Egg

• 1. Introduction

  ›

  An egg is an organic vessel that an animal grows to carry a fertilized egg cell, a zygote. Inside that vessel, an embryo is incubated until it can survive on its own. Then the animal hatches. The largest egg ever recorded came from a whale shark and measured 30 by 14 by 9 centimeters. The smallest known bird egg comes from the bee hummingbird and weighs half of a gram, roughly 0.02 ounces. Between those two extremes lies a vast catalogue of shapes, shells, and survival strategies. How does a single structure stretch across arthropods, fish, reptiles, birds, and even a handful of mammals? Why are some eggs jellylike and shell-less while others are armored in calcium carbonate? And how did this thing on breakfast menus worldwide also become a symbol of life, healing, and rebirth?

• 2. Water And Shore

  ›

  Most fish spawn eggs that are fertilized externally, the male inseminating them after the female lays them. Large fish can produce over 100 million eggs in a single spawning, then leave them to develop without parental care. The eggs of fish and amphibians, the anamniotes, are jellylike. They have no shell and would dry out in the air, which is why even air-breathing amphibians lay their eggs in water. The Coast foam-nest treefrog, Chiromantis xerampelina, instead deposits its eggs in protective foam. When larvae hatch, they often carry the remains of the yolk in a yolk sac that nourishes them for a few days as they learn to swim. Once the yolk is gone, a critical point arrives. They must learn to hunt and feed, or they die. Amniotes broke this dependence on water entirely. They are air-breathing vertebrates with complex eggs containing an amniotic membrane, the feature that gives the group Amniota its name. Forming such an egg requires internal conception, and the shell isolates the developing embryo from the mother. Reptile eggs are leathery for snakes and most lizards, while turtles have a calcareous shell. These shells survive in the air and absorb water from the environment, swelling as the fetus grows. For turtles, tuatara, and most lizards, the temperature of the surroundings determines the sex of the embryo. That single innovation, the shelled egg, let vertebrates carry their young onto dry land.

• 3. Architecture Of The Shell

  ›

  Bird eggs carry a hard shell of calcium carbonate bound with a 5 percent organic matrix. This surface prevents the contents from drying out, limits mechanical damage, and blocks microbes, all while letting gas pass through. Shell thickness ranges from paper thin up to 2.7 in ostriches, and the shell typically makes up 11 of an egg's weight. The variety in appearance is striking. Cormorant eggs are rough and chalky, tinamou eggs are shiny, duck eggs are oily and waterproof, cassowary eggs are heavily pitted, and jacana eggs look lacquered. Tiny pores let the embryo breathe, exchanging oxygen, carbon dioxide, and water with the outside world. A domestic hen's egg has around 7000 pores, and pore size runs inversely to the incubation period. Some shells carry a coating of vaterite spherules, a rare polymorph of calcium carbonate. In the Greater Ani, Crotophaga major, this vaterite coating is thought to act as a shock absorber, protecting the calcite shell from fracture when eggs collide in the nest. The shape varies too. The brown boobook lays a nearly spherical shell, the maleo egg is highly ellipsoidal, and the least sandpiper egg is far more conical. That shape forms as the egg moves through the final part of the oviduct. Cliff-nesting birds often lay highly conical eggs, which roll in a tight circle rather than off the ledge, a trait likely shaped by natural selection. Many hole-nesting birds, by contrast, lay nearly spherical eggs.

• 4. The Painted Egg

  ›

  The default color of an avian egg is white, the color of the calcium carbonate the shell is made from. But some birds, mainly passerines, produce colored eggs. The color comes from pigments laid over that white base. Biliverdin and its zinc chelate, along with bilirubin, give a green or blue ground color, while protoporphyrin IX produces reds and browns as a ground color or as spotting. The same oviduct shell gland that builds the shell secretes these colors. Where soil holds too little calcium, the shell may thin, especially in a circle around the broad end. Protoporphyrin speckling compensates and increases as available calcium falls, and later eggs in a clutch are more spotted than early ones as the female's pigment glands deplete. Color carries a deeper genetic story. Within the common cuckoo lineage, the color of individual eggs appears inherited through the mother only, suggesting the pigment gene sits on the sex-determining W chromosome. For avian hosts of brood parasites, selection pushes the host to evolve distinctive colorations so foreign eggs can be spotted and rejected. The parasite then evolves better mimics, producing an evolutionary arms race between host and parasite. In the common guillemot, which nests in large groups, each female's eggs carry very different markings, making it easier to identify her own among the crowded cliff ledges.

