The great egg strategyAnimals, 14 March 2020
William Harvey (1578-1657) said "Every living animal comes from an egg".
Harvey didn't know about micro-organisms, because bacteria were not discovered until 1676 and protozoa even later than that. He was basically correct, though. Anything much larger than a bacterium, including a human, starts life as an egg.
OnePlanet is in clear English for international readers. The articles use technical words when necessary, but I don't say "neonate" or "juvenile" if I can say "baby", and I don't use idioms like "stave off" or allusions like "Faustian". Read it for fun, or as an easy introduction to academic English.
Asexual reproduction. If you're a bacterium, reproduction is easy. You get a bit bigger, and then you divide in two (fission). The bacterium Escherichia coli can divide every 20 minutes. After 8 hours, a single E.Coli cell can become 224 of them, which is 16,777,216 bacteria. Fission takes almost no energy, you can do it on your own, and it produces an adult. There are no bacteria eggs, and there are no baby bacteria.
Sexual reproduction. If you're much bigger than a bacterium, you're probably either male or female. The process of having a baby starts when a female produces an egg or ovum. This is a single cell, but in some species it's big enough to see without a microscope. A male adds his genetic material (fertilisation). This turns the egg into a zygote, which is a single cell that has genetic material from both parents. The zygote starts to divide and form more cells. It needs oxygen and food (nutrients). In mammals, it gets them from its mother's blood, by attaching to her uterus, and it grows inside her. The zygote becomes a complex embryo. The embryo continues to develop and becomes a foetus. It finally comes out as a baby. This is live birth or viviparity. This strategy works very well for elephants, mice, humans and all other viviparous animals. Mammals are the specialists in viviparous reproduction. Nearly all mammals are viviparous.
In other animals (worms, insects, spiders, crustaceans and other invertebrates; and amphibians, birds, most fish and most reptiles) the embryo-foetus-baby part of the process happens outside the mother's body. This means egg laying; the mother lays eggs. A biologist would call this oviparity; these animals are oviparous.
The picture shows butterfly eggs on a leaf. Different species use different techniques to make sure the embryo has food, oxygen and some protection while it grows. Even so, it's obviously safer for an egg to develop inside its mother until it is big enough to live outside her.
The babies of some viviparous parents are quite big. A baby elephant (an elephant calf) weighs 100-120 kg, and a blue whale baby (a whale calf) weighs more than 2 tonnes. A whale or dolphin calf can swim as soon as it is born. The babies of mammals that walk constantly (elephant, zebra, horses, caribou, musk ox, wildebeest, etc) are often strong enough to walk with their mother a few minutes after they are born, and they can run within a few hours of birth.
Compare this with oviparous species such as birds. Eggs can't move, they can't control their temperature, can't find food, can't protect themselves and can't even call for help. Also, eggs usually produce a much smaller baby than with live birth. What makes this a good strategy?
It obviously is a good strategy, because nearly all animals are oviparous. It works well for invertebrates and for birds, most fish and most reptiles. For millions of years it worked well for dinosaurs and other giant reptiles.
The disadvantages of egg-laying
Eggs that develop outside the mother have certain problems:
- An egg is very soft and delicate. It could get damaged. With larger animals like birds and reptiles, the mother has to make a hard shell around each egg.
- The embryo needs oxygen from the air, so the shell must be gas-permeable, but it must also stop the embryo's water evaporating. The embryo must not get too dry.
- The embryo needs enough food and water to grow for some days or weeks. Eggs must contain contain water, fats, proteins and other nutrients.
- Eggs are full of high-quality food, so a lot of other animals want to eat them. Most predators (foxes, snakes, birds, etc) will take an egg if they can. Some snakes specialise in eating bird eggs, and of course we eat them as omelettes, Egg McMuffins™, cod roe and caviare.
- Most eggs need to be at a constant temperature. It's much easier to keep an embryo at a constant temperature if it's inside the mother's body. A bird embryo in an egg can easily get too cold and die. The parents have to sit on the eggs for a long time to keep them warm (incubation). This makes it easy for predators to eat the parent too.
- Most oviparous mothers can't move their eggs from one place to another, so if there's a problem with predators, or there's a fire, or the river floods, or an elephant knocks her tree down, there's not much the mother can do.
Birds that nest on the ground, like the Eurasian stone curlew (Burhinus oedicnemus - see photos) protect the nest by staying absolutely still. They sit tight unless they know a predator has seen them. In that case, they pretend to have a broken wing (a distraction display or just a broken wing display). The bird runs ahead of the predator for several hundred metres and then flies away.
