The largest recorded egg is from a
whale shark and was in size. Whale shark eggs typically hatch within the
mother. At and up to , 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 long and weighs half of a gram (around 0.02 oz). in which the female lays undeveloped eggs that are externally fertilized by a male. 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. eggs utilize
glycans to form a hydrophilic jelly coat that protects the egg 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,
Amniote eggs and embryos eggs in a nest dug by a female common snapping turtle (
Chelydra serpentina) 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. egg showing the different layers 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.
Marsupial and
placental mammals do not lay eggs, but their unborn young do have the complex tissues that identify amniotes. 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 in
ostriches, and typically form of the egg's weight. For example: •
cormorant eggs are rough and chalky •
tinamou eggs are shiny 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 of an egg 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. Cliff-nesting birds often have highly
conical eggs. They are less likely to roll off, tending instead to roll around in a tight circle; this trait is likely to have arisen due to evolution via natural selection. In contrast, many hole-nesting birds have nearly
spherical eggs.
Colours eggs 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. Non-passerines typically have white eggs, except in some ground-nesting groups such as the
Charadriiformes, 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. 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. Brood parasites include the
cowbird,
black-headed duck,
cuckoo-finch, and three Old World
cuckoo species. 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 (
Acanthodoris lutea), a
nudibranch, in
tide pool laying eggs|right|thumb 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. ==Evolution and structure==