• 5. Hunters And Cheats

  ›

  Many animals feed on eggs. The 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 the long-tailed weasel, M. frenata, steal ducks' eggs. Snakes of the genera Dasypeltis and Elachistodon specialize entirely in eating eggs. Some birds turn other birds' nests into nurseries. Brood parasitism happens when one species lays its eggs in the nest of another, an uncommon behavior practiced by roughly 1 percent of bird species as obligate parasites. In some cases the host's own eggs are removed or eaten by the female, or expelled by her chick. The brood parasites include the cowbird, the black-headed duck, the cuckoo-finch, and three Old World cuckoo species. That parasitic pressure helps explain why a host species would invest so heavily in eggs that no rival could convincingly fake.

• 6. Yolk And Measure

  ›

  More than half the proteins in egg yolk are phosphoglycoproteins, equivalent to milk proteins in mammals or storage proteins in plant seeds. The yolk feeds the growing embryo, and its major precursor is the polypeptide vitellogenin, which occurs in all egg-laying animals and is synthesized in the liver. The insect form is called vitellin. Surrounding the yolk in the egg white is the albumen, a clear hydrophilic layer that serves as a water reservoir. Its predominant protein is ovalbumin, more than half the proteins by mass, though its role is unknown. Ovomucoid, making up 11 percent of the albumen, is the primary egg allergen. Scientists sort vertebrate eggs by how much yolk they carry. Eggs with little yolk are microlecithal, medium ones are mesolecithal, and large ones with a concentrated yolk are macrolecithal. In microlecithal eggs, the yolk is evenly spread, so cleavage divides the egg into cells of fairly similar size. These small eggs can be produced in huge numbers and are the norm where egg mortality is high, as in bivalves and marine arthropods. In placental mammals, where the mother nourishes the embryo throughout the fetal period, the egg holds little if any yolk. Macrolecithal eggs work differently. Their yolk is so large that cell division cannot split the mass, so the fetus develops as a plate on top of the yolk and only envelops it later. These big-yolked eggs appear only in cephalopods, cartilaginous fish, bony fish, reptiles, birds, and monotreme mammals. The eggs of the coelacanths can reach 9 centimeters across, and the young complete full development in the uterus, living on that copious yolk.

• 7. Eggs Among The Mammals

  ›

  Platypuses and two genera of echidna, the spiny anteaters, are Australian monotremes, the only living order of egg-laying mammal. Their eggs are macrolecithal, very much like those of reptiles, and early extinct mammal species also laid eggs, probably the ancestral state. Marsupials lay no eggs, yet their eggs are still macrolecithal, merely small, developing inside the female without forming a placenta. Their young are born at a very early stage and can be classified as a larva in the biological sense. Placental mammals use two types of placenta, the yolk sac and the chorioallantoic. In humans, the first nutrient source is a yolk sac placenta, replaced by a chorioallantoic placenta at around four weeks. Around the eighth week, the yolk sac is absorbed into the umbilical cord, and the fetus finishes developing inside the uterus on nutrients from the mother. The French biologist Thierry Lodé proposed a scheme that divides reproduction by how much development happens before young leave the adult's body. It runs from ovuliparity, where the female spawns unfertilized eggs, through ovoviviparity, where the zygote stays in the body without feeding interactions, to hemotrophic viviparity, where a placenta channels nutrients from the mother's blood. In some sharks and in the black salamander Salamandra atra, embryos feed by consuming other ova or sibling embryos, a form of intra-uterine cannibalism.

• 8. Food, Faith, And Vandalism

  ›

  By a wide margin, the egg most often eaten by people is the chicken egg, typically unfertilized, though duck, roe, and caviar are also popular choices. Under the Kashrut, the set of Jewish dietary laws, eggs are considered pareve, neither meat nor dairy, despite being an animal product, and may be mixed with milk or kosher meat. The chicken egg also became a tool of medicine. In 1931, Alice Miles Woodruff and Ernest William Goodpasture at Vanderbilt University discovered that the rickettsia and viruses behind many diseases will grow in chicken embryos. That finding enabled vaccines against influenza, chicken pox, smallpox, yellow fever, typhus, and Rocky mountain spotted fever, among others. Beyond food and medicine, eggs carry symbolic weight, often standing for life and rebirth, healing and protection, and sometimes appearing in creation myths. Christians view Easter eggs as symbolic of the resurrection of Jesus Christ. The same object also serves as a weapon of nuisance. Throwing raw eggs at houses, cars, or people, known as egging in English-speaking countries, is a minor form of vandalism that can damage vehicle paint and cause serious eye injury. Collecting wild birds' eggs was once a popular hobby, including among European Australians, with the embryo removed before the shell was stored. The practice is now banned in many jurisdictions because it can threaten rare species. In the United Kingdom it is prohibited by the Protection of Birds Act 1954 and the Wildlife and Countryside Act 1981. Those early collections survive as a record. The Australian Museum holds about 20,000 registered clutches, and scientists treat such collections as good natural-history data, because the collectors' notes have helped them understand how birds nest.