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Egg incubation is the process by which an egg, of
oviparous Oviparous animals are animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive method of most fish, amphibians, most reptiles, and all pterosaurs, dinosaurs (including birds), and m ...
(egg-laying) animals, develops an
embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
within the egg, after the egg's formation and
oviposition The ovipositor is a tube-like organ used by some animals, especially insects, for the laying of eggs. In insects, an ovipositor consists of a maximum of three pairs of appendages. The details and morphology of the ovipositor vary, but typical ...
al release. Egg incubation is done under favorable environmental conditions, possibly by brooding and hatching the egg. Multiple and various factors are vital to the incubation of various species of animal. In many species of reptile for example, no fixed temperature is necessary, but the actual temperature determines the sex ratio of the offspring. In birds in contrast, the sex of offspring is genetically determined, but in many species a constant and particular
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
is necessary for successful incubation. Especially in
poultry Poultry () are domesticated birds kept by humans for their eggs, their meat or their feathers. These birds are most typically members of the superorder Galloanserae (fowl), especially the order Galliformes (which includes chickens, qu ...
, 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 Broodiness is the action or behavioral tendency to sit on a clutch of eggs to incubate them, often requiring the non-expression of many other behaviors including feeding and drinking.Homedes Ranquini, J. y Haro-García, F. Zoogenética. 1ra. edi ...
, and most egg-laying breeds of
poultry Poultry () are domesticated birds kept by humans for their eggs, their meat or their feathers. These birds are most typically members of the superorder Galloanserae (fowl), especially the order Galliformes (which includes chickens, qu ...
have had this behavior
selectively bred Selective breeding (also called artificial selection) is the process by which humans use animal breeding and plant breeding to selectively develop particular phenotypic traits (characteristics) by choosing which typically animal or plant ma ...
out of them to increase production.