- An egg can't be very big. A really big egg needs a very thick shell or it will break. If an eggshell is too thick, the embryo will not be able to get enough oxygen, and will not be able to break the shell and get out (hatching). This is an obvious problem if the parent is a very large animal, and we'll look at dinosaur eggs below.
- Mammals and reptiles all breathe air, but some of them live in water. If you breathe air and lay eggs but live in water, you have an extra problem. That's because a very small new baby may drown (die from breathing in water). Whales and dolphins (Cetacea) are marine mammals; they are viviparous, and a whale or dolphin calf is quite big. It can swim immediately. It can go to the surface for air, with little or no help from its mother. Marine turtles (Cryptodira) are reptiles that live their entire lives in the sea. However, they can still move short distances on land, if the land is a sandy beach. Marine turtles come out of the water to lay their eggs in the sand. Sea snakes (Hydrophiinae) are marine reptiles. Some can still move short distances on land, and they lay their eggs on land. The sea snakes that can't move on land keep the baby in the mother's body until it is big enough to swim to the surface for air.
The advantages of egg-laying
With all these problems, there must also be some major advantages to producing eggs that develop outside the mother:
- For a fish, insect or other invertebrate that doesn't give any care, protection or education to its young, laying eggs is a very good solution. They lay their eggs, and then they go away. Problem solved! However there are reptiles, fish and even insects and spiders that do care for their eggs. The tiny Cyclops or water flea is often only 1mm or 2mm long, but the female carries her eggs on her body until they hatch. Earwigs (Dermaptera - see photo) stay with their eggs, clean them to remove fungi, help the young to emerge, and then feed them and protect them from predators as they grow. Many spiders also stay with their eggs to protect them, and some spiders feed their young.
- Some oviparous animals can produce dozens, hundreds or thousands of eggs at one time, and a single sea urchin (Echinoidea - see detail photo) can release a million eggs in a few minutes. If all the animals in the same species produce their eggs in the same region, and in the same period of just one or two days per year, that can mean billions of eggs in the water at the same time. That is too many for predators to eat.
This is a great strategy if you live in water, which is a very dangerous environment. A lot of fish and crustaceans do this.
At one time, every organism on the planet lived in water and breathed water. Although oviparity seems more complex and advanced than viviparity, it is likely that oviparity came first.
- Weight is very important to an animal that needs to run fast, and even more if the animal needs to fly. A viviparous mother with babies inside her will be heavy and slow. Because birds produce eggs instead of live young, the mother doesn't need to carry the babies around all day. Bats (Chiroptera,the mouse-sized flying mammals) do produce live young, which the mother carries with her until they are old enough to fly. However, a bat can only have one or two babies at at time. A bird can easily have a clutch of 9 or 12 eggs. Some birds lay twice as many as that. The ostrich at the top of this page has 30 chicks.
- It takes a lot of energy to produce eggs, so the mother is often very thin after she lays them, but it's quicker to produce eggs than to produce live young. Most oviparous mothers can start to eat a lot as soon as they lay their eggs. By the time the babies emerge, the mother is fit and ready again.
- If a viviparous mother is killed before the birth of the babies, they die too. In many oviparous species it's not a problem if the mother is killed, because the eggs don't need her protection, and the young are completely independent when they come out of the egg. Mother birds do usually protect their eggs and feed the young birds, but if a mother bird is killed while the babies are still in the egg, the male bird can often do her job.
- Producing eggs takes less energy per baby. Mammals usually give birth to between 1 and 15 babies at a time, but some birds lay 15 or 20 eggs; the hawksbill turtle (Eretmochelys imbricata) may lay 140 eggs; and some marine invertebrates can produce a million eggs at a time.
Laying eggs is more energy-efficent than live birth, but being energy-efficient is not the same as being successful. Mammals are very inefficient users of energy compared to reptiles. A mammal has to eat every day just to keep warm, when a snake or a spider is quite happy it if eats once a week. Mammals are just as successful as snakes, though.
Which is the best strategy? Laying eggs or giving birth to live young?
The answer is "it depends".
• Most sharks (Selachii) are oviparous and aplacental, some are viviparous and placental, and some are ovoviviparous. (See Egg Vocabulary, below.) Sharks are an ancient family, and all of these strategies are successful. Click here for pictures of living shark embryos.
• Mammals are the classic example of viviparity, but there are also successful mammals that lay eggs. For example, the platypus (Ornithorhynchus anatinus) in Australia.
