Egg (biology)

In most birds and reptiles, an egg (Latin ovum) is the zygote, resulting from fertilization of the ovum. It nourishes and protects the embryo. Oviparous animals are animals that lay eggs, with little or no other development within the mother. This is the reproductive method of many fish, amphibians and reptiles, all birds, the monotremes, and most insects and arachnids.

Reptile eggs, bird eggs, and monotreme eggs are surrounded by a protective shell, either flexible or inflexible.

The 1.5 kg ostrich egg contains the largest existing single cell currently known, though the extinct Aepyornis and some dinosaurs had larger eggs. The bee hummingbird produces the smallest known bird egg, which weighs half of a gram. The eggs laid by some reptiles and most fish are even smaller, and those of insects and other invertebrates are much smaller still.

The study or collecting of eggs, in particular bird eggs, is called oology.

Bird eggs

Usually after fertilization, the bird egg is laid by the female and is incubated for a time that varies according to the species; then a single young hatches from each egg. Average clutch sizes range from 1 (as in condors) to about 17 (the Grey Partridge). Some birds lay eggs even when not fertilized, and it is not uncommon for pet owners to find their lone bird nesting on a clutch of infertile eggs.

Colors

The default colour of vertebrate eggs is the white of the calcium carbonate from which the shells are made, but some birds, mainly passerines, produce coloured eggs. The pigments biliverdin and its zinc chelate give a green or blue ground colour, and protoporphyrin produces reds and browns as a ground colour or as spotting.

Non-passerines typically have white eggs, except in some ground-nesting groups such as the Charadriiformes, sandgrouse and nightjars, where camouflage is necessary, and some parasitic cuckoos which have to match the passerine host's egg. Most passerines, in contrast, lay coloured eggs, even if, like the tits they are hole-nesters, where there is no need of cryptic colours.

However, a recent study suggests that the protoporphyrin markings on passerine eggs actually act to reduce brittleness by acting as a solid state lubricant. If there is insufficient calcium available in the local soil, the egg shell may be thin, especially in a circle around the broad end. Protoporphyrin speckling compensates for this, and increases inversely to the amount of calcium in the soil.

For the same reason, later eggs in a clutch are more spotted than early ones as the female's store of calcium is depleted.

The colour of individual eggs is also genetically influenced, and appears to be inherited through the mother only, suggesting that the gene responsible for pigmentation is on the sex determining W chromosome (female birds are WZ, males ZZ).

It used to be thought that colour was applied to the shell immediately before laying, but this research shows that colouration is an integral part of the development of the shell, with the same protein responsible for depositing calcium carbonate, or protoporphyrins when there is a lack of that mineral

In species such as the Common Guillemot, which nest in large groups, each female's eggs have very different markings, making it easier for females to identify their own eggs on the crowded cliff ledges on which they breed.

Shell structure

Eggs are usually smooth, but there are exceptions. A cormorant's egg, for example, is quite rough and is very chalky. In contrast, tinamous have very shiny eggs, and ducks have oily and waterproof eggs. Another variation is the very heavily pitted eggs of cassowaries.

There are tiny pores in the shells of eggs to allow the unborn animal to breathe. The domestic hen's egg has around 7500 pores.

Shape

Most bird eggs have a characteristic oval shape, with one end rounded and the other more pointy. This shape results from the egg being forced through the oviduct. Muscles contract the oviduct behind the egg, pushing it forward. The egg's wall is still shapeable, and the pointy end develops at the back side. Highly conical eggs are often seen in cliff-nesting birds. They are less likely to roll off, tending instead to roll around in a tight circle, and thus are believed to have been selected for by evolution. In contrast many hole nesting birds have nearly spherical eggs.

Predation

There are numerous animals that feed on eggs. Principal predators of the Black Oystercatcher's eggs, for example, include raccoons, skunks, mink, river and sea otters, gulls, crows and foxes. The Stoat (Mustela erminea) and Long-tailed Weasel (M. frenata) steal ducks' eggs. The egg-eating snakes (genera Dasypeltis and Elachistodon) specialize in eating eggs.

Fish eggs

The most common reproductive strategy for fish is known as oviparity, in which the female lays undeveloped eggs that are externally fertilized by a male. Typically large numbers of eggs are laid at one time (an adult female cod can produce 4–6 million eggs in one spawning) and the eggs are then left to develop without parental care. When the larvae hatch from the egg, they often carry the remains of the yolk in a yolk sac which continues to nourish the larvae for a few days as they learn how to swim. Once the yolk is consumed, there is a critical point after which they must learn how to hunt and feed or they will die.

A few fish, notably the rays and most sharks use ovoviviparity in which the eggs are fertilized and develop internally. However the larvae still grow inside the egg consuming the egg's yolk and without any direct nourishment from the mother. The mother then gives birth to relatively mature young. In certain instances, the most physically-developed offspring will devour its smaller siblings for further nutrition while still within the mother's body. This is known as intra-uterine cannibalism.

More rarely, some fish such as the hammerhead shark and reef shark are viviparous, with the egg being fertilized and developed internally, but with the mother also providing direct nourishment.

Mammal eggs

Only a few mammal species lay eggs, those in the group known as monotremes, comprised of the platypus and two genera of echidna ('spiny anteaters'). These eggs are not often eaten in western cultures.

Reptile eggs

Reptile eggs are rubbery and are always initially white.

Amphibian eggs

In many ways similar to fish eggs, amphibian eggs are jellylike and are fertilised exterally.

References

Marine Biology notes from School of Life Sciences, Napier University.
Speckles Make Bird Eggs Stronger, Study Finds John Pickrell, National Geographic News, 11 Oct 2005.
Andrew Gosler, Yet even more ways to dress eggs in British Birds, vol 99 no 7, July 2006

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