Egg (biology)
From Free net encyclopedia
In some animals, 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 way 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.
| Image:Tortoise-Hatchling.JPG | Image:Emperor Gum Moth eggs2.jpg | Image:Clupeaharenguskils2.jpg |
Contents |
Bird eggs
Image:LoreleiCockatielNestingEgg.jpg 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
Image:Murre Eggs.jpg Different animals produce different colored eggs. The pigments protoporphyrin, biliverdin, and zinc chelate of biliverdin, are responsible for the diversity of egg color in birds. These pigments are secreted by cells in the oviduct wall, and can cause speckles if color is added right before the egg is laid. The eggs of modern reptiles are all white.
The markings of many birds' eggs may provide camouflage. Cavity-nesting birds such as woodpeckers and kingfishers do not need camouflaged eggs. Their eggs are often bright white, making it easy for the parent to find them. Eggs of almost all owl species are typically pure white.
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 egg.
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.
Ψ===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 peepee. 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.
Image:Salmoneggskils.jpg 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.
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.
References
- Marine Biology notes from School of Life Sciences, Napier University.
See also
- Chicken sexer
- Egg yolk
- Egg white
- Incubate
- Nest
- Oology - the study or collecting of eggs.
- Trophic eggca:Ou (biologia)
cs:Vejce da:Æg (zoologi) de:Ei es:Huevo (biología) eo:Ovo fr:Œuf (biologie) it:Uovo he:ביצה lt:Kiaušinis nl:Ei (dier) ja:卵 no:Egg nn:Egg pl:Jajo (biologia) pt:Ovo fi:Muna su:Endog sv:Ägg uk:Яйце