Eggs are complex structures designed to allow the embryo to develop outside of the mother’s body. An animal egg (Latin, ovum) provides a protective shell in which an embryo can develop. In most birds, reptiles, insects, mollusks, fish, and monotremes (mammals that lay eggs instead of giving birth to live young), the egg contains an ovum or, if fertilized, a zygote. A zygote results from fertilization of an ovum, and develops into an embryo.
After the egg is appropriately produced, it is expelled from the mother’s body. If fertilized, it further develops until the organism can survive on its own. When adequately developed, the embryo breaks out of the egg’s shell to begin its life in the outside world, a process called hatching. Some embryos have a temporary egg tooth with which to crack, or break the egg’s covering or eggshell. A few days after hatching, the egg tooth is no longer needed and is absorbed. Baby animals that have just hatched are called hatchlings, and standard names are used for babies of each particular species, such as “chick” for a baby chicken. Animals that lay eggs are called oviparous and the study of eggs (and also the hobby of collecting eggs, commonly bird eggs) is termed oology. The embryo develops from the small germinal disc located on the egg yoke edge.
Eggs typically have an outer covering called a shell consisting of calcium carbonate. Reptile eggs, bird eggs, and monotreme eggs, which are laid on dry land, are all surrounded by a protective eggshell that can be either flexible or hard and inflexible, as is the familiar chicken egg. The shell of a bird’s egg is a remarkable piece of engineering. It is very lightweight, but the shell is often so strong that it takes some birds over a day to chip their way through it to the outside world.
A shell membrane separates the eggshell from the albumen, or egg white, a gelatin-like substance that provides food for the growing embryo. Two layers of albumen exist, a thick albumen near the yolk and a thin layer near the eggshell. Between the eggshell and the shell membrane is a space called an air chamber designed to hold air. In the center of the egg is the yolk, a yellow liquid, and the germ spot, the zygote. A string like-structure inside at each end of the shell, called the chaloza, is attached to the yolk or nucleus to hold the yolk in the same position no matter how the egg is turned.
The specific construction of bird eggshells varies enormously. For example, duck eggs are oily and waterproof, cormorant (medium-to-large seabirds) eggs are rough and chalky, tinamou eggs are shiny and very colorful, and emu and cassowary eggs are rough, grainy and heavily pitted. The small pores in the hard eggshells allow the embryo to breathe oxygen and expel carbon dioxide. The domestic hen’s egg has around 7,500 microscopic pores and some have as many as 17,000. The small pores also allow pathogens to enter, a problem solved in most vertebrate eggs by the production of lysozymes, an effective anti-bacterial enzyme, and several membranes.
The membranes that surround these eggs are typical of all amniotes (terrestrial tetrapods or four footed creatures that lay eggs, including mammals). Eggs laid on the dry land or in nests are usually kept by the mother within a temperature range that is favorable to the embryo’s development.
The largest known egg is the 1.5 kg (3.3 lb) ostrich egg, although some extinct dinosaurs had larger eggs. The Bee Hummingbird produces the smallest bird egg known, which weighs half a gram. Eggs laid by some reptiles and most fish can be even smaller. Amphibians, including frogs and toads, do not have a hard protective egg shell, nor do fish. The eggs of these latter groups are jellylike.
Eggshells have an amazing variety of solid colors that range from white to bright blue, purple, and even black. They may also have a mixture of colors called spotting. The color of individual eggs is both environmentally influenced and genetically inherited through the mother, suggesting that the gene responsible for egg pigmentation is on the sex-determining W chromosome (female birds are WZ, males ZZ). The default color of all vertebrate eggs is white, produced by the calcium carbonate from which shells are constructed. The green or blue color comes from biliverdin pigments and a brown “ground” color from zinc chelate. Protoporphyrin (a protein that imparts color to the egg) produces a reddish brown color or a spotting paint.
In species which nest in large groups, such as the Common Guillemot, each female’s eggs have very different markings which allow females to identify their own eggs on the crowded cliff ledges on which they breed. Birds typically have white eggs except in certain ground-nesting birds that use egg markings for camouflage. Examples include the colored Charadriiformes (a diverse order of small to medium-large birds, most of which live near water) and non-passerines (birds that have feet designed for specific functions, like webbed feet for swimming or feet for grabbing prey).
When one bird species lays its eggs in the nest of another it is called bird brood parasitism. Some brood parasitic birds, such as cuckoos, have egg coloration that matches eggs of the host passerine (perching birds or, less accurately, songbirds; includes more than half of all bird species). In some cases, the host’s eggs are removed or eaten by the invading female, or expelled from the nest by her chicks. Brood parasites include the cowbirds and many Old World cuckoos. In contrast to the general rule, most passerines lay colored eggs, even if there is no camouflage requirement for cryptic colors (coloration designed for camouflage, from crypsis meaning hiding).
The protoporphyrin markings on passerine eggs function to reduce shell brittleness by acting as a solid-state lubricant. If insufficient calcium exists in the bird’s feed, the eggshells may be too thin, especially in the circle area around the broad end. Protoporphyrin speckling compensates for the brittleness caused by thin eggshells, and protoporphyrin increases inversely to the amount of calcium in a bird’s diet. For this reason eggs laid later in a clutch are more spotted than earlier ones because the female’s calcium store is often increasingly depleted with each egg produced.
It was once believed that the color was applied to the shell immediately before laying, but research has shown that coloration is an integral part of shell development, and the same protein is also responsible for depositing calcium carbonate, or the protoporphyrins, when a lack of that mineral exists.
The Egg Shape
Most bird eggs have an oval shape, with one end rounded and the other end more pointed. This shape results from the egg being forced through the bird’s oviduct by muscles that contract behind the egg, pushing it forward. The egg’s wall is often still slightly malleable when expelled, and the pointed end forms at the back. Cliff-nesting birds often have highly conical eggs because this design of the egg makes it less likely to roll off the cliff. Instead, they roll around in a tight circle. In contrast, many hole-nesting birds tend to have nearly spherical eggs.
The weight bearing capacity of many eggs is well known and derives partly from their shape. The more pointed end of many eggs is a natural example of an arch continued around in three dimensions to form a dome. The eggshell is strong under compression because domes exhibit horizontal and vertical resistance so that compression, applied to any one point, is evenly distributed across the entire surface. The more sharply curved the dome, the stronger its resistance to compression will be. Thus eggs will not be crushed by the weight of the incubating mother bird.
Evolution of the Animal Egg
No evidence exists for the evolution of animal eggs. Sexual reproduction involving the production of eggs has “continued, unaltered in essentials, almost since animal life began” (Robert Burton, 1987, Eggs: Nature’s Perfect Package. New York: Facts on File p. 10). Other than this, little else can be said about egg evolution except that “Whatever the reason for the evolution of sex, it is found at all levels of the animal kingdom and the egg in its many forms is its manifestation” (Burton, p. 12). Many fossil eggs have been found, especially dinosaur eggs, but as far as can be determined from the abundant number of fossils found, eggs have always been close to identical to modern egg types, all wonderfully designed.