• Some reptiles are oviparous, some are viviparous, and some are ovoviviparous. In British Columbia (which has cool summers and long, cold winters) only half the reptile species lay eggs. The northern half of British Columbia is really cold, and none of the reptiles in northern B.C. lay eggs.
Some reptiles even have a choice between being oviparous and truly viviparous, with a placenta that connects the embryo to the mother's blood supply. The yellow-bellied skink (Saiphos equalis) is a small lizard that lives in eastern Australia. It has remarkably small front legs, and may have no back legs at all - see photo. S.equalis that live by the sea lay eggs with shells, but S.equalis that live in the mountains, not very far away, give birth to live young. One of these skinks has even been seen to do both at the same time.
(not in alphabetical order):
Shell (n.): The protective case around an egg. Bird eggs have a hard, rigid shell. The eggs of snakes and other reptiles have a tough, flexible shell. Amphibian and fish eggs usually have just a thin membrane and no tough shell. This may be called an egg sac if it's very thin and transparent, or an egg case if it's tough and flexible like those of the sharks.
Ovum (n.): The biologist's word for an egg. The word ovum (plural ova) is normally used to talk about microscopic unfertilised eggs, not about eggs with a shell.
Fertilisation (n.) or fertilization in American spelling. The moment when male genetic material makes contact with the female genetic material in the egg. An egg can be fertilised inside the female, as with birds, reptiles and mammals; or outside the female, as with most amphibians, fish and crustaceans. These are called internal and external fertilisation.
Embryo (n.): A developing baby. In humans, the baby is an embryo from the time of the zygote (the fertilised egg) until it is 9 weeks old. After that time it is called a foetus. For non-human babies, there is no clear rule about when an embryo becomes a foetus; in fact, a baby developing inside an egg with a shell may be called an embryo right up until it hatches.
Foetus (n.) or fetus in American spelling: The baby as it develops inside the egg. A foetus is more developed than an embryo.
Larva (n.): Insects, crustaceans, fish and amphibians usually have a larval form between the egg and the adult. A larva looks completely different to the adult. A frog larva is called a "tadpole"; it has no legs, and it lives underwater like a fish. (Human babies are not "larvae" because they're not very different from adult humans.)
Yolk (n.): Food for the embryo, inside an eggshell or yolk sac. It is mainly fat and protein, and it is usually yellow.
Egg white (n.) or albumen. Food and protection for the foetus inside an eggshell or egg case. It is mainly water and protein, and it is usually transparent.
Gas exchange (n.): An embryo needs oxygen as it develops. The shell of an egg is porous, to allow oxygen to enter from the outside air, and carbon dioxide to leave.
Cock and hen (n.): Male and female birds. Also male and female salmon, crabs and lobsters.
Cloaca (n.): Birds and reptiles have a single opening for urination, defecation, reproduction and laying eggs, which is why a hen's egg from a farm shop is often a bit ... messy.
Ovipositor (n.): Many invertebrates and some fish have a tube they use to lay eggs. Sometimes they use it to attach their eggs to a leaf or a stone. The seahorse (Hippocampus) is a very strange genus of fish, in which the female uses her ovipositor to lay eggs inside the male seahorse's "brood pouch". Some flying insects use the ovipositor to lay eggs inside other living animals. Ichneumon wasps (Ichneumonidae) use their ovipositor to lay eggs into the larvae of other insects and spiders. The ovipositors of some insects can even make a hole in a tree and lay an egg in or on a beetle larva.
Nest (n.): A typical nest is a bed, shaped like a bowl, that a bird makes for its eggs. Some fish also makes nests for their eggs.
To lay (v.): When the mother puts her eggs into the nest, she lays them. The eggs are laid. If you buy eggs from a farm shop, the box may say "fresh eggs" or "new-laid eggs".
Clutch (n.): All the eggs laid by one female at one time. For example, "she was sitting on a clutch of 6 eggs".
To spawn (v.): Fish and amphibians lay a lot of eggs, and usually they lay them in the water. This is called spawning, and the eggs themselves are called spawn. The male then fertilises the eggs outside her body. Until very recently, every child in Britain knew what "frogspawn" looked like, because you could see it in any quiet pond in spring. Frog eggs are dark in colour, but as the female lays her eggs, she also produces a lot of clear jelly which helps to protect the eggs until the larvae are able to move independently.
