Animals That Lay Eggs: Mammals, Reptiles, Fish and More

Egg-laying and live birth: the key terms

Biologists use three main words to describe how animals reproduce:

• Oviparous animals lay eggs, and the young develop and hatch outside the mother’s body. Birds, crocodiles, turtles and most fish are oviparous.
• Viviparous animals give birth to live young that developed inside the mother, nourished by her body, often through a placenta. Most mammals, as well as some sharks, snakes and lizards, are viviparous.
• Ovoviviparous is an older term for animals whose eggs stay inside the mother until they hatch, so the young are born alive. Many sharks and snakes reproduce this way. Many biologists now simply group it with live birth.

Which animal groups lay eggs?

Mammals that lay eggs: platypus and echidnas

The only egg-laying mammals alive today are the monotremes: the platypus of eastern Australia and four species of echidnas from Australia and New Guinea. A female platypus usually lays two small, leathery eggs, about 17 mm (0.7 in) long, in a burrow and curls around them until they hatch about 10 days later. The short-beaked echidna lays a single rubbery egg, 13–17 mm (0.5–0.7 in) across and weighing 1.5–2 g, directly into a pouch on her belly, where it hatches after about 10 days.

Both feed their young on milk, which seeps from patches of skin because monotremes have no nipples. European scientists argued for decades about whether a furry, milk-producing animal could really lay eggs, until the question was settled in 1884. Learn why monotremes still count as mammals in is a platypus a mammal?

Birds: every species lays eggs

All of the world’s roughly 11,000 bird species lay eggs, and no bird gives birth to live young. Bird eggs have hard shells made mostly of calcium carbonate, and most are kept warm by a parent sitting on them. The male emu incubates the dark green eggs alone, and in many penguins and pigeons the parents take turns.

Bird eggs range from bright blue robin eggs to the speckled eggs of ground-nesting birds, whose colors help hide them. Some birds skip parenthood altogether: the common cuckoo lays its eggs in the nests of other species, which then raise the cuckoo chick as their own.

Reptiles: most lay eggs, but not all

All crocodiles, alligators, turtles, tortoises and the tuatara lay eggs, usually burying them in sand, soil or rotting vegetation. In many turtles and all crocodilians, the temperature of the nest decides whether the hatchlings will be male or female. Tuatara eggs are among the slowest to develop of any reptile, taking more than a year from mating to hatching.

Lizards and snakes are more varied. Roughly a fifth of their species give birth to live young, and live birth has evolved separately in this group many times, especially in cold climates where warm soil for eggs is hard to find. Boas, anacondas, rattlesnakes, garter snakes and the European adder bear live young, while pythons lay eggs and coil around them; a brooding Burmese python even twitches its muscles to warm the clutch. The common lizard of Europe and Asia gives birth to live young across most of its range but lays eggs in a few areas of southern Europe, such as northern Spain and the southern Alps. Read more in do all snakes lay eggs?

Amphibians: from frogspawn to live birth

Most frogs, toads, newts and salamanders lay soft, jelly-coated eggs in water or damp places, and the young hatch as tadpoles or larvae. A common frog can lay a clump of 1,000 to 2,000 eggs, known as frogspawn.

There are striking exceptions. The fire salamander of Europe keeps its eggs inside her body and gives birth to well-developed larvae in streams. The alpine salamander goes further: females give birth on land to one or two fully formed young after a pregnancy of two to four years, one of the longest of any vertebrate. The Surinam toad of South America carries its eggs embedded in the skin of its back, and fully formed froglets emerge after several months.

Fish: billions of eggs and some surprises

Most fish spawn, releasing huge numbers of small eggs into the water. The ocean sunfish holds the vertebrate record: a female can produce up to 300 million eggs at a time.

Many fish give birth to live young instead. Popular aquarium fish such as guppies, mollies and swordtails are livebearers. Most sharks also give birth to live pups, including great white sharks, hammerheads and whale sharks, whose eggs hatch inside the mother. Other sharks, such as catsharks, horn sharks and swellsharks, lay eggs in tough leathery cases, often found washed up on beaches as “mermaid’s purses.” The coelacanth also gives birth to live young, after a pregnancy that a 2021 study estimated may last about five years.

Seahorses have a unique twist. The female lays her eggs, up to around 1,500 of them in some species, into a pouch on the male’s belly. He fertilizes and carries them for about 9 to 45 days, then gives birth to tiny, fully formed seahorses.

Insects and other invertebrates

Nearly all insects lay eggs, often in huge numbers. The queen of a mature termite colony can lay hundreds to thousands of eggs a day for years. Female octopuses lay strings of eggs in a den and guard them until they hatch, usually dying soon afterward.

A few invertebrates give birth instead. In summer, female aphids give birth to live young without mating, and the young are sometimes already pregnant when they are born. Female tsetse flies of Africa nourish a single larva inside their body with a milk-like fluid and give birth to it fully grown. Scorpions also give birth to live young, which climb onto the mother’s back and ride there until their first molt.

Record-breaking eggs

The ostrich lays the largest eggs of any living bird, yet they are tiny compared with the ostrich itself, only about 1–4% of the female’s weight; see how big is an ostrich egg? Kiwis are the opposite extreme. A female kiwi’s egg is so large that it fills much of her body before she lays it, and most kiwis lay just one egg, or occasionally two, per clutch.