Egg

Most arthropods, vertebrates (excluding live-bearing mammals), and mollusks lay eggs, although some, such as scorpions, do not. Reptile eggs, bird eggs, and monotreme eggs are laid out of water and are surrounded by a protective shell, either flexible or inflexible. Eggs laid on land or in nests are usually kept within a warm and favorable temperature range while the embryo grows. When the embryo is adequately developed it hatches; i.e., breaks out of the egg's shell. Some embryos have a temporary egg tooth they use to crack, pip, or break the eggshell or covering.

For people, eggs are a popular food item, and they appear on menus worldwide. Eggs remain an important symbol in folklore and mythology, symbolizing life, healing, and rebirth. They are frequently the subject of decoration. Egg collecting has been popular in some cultures, although the practice is now banned in many jurisdictions. Chicken eggs are used in the production of vaccines for infectious diseases.

Eggs of different animal groups

The largest recorded egg was from a whale shark and was 30 cm × 14 cm × 9 cm (11.8 in × 5.5 in × 3.5 in) in size. Whale shark eggs typically hatch within the mother. At 1.5 kg (3.3 lb) and up to 17.8 cm × 14 cm (7.0 in × 5.5 in), the ostrich egg is the largest egg of any living bird, though the extinct elephant bird and some non-avian dinosaurs laid larger eggs. The bee hummingbird produces the smallest known bird egg, which measures between 6.35–11.4 millimetres (0.250–0.449 in) long and weighs half of a gram (around 0.02 oz). Some eggs laid by reptiles and most fish, amphibians, insects, and other invertebrates can be even smaller.

Several major groups of animals typically have readily distinguishable eggs.

Fish and amphibian 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 (large fish are capable of producing over 100 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 physically most developed offspring will devour its smaller siblings for further nutrition while still within the mother's body. This is known as intrauterine cannibalism.

In certain scenarios, 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.

The eggs of fish and amphibians (anamniotes) are jellylike. Cartilaginous fish (sharks, skates, rays, chimaeras) eggs are fertilized internally and exhibit a wide variety of both internal and external embryonic development. Most fish species spawn eggs that are fertilized externally, typically with the male inseminating the eggs after the female lays them. These eggs do not have a shell and would dry out in the air. Even air-breathing amphibians lay their eggs in water, or in protective foam as with the Coast foam-nest treefrog, Chiromantis xerampelina.

Amniote eggs and embryos

Like amphibians, amniotes are air-breathing vertebrates, but they have complex eggs or embryos, including an amniotic membrane. (The shelled egg is the source for the name Amniota.) The formation of this type of egg requires that conception take place internally, and the shell isolates the embryo development from the mother. Amniotes include reptiles (including dinosaurs and their descendants, birds) and mammals.

Reptile eggs are leathery for snakes and the majority of lizards, while turtles have a calcareous shell. These protective shells are able to survive in the air. They will absorb water from the environment, causing them to swell in size while the fetus is developing. Most reptile eggs are deposited on land, usually in a warm, moist environment, then left alone by the parents. Initially, they are always white. For turtles, tuatara, and most lizards, the sex of the developing embryo is determined by the temperature of the surroundings, with the species determining which gender is favored at cool versus warm temperatures. Not all reptiles lay eggs; some are viviparous ("live birth"). This adaptation may have allowed reptiles to inhabit new habitats, especially in colder climates.

Dinosaurs laid eggs, some of which have been preserved as petrified fossils. Soft-shelled dinosaur eggs are less likely to be preserved, so most of the recovered fossilized egg remains come from calcified eggshells.

Among mammals, early extinct species were found to lay eggs, and was probably the ancestral state. Platypuses and two genera of echidna (spiny anteaters) are Australian monotremes, the only extant order of egg-laying mammal. Marsupial and placental mammals do not lay eggs, but their unborn young do have the complex tissues that identify amniotes.

Bird eggs

Bird eggs are laid by females and incubated for a time that varies according to the species; normally a single young hatches from each egg. Twin yolk eggs have been observed in domestic fowl, but this results in low hatchability. One case of twin geese has been observed to hatch from an elongated egg. Average clutch sizes range from one (as in condors) to about 17–24 (the grey partridge). It is rare for a bird to lay eggs when not fertilized, known as parthenogenesis. One exception is the domestic hen; it is not uncommon for pet owners to find their lone bird nesting on a clutch of unfertilized eggs, which are sometimes called wind-eggs.

Shell

Bird eggs have a hard shell made of calcium carbonate with a 5% organic matrix. This resilient external surface prevents desiccation of the contents, limits mechanical damage, and protects against microbes, all while allowing the exchange of gas with the surrounding atmosphere. They vary in thickness from paper thin up to 2.7 mm in ostriches, and typically form 11%–15% of the egg's weight. Bird eggshells are diverse in appearance and structure. For example:

• cormorant eggs are rough and chalky
• tinamou eggs are shiny
• duck eggs are oily and waterproof
• cassowary eggs are heavily pitted
• jacanas eggs appear lacquered

Tiny pores in bird eggshells allow the embryo to breathe; exchanging oxygen, carbon dioxide, and water with the environment. The pore distribution varies by species, with the pore size being inversely proportional to the incubation period. The domestic hen's egg has around 7000 pores.