To incubate (v.): An oviparous parent incubates its eggs when it stays in physical contact with them, so as to keep them warm. Birds usually incubate their eggs by sitting on them. There are other ways to incubate eggs, such as putting them under sand or vegetation so that they stay at a constant temperature. An egg is not the only thing that is incubated; when a bacteria or virus gets into your body, the time until you show signs of disease is called the "incubation period". In a hospital, a premature baby may be put into a warm box called an "incubator". Many towns now have a large building with a lot of small offices and central photocopying, secretarial and internet services for small new businesses. This may be called a "business incubator".
To brood (v.): An oviparous parent broods its eggs when stays very close to them. This often means the same as incubation, and a chicken with a strong desire to incubate eggs is called a "broody hen" (adj.). A woman who is desperate to have a baby is also sometimes called "broody" ... but probably not if she can hear you. A fish, a reptile, an amphibian or an invertebrate may brood its eggs to protect them from predators, not to keep them warm. Some fish "mouthbrood", which means that the babies stay close to the parent, and if a predator arrives they go into the parent's mouth.
Brood patch (n.): Many birds remove feathers from the part of their body that is in contact with the eggs. This is called a "brood patch". Direct contact between the eggs and the parent's skin helps keep the embryos warm and hydrated.
Brood pouch (n.) The male seahorse (Hippocampus) is famous for having a "brood pouch". The female lays her eggs into the brood pouch, and they develop inside the male's body until they hatch.
Brood (n.): I said that a clutch is all the eggs laid by one female at a time; her "brood" is all the baby birds that come from those eggs. So, one particular hen could have a clutch of 6 eggs and, three weeks later, a brood of 5 or 6 chicks. (The equivalent word for mammals is a "litter". So, one particular cat could have a litter of four or five kittens.) To hatch (v.): When the baby leaves the egg, it hatches. It is then a chick (for birds) or a hatchling (for reptiles) until it gets a bit bigger. Hatching is not easy, and a baby often needs help. Many baby reptiles and birds have a temporary "egg tooth" (a hard, sharp point on their nose or beak) to help them break the egg. Mother crocodiles will carefully help a baby to get out of its egg.
Hatchling (n.): A baby that has just hatched.
Nestling (n.): A baby bird that is too young to leave the nest.
Chick (n.): An older baby bird that can walk but not fly. The word is used mainly for baby chickens, as in the "brood" photo on the right. If it's a baby duck, it's a duckling.
Fledgling (n.): An older baby bird that has most of its feathers and will soon learn to fly.
Yolk sac (n.): In biology, a sac is a soft, flexible, transparent bag (not a sack, which is a cheap cloth bag for carrying potatoes and coal). Both oviparous and viviparous embryos have a yolk sac containing nutrients. Human embryos have a yolk sac for the first few weeks, but then they connect to the mother's uterus with a placenta. Some baby fish hatch with a sac of food attached to their stomach. These babies are called "yolk sac larvae" or "alevins".
Oviparous (adj.): The mother produces an egg with yolk (and usually a shell). She then lays the egg, which hatches outside her body. Birds are oviparous.
Viviparous (adj.): The egg has no shell and very little yolk. It develops inside the mother's body, in her uterus (her womb). She gives birth to a live baby. The embryo may be connected to her uterus by a placenta so it can use her blood for food and oxygen. Humans are viviparous and placental. A placenta is not the only way for a viviparous mother to feed her embryos. Some sharks produce a kind of milk through the wall of the uterus, and some produce extra unfertilised eggs for the embryos to eat while they are in the uterus.
Ovoviviparous (adj.): The mother produces eggs with yolk but they develop inside her body. She keeps the eggs inside her body until the babies are ready to emerge. Most reptiles are oviparous, but vipers and some other snakes and lizards are ovoviviparous. So are some fish. The word "ovoviviparous" is going out of fashion, and it's better to talk about "oviparous egg retention".
Placenta (n.): The structure that connects a viviparous embryo or foetus to its mother for nutrients and gas exchange; or the structure that connects a plant seed to the parent plant.
To give birth (v.): When a baby, not in a shell, emerges from the body of a viviparous mother, she "gives birth". And as you know, your birthday is the day you were born on. Viviparous mothers "bear" live young. (Pronunciation: That's beə(r) as in air, wear, tear and pear, not as in beer, clear, near or dear).
Food for embryos
In the hen's egg that we all know, the zygote starts life in the middle of a food supply - the yolk and the albumen. The hen first has to find food and eat it, and then process the food into yolk and albumen, and then add a hard shell. This process takes time and energy, and it's quite complex, so there are a lot of things that can go wrong. The eggs of all vertebrates (birds, reptiles and mammals) have yolk.
