The biology of eggs

For thoughtful child sponsors

In some animals, an egg is the zygote, resulting from fertilization of the ovum, which nourishes and protects the embryo.

Reptile eggs, bird eggs, and mammalian monotreme eggs are surrounded by a protective shell, either flexible or inflexible. The study of Eggs is called Oology.

An average Whooping Crane egg is 102 mm long, and weighs 208 grams

Table of contents

1 Animal's eggs
2 Colors
3 Egg structure
4 Predation

Animal's eggs

Eggs are usually incubated by the female. In some species, such as the Whooping Crane, both the male and the female take turns incubating the egg. The cassowary's eggs, however, are incubated only by the male.

The 3.3 pound ostrich egg is the largest existing single cell known to man. (Some dinosaurs had larger eggs.) The hummingbird produces the smallest known egg which weighs half of a gram.

Different animals produce different colored eggs. The pigments protoporphyrin, biliverdin, and zinc chelate of biliverdin, are responsible for the diversity of egg color. These pigments are secreted by cells in the oviduct wall, and can cause speckles if color is added right before the egg is laid. Reptiles' eggs are all white and it is thought that this was true for all animals long ago.

The eggs of cavity nesting birds such as the woodpecker and the kingfisher, are often bright white, making it easy for the parent to locate them.

In species such as the Common Murre, which nest in large groups, each female's eggs have very different markings, making it easier for females to identify their own egg.

Egg structure

Eggs are usually smooth, but there are exceptions. The cormorant's egg, for example, is quite rough and is very chalky. In contrast, the tinamous has very shiny eggs and the duck has oily and waterproof eggs. Another variation is the cassowary' very heavily pitted eggs.

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

Predation

Brown-headed cowbird eating another bird's egg

There are quite a few animals that feed on eggs. Principle predators of the Black Oystercatcher's eggs, for example, include racoons, skunks, minks, river and sea otters, gulls, crows and foxes. The Ermine (Mustela erminea) and long-tailed weasel (M. frenata) steal ducks' eggs.