While birds are well-known for laying eggs, the animal kingdom features a wide array of species that reproduce by laying eggs. This reproductive strategy, known as oviparity, is common across many different animal groups.
Reptiles and Amphibians
Many reptiles, such as snakes, lizards, turtles, and crocodiles, lay eggs. These eggs have a leathery or sometimes hard shell that protects the developing embryo from desiccation, allowing them to be laid on land. Reptile eggs are often buried in soil, sand, or decaying vegetation, with some species, like crocodiles, guarding their nests. Unlike birds, reptile eggs do not require a specific temperature range for incubation; instead, the surrounding environment’s temperature often determines the sex of the hatchlings in many species.
Amphibians, including frogs, toads, and salamanders, also lay eggs. Unlike reptiles, their eggs lack a hard shell and are encased in a gelatinous substance. These eggs are laid in water or moist environments. The larvae, such as tadpoles, hatch from these eggs and undergo metamorphosis to transform into their adult form.
Fish and Invertebrates
The majority of fish species lay eggs, which are often referred to as roe. These eggs are usually fertilized externally, with females releasing eggs into the water and males releasing sperm. Fish exhibit diverse egg-laying strategies, ranging from scattering millions of eggs into open water, as seen with cod and salmon, to building nests or attaching eggs to aquatic vegetation. Some oviparous shark species also lay eggs, which are sometimes encased in distinctive, protective cases often called “mermaid’s purses.”
Invertebrates represent an enormous and varied group, with most species reproducing through eggs. Insects, for example, lay eggs in countless ways and locations, including on leaves, in soil, or even inside other organisms. Butterflies, beetles, and ants are common examples of insects that begin their life cycle as eggs. Mollusks, such as snails and squid, often lay their eggs in protective masses, while arachnids like spiders typically lay their eggs within silk sacs.
Mammals That Lay Eggs
Perhaps the most unexpected animals that lay eggs are monotremes, a unique group of mammals. There are only five known living species of monotremes: the platypus and four species of echidnas. These animals are distinct from other mammals because they do not give birth to live young; instead, they lay eggs, yet they still feed their offspring with milk, a defining mammalian trait.
The platypus, a semi-aquatic monotreme found in Australia, typically lays one to three small, leathery eggs, though two eggs are most common. The female incubates these eggs in a specially constructed burrow, often curling her body around them to keep them warm. After hatching, the tiny, undeveloped young, sometimes called puggles, lap milk that is secreted from patches on the mother’s skin, as monotremes lack nipples.
Echidnas, also known as spiny anteaters, are terrestrial monotremes. The female echidna lays a single, soft-shelled, leathery egg about 22 days after mating. This egg is then deposited into a temporary pouch on her belly, where it is incubated. The puggle hatches after about 10 days and remains in the mother’s pouch, feeding on milk secreted from mammary glands, until it develops spines and is too large to stay.