Many insects have a much more economical system. They lay eggs directly onto something the larvae can eat. For example, the European/ Asian peacock butterfly (Aglais io) lays its eggs on the leaves of the stinging nettle (Urtica dioica). When the eggs hatch into caterpillars, they can start eating the leaf straight away.
It isn't easy to see peacock butterfly eggs because they are the same colour as the nettle leaf they are stuck to, but lacewings (Neuroptera) lay white eggs that are not stuck directly to the leaf. There are at least 85 species of Neuropteran, such as Chrysoperla carnea. A lacewing mother attaches the eggs to the leaf or stem of a plant. Each egg is on the end of white stalk that may be 10mm long. The stalk is perhaps to protect the eggs from predators, but lacewing larvae are tiny predators themselves. They eat a lot of aphids, so that gardeners often call them the "aphid wolf" or the "aphid lion". They also eat caterpillars larger than themselves, and sometimes even try to bite a human finger, without success.
Many flies (Diptera) lay their eggs directly onto a dead animal, so the fly larvae (maggots) can eat it as they develop. The adults are astonishingly fast at finding dead animals, and have often been observed to arrive within 2 minutes of a mouse dying in a mousetrap. Some of the Diptera don't wait until the animal is dead. Blowfly strike or myiasis is when fly larvae start to eat an animal alive. This is typically a large animal such as a deer, a sheep or a cow, and sometimes a human. Maggots of the bluebottle (Calliphora erythrocephala) will eat only dead tissue, but the greenbottle (Lucilia sericata) and blackbottle (Phormia terraenovae) will also eat living tissue.
Many wasps, especially ichneumon wasps (Ichneumonidae) and spider wasps (Pompilidae) lay eggs in or on other insects and spiders. Spider wasps sting the spider so that it is alive but can't move. They put it in a safe place, such as the hole the spider lived in, and lay an egg on it. The wasp then covers the entrance to the hole with earth to keep it safe. After the egg hatches, the wasp larva eats the living spider.
Shark eggs start life with yolk, but many of them finish the yolk while still in their mother, and then connect to their mother's blood supply through a placenta. The embryo of the grey nurse shark or sand tiger shark (Carcharias taurus) does develop inside its mother until it is born, but it does not develop a placenta. Instead, it eats the mother's unfertilised eggs - and it also eats fertilised eggs, and even its brothers and sisters until there is only one baby (a shark pup) left. This has two advantages. First, that one pup is big and strong when it is born. Second, the other pups often had a different father. This cannibalism helps ensure that only the strongest pups with the best genes survive.
What happens after an animal lays its eggs? A lot of fish release their eggs into water, so that the eggs go off individually and the mother never sees them again. Some animals lay their eggs in one place, go away and never come back. A marine turtle, for example. Some lay the eggs and then go away but not too far, and come back when the eggs start to hatch. For example, a crocodile. And some stay with the eggs to protect them until they hatch, like a bird.
Eggs usually need a specific temperature. If they get too hot or too cold, the foetus will die. Birds in cool climates have to sit on their eggs almost the whole time until they hatch.
Megapodes (Megapodiidae) look like large chickens, and live in Australia and on islands in the western Pacific. They don't incubate their eggs by sitting on them. Instead, they leave them in warm places. These places can be warm because of the sun; or because of geothermal heat from under the ground. They also create warmth using wet leaves. If you make a big pile or mound of wet grass or leaves, it soon gets warm. The leaves create heat as they decay, and the male bird adds leaves or takes them away to make sure the eggs stay at the correct temperature. Megapodes are sometimes called "incubator birds" or "mound builders".
The Emperor penguin (Aptenodytes forsteri) lives in the Antarctic. For 10 weeks in winter, each male holds one egg on top of his feet and under his stomach, to keep it at 38° in temperatures of -20° and sometimes -35° Celsius, often with snow and 80 km/h winds. It would be a lot easier if the embryo developed inside the female's body, but all birds are oviparous.
Mammals produce milk for their young, but so do some non-mammalian species.
In pigeons and doves (the Columbidae), both parents produce a kind of milk for their young. It comes from the part of the throat (the interior of the neck) called the crop, so it is called crop milk. Cow's milk is a liquid, but crop milk is almost solid, like soft cheese. It contains a lot of fat and protein. Young pigeons and doves eat nothing but crop milk for the first week or two of their lives. It's not only pigeons and doves; flamingos and the male Emperor penguin also produce crop milk for their young.
