The term used to describe any animal that reproduces by laying eggs is “oviparous.” This reproductive strategy is the rule, representing the ancestral and most common method of propagation across the vast majority of species, including insects, fish, reptiles, and the unusual egg-laying mammals. While birds are the most famous oviparous creatures, the true diversity of egg-laying life exists in groups that have adapted this process to every environment.
Aquatic Egg Layers
The most numerous oviparous animals rely on water, either marine or freshwater, for their eggs to develop. Fish typically release a vast number of small, shell-less eggs, often relying on external fertilization where the male releases sperm over the eggs after they are laid. Many species, such as salmon, scatter sticky eggs in the water column or on substrate to prevent them from being swept away. The egg’s protective layer, called the chorion, is rigid and shields the embryo from damage and pathogens.
Amphibians, including frogs, toads, and salamanders, also depend on water, reflecting their evolutionary link to fish. Their eggs are gelatinous, consisting of an embryo surrounded by multiple layers of jelly-like material that swells in water. This soft, permeable covering prevents the embryo from drying out, as the egg lacks a shell to retain moisture. Amphibians often lay their eggs in large masses in shallow, oxygen-rich water, where the young hatch as aquatic larvae before undergoing metamorphosis to a terrestrial adult form.
Terrestrial Egg Layers
Moving away from the water required the amniotic egg, a biological innovation that allowed life to fully colonize dry land. This structure is characteristic of reptiles, the first vertebrates to develop a reproductive unit independent of an aquatic environment. The reptile egg is enclosed in a protective shell, which can be leathery and flexible (in most snakes and lizards) or harder and calcified (in some turtles and crocodiles). The shell is porous, allowing for the exchange of oxygen and carbon dioxide while preventing excessive water loss.
Within the shell, specialized membranes support the embryo: the amnion encloses it in a fluid-filled sac; the yolk sac provides nutrients; and the allantois acts as a waste disposal and respiratory organ. Many reptiles, such as sea turtles, must return to land to bury their eggs, relying on environmental temperature for incubation. Insects and other arthropods, the largest group of terrestrial oviparous animals, also lay eggs adapted for land. Their eggs are structurally simpler and often laid in colossal numbers or encased in a protective structure, like the foam-like ootheca of a praying mantis.
The Mammalian Exception
The most surprising egg-laying animals are the monotremes, a small order of mammals represented by the platypus and four species of echidnas. Monotreme eggs are small, spherical, and have a leathery shell, resembling those of reptiles. The female platypus typically lays one to three eggs in a burrow nest, incubating them by curling around them.
Echidnas develop a temporary pouch formed by specialized muscles in which they incubate a single egg. The incubation period is short, usually lasting only about ten days before the young, often called puggles, hatch in a very underdeveloped state. After hatching, the young are not fed through a nipple; instead, the mother’s milk is secreted from specialized mammary gland pores onto a patch of skin, which the puggle laps up.