Many animals across diverse groups reproduce by laying eggs, a strategy known as oviparity. While birds are widely recognized for this method, egg-laying is a common and effective way for a vast array of species to produce young. This approach allows offspring to develop outside the parent’s body, often providing opportunities for larger numbers of progeny.
Mammals That Lay Eggs
A small, distinctive group of mammals, known as monotremes, stands out for their egg-laying reproduction. This group includes the platypus and four species of echidnas. These unique creatures are found exclusively in Australia and New Guinea.
Female platypuses typically lay one to three eggs in secluded dens. These eggs have a leathery shell, unlike the hard, calcified shells of bird eggs. Echidnas, conversely, usually lay a single soft-shelled, leathery egg, which the female places into a temporary pouch on her belly for incubation. Despite their egg-laying habit, monotremes are classified as mammals because they possess fur, are warm-blooded, and produce milk to feed their young. Unlike most mammals, they secrete milk through pores in their skin rather than from nipples, with the young licking it directly from the fur.
Reptiles and Amphibians
Many reptiles and amphibians rely on egg-laying for reproduction. Reptile eggs feature a leathery or parchment-like shell, which is flexible and porous, allowing them to absorb moisture as the embryo grows. Snakes, for instance, lay oblong or elliptical eggs with a soft, pliable texture.
Lizards also lay leathery-shelled eggs. Lizard eggs vary in appearance, with some hardening after deposition. Crocodiles, in contrast, lay hard-shelled eggs. Female crocodiles lay clutches of 12 to 48 eggs in nests dug into the ground or built as mounds of vegetation near water sources. Amphibians, such as frogs, toads, and salamanders, deposit their jelly-like eggs directly in water or moist terrestrial environments. These eggs lack a hard shell, relying on their gelatinous coating for protection and moisture retention.
Underwater and Spineless Egg Layers
The aquatic world teems with egg-laying animals, particularly fish. Many shark species, and all true skates, reproduce by laying eggs encased in tough, leathery capsules often called “mermaid’s purses”. These egg cases vary in shape and size, with some featuring tendril-like structures to anchor them to underwater vegetation or rocks. Bony fish are also prolific egg-layers, with females often releasing thousands or even millions of small, simple eggs into the water column, relying on external fertilization and large numbers to ensure survival.
Beyond the vertebrates, the spineless world of invertebrates showcases an immense diversity of egg-laying strategies. Insects lay eggs that exhibit a remarkable range of shapes, sizes, and colors, often adapted to their specific habitats and survival needs. These eggs can be spherical, elongated, or even intricately ornamented, with outer layers providing protection against environmental stresses.
Spiders lay their eggs within silk sacs, which can vary in color, shape, and size, ranging from round to teardrop-shaped. A single spider egg sac can contain dozens to hundreds of eggs, and some female spiders carry these sacs with them or attach them to secluded locations.
Mollusks also include numerous egg-laying species, such as snails, slugs, and octopuses. Deep-sea octopuses are known for their exceptionally long brooding periods, with females guarding tear-drop-shaped eggs for over four years, meticulously cleaning and protecting them until they hatch.