A Japanese jumping spider, Toxeus magnus, produces "milk" for its babies for 40 days after they hatch. This spider milk contains fat, and four times as much protein as cows' milk.
Some species of shark and ray (Batoidea) retain their eggs and produce a kind of mucous milk for embryos in their uterus or even produce semi-solid structures (trophonemata) for the embryos to eat. That's called "histotrophy".
Histo- means "living tissue", as in "histology" which means studying tissue under a microscope. The word endings -troph, -trophic and -trophy mean eating or growing. For example, green plants are "autotrophs" because they make their own food from water, carbon dioxide and sunlight. If we think of life as a pyramid with green plants at the bottom and apex predators like lions and tigers at the top, "trophic level" describes where any living organism is in that pyramid. "Matrotrophy" means getting food direct from the mother's body, which is what viviparous animals do. In some species of spider, the young kill and eat their mother. This is called "matriphagy".
Caecilians are tropical amphibians that look like fat blue worms (if small) or wet blue snakes (if large). They lay eggs. Some caecilians stay with their young and produce extra skin for them to eat. So, is that histotrophy, matrotrophy or matriphagy? Or all three?
"Baby" vs neonate and juvenile
Scientists shriek with horror if they read the word "baby" in an article about animals, because it's unscientific. It's unscientific because it's anthropomorphic. In other words, it means thinking about animals as if they're like humans. This is an interesting area.
Many religions regard animals as furry robots because they don't have a soul.
Many scientists regard animals as furry robots because they don't have a mind.
This has some advantages for scientific research, but it creates problems. First, there is the problem of mind. There is a lot of evidence that animals have memory and intelligence, and individual preferences and personalities, so what is mind? If it's something extra that only humans have, it starts to sound like soul.
Second, there is an ethical problem because robots have no personality, no emotions and do not feel pain, so you can do anything you want to them. The friends of René Descartes (1596-1650, "I think therefore I am") used to nail living dogs to a table and cut them open for science, without anaesthetic. It reminds me of the old joke about the farmer who castrated bulls by crushing their testicles between two bricks. Somebody asked him "Doesn't it hurt?" and he said "Only if you hit your thumb."Third, there is a scientific problem, because the rule against anthropomorphism means that a scientist must not consider any "anecdotal evidence" of animal behaviour. In other words, if you observe an animal do something once, that's not enough. You must be able to repeat the observation or it's not scientifically valid. However, animals often do something only once. You can't make your Siamese cat repeat the clever thing it once did with the refrigerator. Anybody who rides horses regularly knows that horses like some people more than others. If a horse likes you, it will catch you when you nearly fall off. If it doesn't like you, it waits for an opportunity to make you fall off. These are important aspects of animal behaviour that science traditionally does not consider, because they are not repeatable and they depend on the anthropomorphic idea that a horse can have preferences.
We'll look at anthropomorphism again. In the meantime, it's OK for a science writer to call baby animals "neonates" when they are very new babies, then "juveniles", then "sub-adults", then "adults", and you can use the adjective "immature". Perhaps also "adolescents", if the particular species has a stage when they behave like adolescent humans. For example, dolphins, elephants, wolves, chimpanzees and gorillas. Scientists aren't very happy about using the word "mother", either. To avoid using it, they sometimes say "maternal parent" or "female parent" or just "parent". Ah, academic English...
There are hundreds of words in English for different stages in the development of baby animals. Some of them are the traditional words of fishermen, hunters or farmers. Egg, alevin, fingerling, parr and smolt are all words for different ages of baby salmon (Salmo salar). Most baby fish are just called fry, but salmon and trout are commercially important so they get more words. There are also scientific words for stages that are of no interest to farmers, such as larva, embryo and foetus, and nymph, pupa, instar and imago for various small invertebrates. Many scientific words are for baby stages that are so small that no human eye saw them until the microscope was invented in the late 17th century, such as nauplius, cypris and zoea. These are larval stages of various marine crustaceans.
More about eggs and embryos
The pictures show the embryo of a catshark (Scyliorhinidae) in a transparent egg case; and embryos of the lesser spotted dogfish (Scyliorhinus canicula, another kind of catshark) in translucent egg cases in an aquarium. You can often find these egg cases on Atlantic beaches. They are dark brown, strong but flexible, square like an envelope, with a short tendril at each corner, and they are called "mermaids' purses".
