed, and the extent to which their nest protects the egg from the sun.

The most spherical eggs were typically found in desert species nesting in open nests, where the embryo inside the egg is likely to be damaged by the heat from the sun and dehydration. The most elongated or ‘egg-shaped’ eggs were found in moist environments, or in species that have enclosed nests, which are not exposed to the elements.

Lead author Daisy Duursma from the Department of Biological Sciences said as Easter approaches and the shops are full of chocolate eggs, it is a good time to be wondering why bird eggs are egg shaped and not nice round balls.

“There is a tension between allowing oxygen to get inside the egg so the embryo can breathe, but at the same time not allow too much moisture to escape and risk dehydration”.

The classically-shaped eggs with which we are so familiar have a greater surface area to volume ratio than a spherical egg, allowing for a greater level of gas exchange between the embryo and the environment.

The classic egg-shaped eggs are laid in relatively protected environments and prioritise gas exchange for the developing embryo to breathe, in a context where dehydration is less of a concern. The opposite is true in harsh environments where eggs are exposed to the elements.

“The egg shape is an adaptation that balances the need for the embryo to breathe without losing too much water, and can be nicely seen by looking at the eggs that are laid by different species across the diverse continent of Australia,” concluded Professor Simon Griffith, the leader of the project.

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