Apps

egg — news, coverage & facts — IJR

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.

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.

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.

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.

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.

Evolution and structure

All sexually reproducing life, including both plants and animals, produces gametes. The male gamete cell, sperm, is usually motile whereas the female gamete cell, the ovum, is generally larger and sessile. The male and female gametes combine to produce the zygote cell. In multicellular organisms, the zygote subsequently divides in an organised manner into smaller more specialised cells (Embryogenesis), so that this new individual develops into an embryo. In most animals, the embryo is the sessile initial stage of the individual life cycle, and is followed by the emergence (that is, the hatching) of a motile stage. The zygote or the ovum itself or the sessile organic vessel containing the developing embryo may be called the egg.

A 2011 proposal suggests that the phylotypic animal body plans originated in cell aggregates before the existence of an egg stage of development. Eggs, in this view, were later evolutionary innovations, selected for their role in ensuring genetic uniformity among the cells of incipient multicellular organisms.

The yolk component of the egg provides the nutrients needed for the growth of the embryo. More than half the proteins in egg yolk are phosphoglycoproteins, which are equivalent to milk proteins in mammals or storage proteins in plant seeds. The polypeptide vitellogenin (Vtg) is the major precursor of the lipoproteins and phosphoproteins that make up most of the protein content of yolk. It occurs in all egg-laying animals; the insect form is called vitellin. The synthesis of yolk protein components occurs in the liver. In an amniote egg, the yolk is surrounded by a membranous yolk sac that attaches to the embryo.

The albumen, or egg white, is a clear liquid layer that surrounds the yolk. This material is hydrophilic and serves as a water reservoir for the embryo. The predominant protein in egg whites is ovalbumin, forming more than half the proteins by mass. The role of this protein is unknown. Ovomucoid, which makes up 11% of the albumen, is the primary egg allergen.

Egg size and yolk

Vertebrate eggs can be classified by the relative amount of yolk; Simple eggs with little yolk are called microlecithal, medium-sized eggs with some yolk are called mesolecithal, and large eggs with a large concentrated yolk are called macrolecithal. This classification of eggs is based on the eggs of chordates, though the basic principle extends to the whole animal kingdom. Within the egg cell cytoplasm, a uniform distribution of yolk is termed isolecithal, while an uneven distribution is telolecithal. Mammal eggs are isolecithal with small amounts of yolk, while bird and reptile eggs are telolecithal.

Small eggs with little yolk are called microlecithal. The yolk is evenly distributed, so the cleavage of the egg cell cuts through and divides the egg into cells of fairly similar sizes. In sponges and cnidarians, the dividing eggs develop directly into a simple larva, rather like a morula with cilia. In cnidarians, this stage is called the planula, and either develops directly into the adult animals or forms new adult individuals through a process of budding.

Microlecithal eggs require minimal yolk mass. Such eggs are found in flatworms, roundworms, annelids, bivalves, echinoderms, the lancelet and in most marine arthropods. In anatomically simple animals, such as cnidarians and flatworms, the fetal development can be quite short, and even microlecithal eggs can undergo direct development. These small eggs can be produced in large numbers. In animals with high egg mortality, microlecithal eggs are the norm, as in bivalves and marine arthropods. However, the latter are more complex anatomically than e.g. flatworms, and the small microlecithal eggs do not allow full development. Instead, the eggs hatch into larvae, which may be markedly different from the adult animal.

In placental mammals, where the embryo is nourished by the mother throughout the whole fetal period, the egg possesses little if any yolk.

Mesolecithal eggs have comparatively more yolk than the microlecithal eggs. The yolk is concentrated in one part of the egg (the vegetal pole), with the cell nucleus and most of the cytoplasm in the other (the animal pole). The cell cleavage is uneven, and mainly concentrated in the cytoplasma-rich animal pole.

The larger yolk content of the mesolecithal eggs allows for a longer fetal development. Comparatively anatomically simple animals will be able to go through the full development and leave the egg in a form reminiscent of the adult animal. This is the situation found in hagfish and some snails. Animals with smaller size eggs or more advanced anatomy will still have a distinct larval stage, though the larva will be basically similar to the adult animal, as in lampreys, coelacanth and the salamanders.

Eggs with a large yolk are called macrolecithal. The eggs are usually few in number, and the embryos have enough food to go through full fetal development in most groups. Macrolecithal eggs are only found in selected representatives of two groups: Cephalopods and vertebrates.

Macrolecithal eggs go through a different type of development than other eggs. Due to the large size of the yolk, the cell division can not split up the yolk mass. The fetus instead develops as a plate-like structure on top of the yolk mass, and only envelopes it at a later stage. A portion of the yolk mass is still present as an external or semi-external yolk sac at hatching in many groups. This form of fetal development is common in bony fish, even though their eggs can be quite small. Despite their macrolecithal structure, the small size of the eggs does not allow for direct development, and the eggs hatch to a larval stage ("fry"). In terrestrial animals with macrolecithal eggs, the large volume to surface ratio necessitates structures to aid in transport of oxygen and carbon dioxide, and for storage of waste products so that the embryo does not suffocate or get poisoned from its own waste while inside the egg, see amniote.

In addition to bony fish and cephalopods, macrolecithal eggs are found in cartilaginous fish, reptiles, birds and monotreme mammals. The eggs of the coelacanths can reach a size of 9 cm (3.5 in) in diameter, and the young go through full development while in the uterus, living on the copious yolk.

Egg-laying reproduction

Animals are commonly classified by their manner of reproduction, at the most general level distinguishing egg-laying (Latin. oviparous) from live-bearing (Latin. viviparous). French biologist Thierry Lodé proposed a classification scheme that further divides the reproduction types according to the development that occurs before the offspring are expelled from the adult's body:

Ovuliparity means the female spawns unfertilized eggs (ova), which must then be externally fertilised. Ovuliparity is typical of bony fish, anurans, echinoderms, bivalves and cnidarians. Most aquatic organisms are ovuliparous. The term is derived from the diminutive meaning "little egg".

Oviparity is where fertilisation occurs internally and so the eggs laid by the female are zygotes (or newly developing embryos), often with important outer tissues added (for example, in a chicken egg, no part outside of the yolk originates with the zygote). Oviparity is typical of birds, reptiles, some cartilaginous fish and most arthropods. Terrestrial organisms are typically oviparous, with egg-casings that resist evaporation of moisture.

Ovo-viviparity is where the zygote is retained in the adult's body but there are no trophic (feeding) interactions. That is, the embryo still obtains all of its nutrients from inside the egg. Most live-bearing fish, amphibians or reptiles are actually ovoviviparous. Examples include the reptile Anguis fragilis, the sea horse (where zygotes are retained in the male's ventral "marsupium"), and the frogs Rhinoderma darwinii (where the eggs develop in the vocal sac) and Rheobatrachus (where the eggs develop in the stomach).

Histotrophic viviparity means embryos develop in the female's oviducts but obtain nutrients by consuming other ova, zygotes or sibling embryos (oophagy or adelphophagy). This intra-uterine cannibalism occurs in some sharks and in the black salamander Salamandra atra. Marsupials excrete a "uterine milk" supplementing the nourishment from the yolk sac.

Hemotrophic viviparity is where nutrients are provided from the female's blood through a designated organ. This most commonly occurs through a placenta, found in most mammals. Similar structures are found in some sharks and in the lizard Pseudomoia pagenstecheri. In some hylid frogs, the embryo is fed by the mother through specialized gills.

The term hemotrophic derives from the Latin for blood-feeding, contrasted with histotrophic for tissue-feeding.