Bird eggs form in the mother's body before they are fertilised. After fertilisation, the mother's body has only to add the shell. This may happen even if the egg has not been fertilised. That's why shops can sell unfertilised eggs from chickens. In the normal social structure for chickens, hens are regularly inseminated by the cock bird, so their body just goes ahead and makes eggs, on the assumption that they will be fertilised. Farms that produce eggs for sale in food shops simply keep the hens separate from the cock, or refrigerate the eggs as soon as the hen lays them.
The cuckoo (Cuculus canorus) is a bird famous for its "cuck-oo" song, and for laying its single egg in the nests of other birds. Cuckoos lay eggs that look like those of the other birds. When the baby cuckoo hatches out, it throws the other eggs out of the nest. The cuckoo is a medium-sized bird, and its mother often lays her egg in the nest of a small bird, so the baby cuckoo is often bigger than the "parents" who are feeding it. Biologists are not sure why the "parents" do not understand that this monster baby is not theirs. Another species of cuckoo, the great spotted cuckoo (Clamator glandarius), normally lays its egg in the nest of a pair of magpies. The magpie (Pica pica) is an intelligent bird and often sees that there is a problem. However, there is some evidence that if the magpies throw the cuckoo chick out of the nest, the mother cuckoo comes back, kills the magpie chicks, destroys the nest and possibly attacks the adult magpies. Biologists call this "the Mafia hypothesis".
Some species of insect use a similar tactic, for example "cuckoo bees" (Psithyrus) and "cuckoo wasps" (Chrysididae).
Birds can't move their eggs if a predator arrives. All they can do is try to make the predator go away by attacking it. Some sea birds, especially terns like the Arctic tern (Sterna paradisaea) will attack people who go near their nests by diving at the person's head.
Small birds that see a predator will make alarm calls, and this may collect a group (a "mob") of other small birds who all make noise and dive at the predator ("mobbing").
The largest bird eggs were those of the extinct elephant birds from Madagascar, like Aepyornis maximus and Vorombe titan. The adults could weigh 750kg, and some eggs were 34cm long and weighed 10 kg. Even that largest dinosaur eggs were not much bigger than that.
Dinosaurs certainly laid eggs; as far as we know, no dinosaurs gave birth to live young. Dinosaurs were reptiles, but it is modern birds that are the last of the dinosaurs. Birds evolved from small dinosaurs and they are members of the Dinosauria clade, so it's not surprising that all birds lay eggs too.
A large dinosaur had a similar problem with laying eggs that a giraffe has when giving birth; it's a long way to the ground. Some mother giraffes stand with their legs as far apart as possible, to reduce the distance the baby has to fall; other mothers just let the baby fall on its head. Giraffe babies are born head first, and their necks are so long that the head doesn't fall very far, but the baby's hind feet may fall 2 metres.
Dinosaur egg shells were up to 4.75mm thick. Inside an egg with a really thick shell, it must have been difficult for the embryo to get enough oxygen. Dinosaur egg shells often had a very complex structure of pores and channels so that oxygen could get in and carbon dioxide could get out. A very thick shell was also difficult for the baby to break when hatching. However, a shell 4.75mm thick would not have been strong enough to protect its 20 kg contents in a fall of 2 or 3 metres onto hard ground. Nobody knows how big dinosaurs solved this problem. Unlike bones and teeth, soft tissue such as skin does not often get fossilised. Possibly the females had a long tube of skin on the cloaca (an ovipositor) and eggs slid slowly down to the ground inside the tube.
Plesiosaurs lived at the same time as the dinosaurs, but they weren't members of the Dinosauria. They were marine reptiles, and we now know that some of them gave birth to live young. Plesiosaurs were fast swimmers that ate fish and other large marine creatures. They could be anything from 1.5 metres to 15 metres long (as big as a medium-sized whale). They breathed air, like marine turtles, whales and dolphins. Plesiosaurs had longer, stronger flippers than whales or dolphins. Some of them may have been able to move a short distance on land, like the marine turtles today that use their flippers to go up a beach to lay eggs in the sand. Some experts thought that plesiosaurs probably did this too, but others thought they were like whales and dolphins, and could not move around out of the water, especially the whale-sized species.
In 1987 in Kansas, the fossil skeleton of one Polycotylus latippinus plesiosaur, an adult female 5 metres long, was found with the skeleton of another plesiosaur inside it. The smaller one would have been 1.5 metres long. In 2011, researchers announced that they were mother and baby. The "pregnant plesiosaur" is a star exhibit at the Natural History Museum of Los Angeles.
