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Incubation refers to the process by which certain oviparous (egg-laying) animals hatch their eggs, and to the development of the embryo within the egg. The most vital factor of incubation is the constant temperature required for its development over a specific period. Especially in domestic fowl, the act of sitting on eggs to incubate them is called brooding. The action or behavioral tendency to sit on a clutch of eggs is also called broodiness, and most egg-laying breeds of poultry have had this behavior selectively bred out of them to increase production.
A wide range of incubation habits is displayed among birds. In warm-blooded species such as bird species generally, body heat from the brooding parent provides the constant temperature, though several groups, notably the megapodes, instead use heat generated from rotting vegetable material, effectively creating a giant compost heap while crab plovers make partial use of heat from the sun. The Namaqua sandgrouse of the deserts of southern Africa, needing to keep its eggs cool during the heat of the day, stands over them drooping its wings to shade them. The humidity is also critical, and if the air is too dry the egg will lose too much water to the atmosphere, which can make hatching difficult or impossible. As incubation proceeds, an egg will normally become lighter, and the air space within the egg will normally become larger, owing to evaporation from the egg.
In the species that incubate, the work is divided differently between the sexes. Possibly the most common pattern is that the female does all the incubation, as in the coscoroba swan and the Indian robin, or most of it, as is typical of falcons. In some species, such as the whooping crane, the male and the female take turns incubating the egg. In others, such as the cassowaries, only the male incubates. The male mountain plover incubates the female's first clutch, but if she lays a second, she incubates it herself. In hoatzins, some birds (mostly males) help their parents incubate later broods.
The incubation period, the time from the start of uninterrupted incubation to the emergence of the young varies from 11 days (some small passerines and the black-billed and yellow-billed cuckoos) to 85 days (the wandering albatross and the brown kiwi). In these latter, the incubation is interrupted; the longest uninterrupted period is 64 to 67 days in the emperor penguin. In general smaller birds tend to hatch faster but there are exceptions, and cavity nesting birds tend to have longer incubation periods. It can be an energetically demanding process, with adult albatrosses losing as much as 83 g of body weight a day. Megapode eggs take from 49 to 90 days depending on the mound and ambient temperature. Even in other birds, ambient temperatures can lead to variation in incubation period. Normally, of course, the egg is incubated outside the body. However in one recorded case, the egg incubation occurred entirely within a chicken. The chick hatched inside and emerged from its mother without the shell, leading to internal wounds that killed the mother hen.
Embryo development remains suspended until the onset of incubation. The freshly laid eggs of domestic fowl, ostrich and several other species can be stored for about two weeks when maintained under 5 C. Extended periods of suspension have been observed in some marine birds. Some species begin incubation with the first egg, causing the young to hatch at different times; others begin after laying the second egg, so that the third chick will be smaller and more vulnerable to food shortages. Some start to incubate after the last egg of the clutch, causing the young to hatch simultaneously.
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Very few mammals lay eggs. In perhaps the best known example, the platypus, the eggs develop in utero for about 28 days, with only about 10 days of external incubation (in contrast to a chicken egg, which spends about one day in tract and 21 days externally). After laying her eggs, the female curls around them. The incubation period is divided into three phases. In the first phase, the embryo has no functional organs and relies on the yolk sac for sustenance. The yolk is absorbed by the developing young. During the second phase, the digits develop and, in the last phase, the egg tooth appears. The only other egg-laying mammal is the echidna.
Methods of incubation vary widely among the many different kinds of reptiles.
Various species of sea turtles bury their eggs on beaches, under a layer of sand that provides both protection from predators and a constant temperature for the nest.
Snakes may lay eggs in communal burrows, where a large number of adults combine to keep the eggs warm.
Fish generally do not incubate their eggs.
Some amphibians brood their eggs. The female salamander Ensatina (Ensatina eschscholtzii) curls around the clutch of eggs and massages individual eggs with her pulsating throat. Some aquatic frogs such as the Surinam toad (Pipa pipa) have pouches in their skin into which the eggs are inserted. Other neotropical frogs in the family Hemiphractidae also have pouches in which the eggs develop, in some species directly into juvenile frogs and in others into tadpoles that are later deposited in small water bodies to continue their development. The male Darwin's frog carries the eggs around in his mouth until metamorphosis and the female stomach-brooding frog of Australia swallows the eggs which develop in her stomach.
Brooding occurs in some invertebrates when the fertilised eggs are retained inside or on the surface of the parent, usually the mother. This happens in some cnidarians (sea anemones and corals), a few chitons, some gastropod molluscs, some cephalopods, some bivalve molluscs, many arthropods, some entoproctans, some brachiopods, some bryozoans and some starfish. 
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