Many animals across the globe reproduce by laying eggs, a biological strategy known as oviparity. While birds are widely recognized for this reproductive method, numerous other animal groups also employ oviparity. This approach allows for the development of offspring outside the parent’s body.
Reptiles and Amphibians
Reptiles, a diverse group including snakes, lizards, turtles, and crocodiles, predominantly lay eggs. These eggs are often adapted for terrestrial environments, featuring leathery or hard shells that provide protection while allowing for gas exchange. For instance, many snakes and lizards produce flexible, leathery-shelled eggs that can absorb moisture from their surroundings as the embryo grows. Crocodilians and some turtles lay eggs with harder, more rigid shells. The incubation temperature for many reptile eggs, such as those of turtles and some lizards, can determine the sex of the hatchlings.
Amphibians, including frogs, toads, and salamanders, also lay eggs, but their eggs typically differ significantly from those of reptiles. Amphibian eggs are usually jelly-covered and lack a hard shell, requiring a moist or aquatic environment to prevent desiccation. For example, frogs often lay their eggs in large, gelatinous masses in water, where they develop into tadpoles. Salamanders may lay eggs singly or in smaller clusters, often attached to submerged vegetation or debris, encased in a thick jelly layer.
Fish and Aquatic Invertebrates
The majority of fish species reproduce by laying eggs, exhibiting diverse strategies. Most fish engage in external fertilization, where females release eggs and males simultaneously release sperm into the water. Some species, known as egg scatterers, release their eggs broadly into the water column, while others are egg depositors, carefully placing sticky eggs on substrates or within nests.
Aquatic invertebrates also rely on egg-laying for reproduction. Many mollusks, such as snails and clams, release their eggs into the water, sometimes encased in protective jelly. Crustaceans, including shrimp and crabs, often lay eggs and then carry them attached to their bodies, such as under their tails, until they hatch. Echinoderms, like sea stars and sea urchins, release eggs and sperm directly into the water for external fertilization.
Terrestrial Invertebrates
Terrestrial invertebrates, a vast group, display immense diversity in their egg-laying adaptations for land environments. Insects, such as butterflies, beetles, ants, and grasshoppers, lay eggs with specialized coverings that protect against desiccation. Many insect mothers will select specific oviposition sites or even cover their eggs with plant material or secretions to shield them from environmental stresses.
Arachnids, including spiders and scorpions, also lay eggs. Female spiders typically enclose their eggs within protective silk sacs. Some spiders carry these sacs with them for protection, while others hide them in secluded locations or suspend them in their webs. Other terrestrial invertebrates, such as land snails and many types of worms, also lay eggs, often in moist soil or decaying organic matter.
Egg-Laying Mammals
Among mammals, a unique group known as monotremes are the only ones that lay eggs. This ancient lineage includes the platypus and four species of echidnas. Unlike most mammals that give birth to live young, monotremes retain a more ancestral reproductive trait.
Monotreme eggs have a leathery shell, similar to those of reptiles, rather than the hard, brittle shells of bird eggs. The female platypus lays her eggs in a burrow, typically one to three at a time, and incubates them for about ten days by holding them between her body and tail. Echidnas lay a single egg into a temporary pouch on their abdomen, where it hatches after about ten days. After hatching, the young monotremes are nourished with milk secreted from mammary glands on the mother’s skin.