Of course, there is more than one way to end up inside a large carnivorous animal, but the smaller plesiosaur was probably too big to swallow in one piece; it was the same species as the larger one; its bones were still soft (not fully ossified) when it died; they were not damaged by teeth; and some of its bones are stuck to the inside of the larger plesiosaur's pelvis. So, it looks as if plesiosaurs were viviparous. They gave birth to a single, large baby which was able to swim to the surface and breathe air.
A baby 1.5 metres long was a big investment for the mother plesiosaur, unlike a mother insect, fish or mouse which may have hundreds or thousands of offspring. Plesiosaurs were K-selected or K-strategists like us; mice are r-selected or r-strategists. In a K-selected species, the mother has only a few babies, which are quite big, and she protects and educates them for months or years. That means plesiosaurs were probably quite intelligent, lived in groups and had complex social lives.
Compare that with the oviparous dinosaurs. They laid eggs in clutches of up to 40 eggs. The largest dinosaur egg was 50cm-60cm long. That's very small for an animal that could have weighed two or three tons, and the largest dinosaur weighed 50 tonnes or more. It is difficult to imagine that a 10 kg or even 20 kg baby could have lived with adults that size. A small baby can't walk as fast as a colossal adult, it can be killed by predators that the adult is too big to see properly, and the adult may kill the baby accidentally. The big dinosaurs probably did very little baby care, protection or education. Instead, the babies had to grow up alone.
The babies (OK, "juveniles") of the giant dinosaurs probably didn't even eat the same food as the adults. The skull shape of a juvenile Diplodocus is very different to the skull shape of the adults. Probably the adult Diplodocus simply ate all the vegetation it could find, including the tops of trees, but the juveniles looked for young leaves near the ground. You won't find that in the middle of a forest, so probably the juveniles lived on the edge of woodland.
There were hardly any dinosaur species with adults that weighed between 2kg and 100kg, probably because the juveniles of the giant dinosaurs filled those ecological niches. Instead of the hundreds of thousands of species that exist today, each with its own little ecological niche, there were only a few species of dinosaur, and each individual of the giant dinosaurs occupied several different niches as it got bigger. That meant a lack of biodiversity. It is biodiversity that makes it possible for a particular class of animals (mammals, or reptiles, or birds, for example) to survive a disaster such as rapid climate change or a meteor strike.
The extinction of the dinosaurs is all guesswork, though. You don't see many plesiosaurs around today either!
1: Ostrich and chicks, Phil Kates on flickr, CC BY-SA 2.0
2: Stink bug eggs and hatchlings, Heinz Anton Meier on Pixabay
3: Butterfly eggs, Rolf Dietrich Brecher on flickr, CC BY-SA 2.0
4: Gull's eggs, Brett Hondow on Pixabay
5: Stone curlew sitting tight, Karsten Paulick on Pixabay
6: Stone curlew broken wing display, sipa on Pixabay
7: Earwig on nest, Jean Hort on flickr, CC BY-SA 2.0
8: Sea urchin (detail), Bernd Thaller on flickr, CC BY 2.0
9: Gaping nestlings, Franck Barske on Pixabay
10: Yellow-bellied skink, Ray Mjadwesch and Doug Beckers on flickr, CC BY-SA 2.0
11: Snake with eggs, Joshua Ganderson on flickr, CC BY 2.0
12: Tortoise hatching, Skeeze on Pixabay
13: Ichneumon wasp, Katya on flickr, CC BY-SA 2.0
14: Frogspawn, Adege on Pixabay
15: Brood of chicks, Eveline de Bruin on Pixabay
16: Yolk-sac larva, California Department of Fish and Wildlife on flickr, CC BY 2.0
17: Lacewing (Neuroptera) eggs, Dinesh Valke on flickr, CC By-SA 2.0
18: Greenbottle (Lucilia sericata), Nicki Dugan Pogue on flickr, CC BY-SA 2.0
19: Dove and chick, AnnCam on flickr, CC BY 2.0
20: Caecilian in Khao Yai national park, Thailand. Tontan Travel on flickr, CC BY-SA 2.0
21: Catshark embryo, NOAA Ocean Exploration & Research on flickr, CC BY-SA 2.0
22: Dogfish embryos, Chris Favero on flickr, CC BY-SA 2.0
23: Arctic tern dive-bombing, Jitze Couperus on flickr, CC BY 2.0
24: Dinosaur graphic, Clker-Free-Vector-Images on Pixabay
25: Dinosaur nest fossil, Gary Todd on flickr, public domain.
26: Plesiosaur fossil on display in Natural History Museum of Los Angeles County