Many animals reproduce by laying eggs, a strategy known as oviparity. This method is found across diverse species from various animal groups, with eggs reflecting unique environments and adaptations.
Reptiles: Terrestrial Egg Layers
Reptiles are a prominent group of egg-laying animals, with most species depositing their eggs on land. Their eggs feature leathery shells, which offer protection and allow for gas exchange. Some reptiles, like sea turtles and certain lizards, produce eggs with hard, calcified shells that prevent desiccation and protect against predators.
Snakes lay their eggs in secluded, damp, and warm locations, such as abandoned burrows, decaying vegetation, or beneath rocks and logs. Most snake species do not guard their eggs after laying them. Crocodiles and alligators, however, exhibit parental care, with females guarding clutches of 30 to 60 hard-shelled eggs in nests. The incubation period for crocodile eggs ranges from 55 to 100 days, and temperature determines the sex of the hatchlings.
Amphibians: Eggs in Water and on Land
Amphibians, including frogs, toads, and salamanders, reproduce through egg-laying. Their eggs are jelly-like, lacking the hard or leathery shells found in reptiles. Amphibian eggs require a moist or aquatic environment for development.
Many species deposit eggs directly into water bodies, where larvae develop. Other amphibians, such as certain caecilians, lay their eggs in damp terrestrial nests. Female caecilians often guard these terrestrial clutches, providing parental care until the eggs hatch.
Fish: Aquatic Egg Layers
Fish eggs are broadly categorized into pelagic and demersal types. Pelagic eggs are buoyant with thin outer layers, floating widely in the water column. Demersal eggs are denser and sink, often possessing thicker outer layers that resist mechanical damage. These eggs are laid on the ocean or river floor, or attached to structures.
Behaviors range from broadcast spawning, where eggs and sperm are released into open water, to complex strategies like nest building. Some fish species construct elaborate nests to protect their eggs, while others engage in mouthbrooding, carrying the eggs in their mouths for incubation. Certain sharks lay distinctive leathery egg cases, often called “mermaid’s purses,” which are anchored to the seafloor or submerged vegetation. Examples include catsharks, zebra sharks, and horn sharks.
Invertebrates: A Diverse World of Eggs
Insects produce eggs that exhibit diversity in size, shape, and surface ornamentation. These eggs can range from barely visible to several millimeters in diameter, with structures adapted to various environments. Insects choose egg-laying sites that provide protection, food sources, and suitable microclimates for their offspring.
Spiders lay their eggs within silk egg sacs, which the female often guards. Snails lay their eggs, often in moist soil, either in holes they dig or under logs and debris. A single snail can lay a clutch of 100 to 400 tiny, pearly white eggs. Many marine invertebrates, such as squid, octopus, and some crustaceans, also lay eggs, often in large masses.
Mammals: The Unexpected Egg Layers
Among mammals, only monotremes reproduce by laying eggs. The two living examples are the platypus and the echidnas, both found exclusively in Australia and New Guinea.
Monotreme eggs have a leathery shell, similar to those of reptiles, rather than a hard, calcified shell like bird eggs. The female retains the eggs internally before laying them.
A platypus lays its eggs in a burrow and incubates them by curling its tail to hold the eggs against its warm belly. Echidnas store their eggs in a temporary pouch on their body. Despite laying eggs, monotremes are mammals because they produce milk to feed their young, secreting it from a specialized patch on their skin.