Avian incubation

A wide range of incubation habits is displayed among birds. In
warm-blooded Warm-blooded is an informal term referring to animal species which can maintain a body temperature higher than their environment. In particular, homeothermic species maintain a stable body temperature by regulating metabolic processes. The on ...
species such as
bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
species generally, body heat from the
brooding patch A brood patch is a patch of featherless skin on the underside of birds during the nesting season. Feathers act as inherent insulators, and prevent efficient incubation. Birds have solved this evolutionary dilemma by developing dedicated brood patc ...
of the brooding parent provides the constant temperature. Several groups, notably the
megapode The megapodes, also known as incubator birds or mound-builders, are stocky, medium-large, chicken-like birds with small heads and large feet in the family Megapodiidae. Their name literally means "large foot" and is a reference to the heavy legs ...
s, instead use heat generated from rotting vegetable material, effectively creating a giant
compost Compost is a mixture of ingredients used as plant fertilizer and to improve soil's physical, chemical and biological properties. It is commonly prepared by decomposing plant, food waste, recycling organic materials and manure. The resulting ...
heap while
crab plover The crab-plover or crab plover (''Dromas ardeola)'' is a bird related to the waders, but sufficiently distinctive to merit its own family Dromadidae. Its relationship within the Charadriiformes is unclear, some have considered it to be closely ...
s make partial use of heat from the sun. The
Namaqua sandgrouse The Namaqua sandgrouse (''Pterocles namaqua''), is a species of ground-dwelling bird in the sandgrouse family. It is found in arid regions of south-western Africa. Taxonomy The Namaqua sandgrouse was formally described in 1789 by the German na ...
of the deserts of southern
Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...
, 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, because 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. Experiments with
great tit The great tit (''Parus major'') is a passerine bird in the tit family Paridae. It is a widespread and common species throughout Europe, the Middle East, Central Asia and east across the Palearctic to the Amur River, south to parts of North Af ...
s show that females compensate for the potential effects of differential heating by moving the eggs homogeneously within the clutch. 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
Atlantic canary The Atlantic canary (''Serinus canaria''), known worldwide simply as the wild canary and also called the island canary, common canary, or canary, is a small passerine bird belonging to the genus '' Serinus'' in the finch family, Fringillidae. I ...
and the
Indian robin The Indian robin (''Copsychus fulicatus'')Rasmussen & Anderton emend the species epithet from ''fulicata'' to ''fulicatus'' since ''Saxicola'' is masculine and the ''-oides'' ending is always masculine according to ICZN Code 30.1.4.4ICZN Code. See ...
, or most of it, as is typical of
falcon Falcons () are birds of prey in the genus ''Falco'', which includes about 40 species. Falcons are widely distributed on all continents of the world except Antarctica, though closely related raptors did occur there in the Eocene. Adult falcons ...
s. In some species, such as the
whooping crane The whooping crane (''Grus americana'') is the tallest North American bird, named for its whooping sound. It is an endangered crane species. Along with the sandhill crane (''Antigone canadensis''), it is one of only two crane species native to ...
, the male and the female take turns incubating the egg. In others, such as the
cassowaries Cassowaries ( tpi, muruk, id, kasuari) are flightless birds of the genus ''Casuarius'' in the order Casuariiformes. They are classified as ratites (flightless birds without a keel on their sternum bones) and are native to the tropical fo ...
, 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
hoatzin The hoatzin ( ) or hoactzin ( ), (''Opisthocomus hoazin''), is the only species in the order Opisthocomiformes. It is a species of tropical bird found in swamps, riparian forests, and mangroves of the Amazon and the Orinoco basins in South Ameri ...
s, 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
passerine A passerine () is any bird of the order Passeriformes (; from Latin 'sparrow' and '-shaped'), which includes more than half of all bird species. Sometimes known as perching birds, passerines are distinguished from other orders of birds by th ...
s and the black-billed and yellow-billed cuckoos) to 85 days (the
wandering albatross The wandering albatross, snowy albatross, white-winged albatross or goonie (''Diomedea exulans'') is a large seabird from the family Diomedeidae, which has a circumpolar range in the Southern Ocean. It was the last species of albatross to be desc ...
and the
brown kiwi Kiwi ( ) are flightless birds endemic to New Zealand of the order Apterygiformes. The five extant species fall into the family Apterygidae () and genus ''Apteryx'' (). Approximately the size of a domestic chicken, kiwi are by far the smallest ...
). In these latter, the incubation is interrupted; the longest uninterrupted period is 64 to 67 days in the
emperor penguin The emperor penguin (''Aptenodytes forsteri'') is the tallest and heaviest of all living penguin species and is endemic to Antarctica. The male and female are similar in plumage and size, reaching in length and weighing from . Feathers of t ...
. 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 The megapodes, also known as incubator birds or mound-builders, are stocky, medium-large, chicken-like birds with small heads and large feet in the family Megapodiidae. Their name literally means "large foot" and is a reference to the heavy legs ...
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 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. Incubation periods for birds:


Mammalian incubation

Very few
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...
s lay eggs. In perhaps the best known example, the
platypus The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal endemic to eastern Australia, including Tasmania. The platypus is the sole living representative or mono ...
, the eggs develop ''
in utero ''In Utero'' is the third and final studio album by American rock band Nirvana. It was released on September 21, 1993, by DGC Records. After breaking into the mainstream with their second album, ''Nevermind'' (1991), Nirvana hired Steve Albin ...
'' 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 An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
has no functional organs and relies on the
yolk sac The yolk sac is a membranous wikt:sac, sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac' ...
for sustenance. The yolk is absorbed by the developing young. During the second phase, the digits develop. In the last phase, the
egg tooth An egg tooth is a temporary, sharp projection present on the bill or snout of an oviparous animal at hatching. It allows the hatchling to penetrate the eggshell from inside and break free. Birds, reptiles, and monotremes possess egg teeth as ha ...
appears. The only other egg-laying mammal is the
echidna Echidnas (), sometimes known as spiny anteaters, are quill-covered monotremes (egg-laying mammals) belonging to the family Tachyglossidae . The four extant species of echidnas and the platypus are the only living mammals that lay eggs and the ...
. New science research has been found that eggshells have a nanostructure that has inner and outer layers. The structure of this shell contains a protein known as osteopontin which is also found in tooth and bone. What researchers found was that the inner layers of the shell were thinner than the outer shells. This is because in the process of the egg being incubated the chicken embryos are taking the protein from the shell making the chicks skeleton stronger.


