Most eggs in nature, such as those from common poultry, are asymmetrical ovoids, often described as elliptical or pear-shaped. Despite this typical form, many diverse animal species produce eggs that are perfectly or near-perfectly spherical. A “round egg” refers to shelled eggs, whether they possess a hard, calcified exterior or a soft, leathery one, distinguishing them from the gelatinous spawn of fish or amphibians. This spherical shape represents a distinct biological strategy, appearing across the animal kingdom in certain birds, various reptiles, and a unique order of mammals. This geometry reflects specific evolutionary and biomechanical pressures related to the animal’s physiology and nesting environment.
Round Eggs in the Avian World
Most bird species lay asymmetrical eggs, featuring one end broader than the other, but a few avian groups deviate toward the spherical shape. Birds that nest in enclosed spaces, such as tree cavities or burrows, often produce significantly rounder eggs than open-nesting species. This is a direct result of the reduced risk of the egg rolling out of a contained nest site.
Species like the Little Bee-eater, which nests in tunnels, lay almost perfectly spherical eggs, as do many species of Kingfishers that excavate nests in riverbanks. Owls, particularly the Brown Hawk-Owl, are also known for their nearly round eggs. The lack of a pointed end means the eggs do not need the stabilizing shape found in open-nesting birds, such as the Common Murre, whose conical eggs roll in a tight circle.
The eggs of cavity-nesting birds are also commonly white, lacking the pigments required for camouflage in open nests. Since the eggs are laid in a secure environment, the evolutionary pressure shifts away from camouflage and stability toward maximizing volume or optimizing shell structure. This results in a simple shape highly adapted to its specific, protected nesting location.
Reptilian and Monotreme Egg Shapes
In contrast to birds, many species of reptiles commonly produce spherical or near-spherical eggs. This is particularly true among hard-shelled eggs laid by turtles and tortoises, where the round shape is the standard morphology. The common snapping turtle and various tortoise species, for example, deposit eggs that are nearly equidistant in all dimensions.
The eggs of most reptiles also differ fundamentally from those of birds by possessing a soft, flexible, or leathery shell. This shell is porous and allows for the exchange of water with the surrounding substrate. This is a characteristic feature of many reptile eggs, including those of numerous gecko species, such as the Uroplatus genus, which lay hard, white, spherical eggs.
The monotremes, a unique order of egg-laying mammals that includes the Platypus and the Echidna, also produce spherical eggs. These eggs are small, typically around 13 to 15 millimeters in diameter, and are covered by a soft, compressible, leathery shell, similar to those of many reptiles. The female Platypus will lay her one to three eggs in a nesting burrow, while the Echidna deposits its single egg into a temporary pouch.
The Biology of Egg Shape
The specific shape of an egg is a compromise between several competing biomechanical and evolutionary factors, all driven by the organism’s physiology and environment. The simple geometry of a sphere is the most efficient shape for maximizing internal volume relative to external surface area. This means a spherical egg requires the least amount of calcium carbonate to form a shell around a given volume of contents, which is advantageous for species with limited access to calcium resources.
However, the oviduct, the canal through which the egg travels, imposes physical constraints that often push the egg toward a more elongated or asymmetrical form. Studies suggest that the final egg shape is largely determined early in the oviduct by the formation of the egg membrane and the pressure exerted upon it. Variations in the membrane’s thickness and elasticity influence how the egg expands, leading to the wide spectrum of shapes seen in nature.
The evolution of more elliptical or asymmetrical eggs in high-flying birds, for instance, is linked to the need for a streamlined body shape, which results in a narrower oviduct. The elongated egg shape allows these birds to maximize the egg’s contents without increasing its width, enabling it to pass through a constrained pelvic canal. Conversely, species with less demanding flight requirements or those with secure nests face fewer such constraints. This allows the egg to default to the energetically efficient, volume-maximizing spherical shape, which is inherently stable in a static environment and minimizes the chance of breakage.