Some bird eggshells have a coating of vaterite spherules, which is a rare polymorph of calcium carbonate. In Greater Ani Crotophaga major this vaterite coating is thought to act as a shock absorber, protecting the calcite shell from fracture during incubation, such as colliding with other eggs in the nest.

Shape

Bird egg shapes are ovoid and axisymmetrical in form, but vary by ellipticity and asymmetry depending on the bird species. Thus, the brown boobook species has a nearly spherical shell, the maleo egg is highly ellipsoidal, and the least sandpiper egg is much more conical. The shape is likely formed as the egg moves through the final part of the oviduct, being initially more spherical in form. Ellipticity is introduced by the egg being easier to stretch along the oviduct axis. The eggs of birds that have adapted for high-speed flight often have a more elliptical or asymmetrical form. Thus, one hypothesis is that long, pointy eggs are an incidental consequence of having a streamlined body typical of birds with strong flying abilities; flight narrows the oviduct, which changes the type of egg a bird can lay.

Colours

The default colour of avian eggs is the white of the calcium carbonate from which the shells are made, but some birds, mainly passerines, produce coloured eggs. The colour comes from pigments deposited on top of the calcium carbonate base; biliverdin and its zinc chelate, and bilirubin, give a green or blue ground colour, while protoporphyrin IX produces reds and browns as a ground colour or as spotting. Shell colours are secreted by the same oviduct shell gland that generates the egg shell, and thus can be deposited throughout the shell. When a chalky covering is added, it is the final step in the process.

Non-passerines typically have white eggs, except in some ground-nesting groups such as the Charadriiformes, sandgrouse, and common terns, 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 there is no need of cryptic colours. However, some have suggested 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. Later eggs in a clutch are more spotted than early ones as the female's pigment glands become depleted.

Within the common cuckoo lineage, the colour of individual eggs is genetically influenced, and appears to be inherited through the mother only. This suggests that the gene responsible for pigmentation is on the sex-determining W chromosome (female birds are WZ, males ZZ). However, egg colour in other species is most likely inherited from both parents. For chickens, egg colour appears determined from the hen's genome, diet, and stress factors like disease. With American robins, there is some evidence that the brightness of the egg colouration may influence male parental care of the nestlings.

Evolutionary factors can drive egg colouration, such as predation selecting for cryptic colouration, or colourful eggs possibly being used to coerce males into providing additional care during incubation – the blackmail hypothesis. For avian species that play host to brood parasite eggs, selection pressure drives the host species to evolve distinctive egg colourations so that foreign eggs can be identified and rejected. Likewise, the brood parasite species evolve eggs that better mimic those of the host. The result is an egg colouration evolutionary arms race between the host and parasite. 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.

Yolks of birds' eggs are yellow from carotenoids, it is affected by their living conditions and diet.

Predation

Many animals feed on eggs. For example, principal predators of the black oystercatcher's eggs include raccoons, skunk, mink, river and sea otters, gulls, crows and foxes. The stoat (Mustela erminea) and long-tailed weasel (M. frenata) steal ducks' eggs. Snakes of the genera Dasypeltis and Elachistodon specialize in eating eggs.

Brood parasitism occurs in birds when one species lays its eggs in the nest of another. This is an uncommon behavior, with 1% of bird species being obligate parasites. In some cases, the host's eggs are removed or eaten by the female, or expelled by her chick. Brood parasites include the cowbird, black-headed duck, cuckoo-finch, and three Old World cuckoo species.

Mammalian eggs

The eggs of the egg-laying mammals (the platypus and the echidnas) are macrolecithal eggs very much like those of reptiles. The eggs of marsupials are likewise macrolecithal, but rather small, and develop inside the body of the female, but do not form a placenta. The young are born at a very early stage, and can be classified as a "larva" in the biological sense.

In placental mammals, there are two types of placenta: the yolk sac and the chorioallantoic. In humans, the initial nutrient source is a yolk sac placenta that is replaced by a chorioallantoic placenta at around four weeks. Around the eighth week, the yolk sac is absorbed into the umbilical cord. Receiving nutrients from the mother, the fetus completes the development while inside the uterus.

Invertebrate eggs

Eggs are common among invertebrates, including insects, spiders, mollusks, and crustaceans. Eggs deposited on land or in fresh water tend to have more yolk, which allows longer development in the egg before hatching. Eggs with little yolk hatch more rapidly into larval form that can seek out food. Some land invertebrates are viviparous, developing offspring within the body of the mother that are supplied nutrition by the host. Examples include the tsetse fly and some peripatus species.

Parental care does occur in some invertebrate species, although rarely by the male; the addition of paternal care usually doesn't provide sufficient evolutionary advantage for it to evolve with any frequency. A counter-example is the dung beetle, where the male and female cooperate to bury balls of dung where the female can lay her eggs. Examples of invertebrates that provide parental care include the treehopper and velvet spider. Female jumping spiders provide milk for their offspring.

Many insect species and other invertebrate taxa are capable of parthenogenesis, which is the production of offspring using an unfertilized egg. In the subterranean termite, the queen produces new queen eggs via parthenogenesis but the soldiers and workers are created via sexual reproduction. Unisexual reproduction is uncommon in vertebrates, but has been observed in some fish, reptile, and amphibian taxa.