Reptilian incubation

Methods of incubation vary widely among the many different kinds of reptiles. Various species of
sea turtle Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the flatback, green, hawksbill, leatherback, loggerhe ...
s bury their eggs on beaches under a layer of sand that provides both protection from predators and a constant temperature for the nest.
Snake Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more ...
s may lay eggs in communal burrows, where a large number of adults combine to keep the eggs warm.
Alligator An alligator is a large reptile in the Crocodilia order in the genus ''Alligator'' of the family Alligatoridae. The two extant species are the American alligator (''A. mississippiensis'') and the Chinese alligator (''A. sinensis''). Additional ...
s and
crocodile Crocodiles (family Crocodylidae) or true crocodiles are large semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term crocodile is sometimes used even more loosely to include all extant me ...
s either lay their eggs in mounds of decomposing vegetation or lay them in holes they dig in the ground.


Incubation by other vertebrates

Fish generally do not incubate their eggs. However, some species mouthbrood their eggs, not eating until they hatch. 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 The pipa, pípá, or p'i-p'a () is a traditional Chinese musical instrument, belonging to the plucked category of instruments. Sometimes called the "Chinese lute", the instrument has a pear-shaped wooden body with a varying number of frets ran ...
'') have pouches in their skin into which the eggs are inserted. Other neotropical frogs in the family
Hemiphractidae The Hemiphractidae are a family of frogs from South and Central America. Previously, this group had been classified as a subfamily (Hemiphractinae) under family Hylidae. More recent research classifies these genera into their own family, or som ...
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 Metamorphosis is a biological process by which an animal physically develops including birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some inse ...
, and the female stomach-brooding frog of Australia swallows the eggs, which develop in her stomach.


Incubation by invertebrates

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
cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that ...
ns (
sea anemone Sea anemones are a group of predatory marine invertebrates of the order Actiniaria. Because of their colourful appearance, they are named after the '' Anemone'', a terrestrial flowering plant. Sea anemones are classified in the phylum Cnidaria, ...
s and
coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and se ...
s), a few
chiton Chitons () are marine molluscs of varying size in the class Polyplacophora (), formerly known as Amphineura. About 940 extant and 430 fossil species are recognized. They are also sometimes known as gumboots or sea cradles or coat-of-mail she ...
s, some gastropod molluscs, some
cephalopod A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...
s, some bivalve molluscs, many
arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
s, some
entoprocta Entoprocta (), or Kamptozoa , is a phylum of mostly sessile aquatic animals, ranging from long. Mature individuals are goblet-shaped, on relatively long stalks. They have a "crown" of solid tentacles whose cilia generate water currents that ...
ns, some
brachiopod Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...
s, some
bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a ...
ns, and some
starfish Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish a ...
.


See also

* Artificial incubation *
Broodiness Broodiness is the action or behavioral tendency to sit on a clutch of eggs to incubate them, often requiring the non-expression of many other behaviors including feeding and drinking.Homedes Ranquini, J. y Haro-García, F. Zoogenética. 1ra. edi ...
*
Accumulated thermal unit An accumulated thermal unit, is a unit of measurement used to describe the cumulative effect of temperature over time. 1 ATU is equal to 1  degree Celsius for 1 day. For example, in an environment at a constant temperature of 5&n ...


References

*
Christopher Perrins Christopher Miles Perrins, (born 11 May 1935) is Emeritus Fellow of the Edward Grey Institute of Field Ornithology at the University of Oxford, Emeritus Fellow at Wolfson College, Oxford and Her Majesty's Warden of the Swans since 1993. Educat ...
(editor), ''Firefly Encyclopedia of Birds'',


External links

*Scarlet macaw from Guide to macaws *Incubation periods in table fro
PennState Extension
{{Authority control Bird breeding Eggs Reproduction in animals Ethology sv:Häckning#Ruvning