Animals that lay eggs are called oviparous animals and form entirely inside of the egg shell with no other development. Such animals may be reptiles, birds, sea creatures and even some mammals. Most mammals do not lay eggs, however. While oviparous animals lay eggs, the way in which they protect their eggs may vary from species to species.
Birds are the most commonly thought of animal that lay eggs. A bird egg has four components, which are the yolk, germinal disc, white part and shell. Birds may lay their eggs in nests or in other areas away from predators. Birds may use sticks, leaves and other debris found in nature to construct their nest. Birds generally sit on their eggs to protect and keep them warm until they hatch.
While some reptiles like some varieties of lizards, snakes and chameleons give birth to their young live, many species of reptiles lay eggs. All turtles, tortoises and crocodiles lay eggs in addition to some snakes and lizards. These reptiles lay their eggs out of water. Similar to birds, oviparous reptiles seek to hide their eggs from predators. To accomplish this, they will often dig holes in moist soil and bury their eggs there. Reptile eggs can incubate from 45 days to one year and the temperature of the incubation site can influence the sex of the hatchling in some cases.
Most fish lay eggs underwater. Fish will generally lay millions of eggs, which will develop without parental involvement. Males may also fertilize these eggs externally. When fish hatch, they survive off of the yolk of their egg until they learn to hunt and feed themselves. Amphibians, like toads and frogs, may also lay eggs in water, which later hatch on their own. Rays and sharks, on the other hand, fertilize and nurture their eggs internally before giving birth to live young.
While most all mammals give birth to live young, other species of mammals lay eggs. These species are called monotremes and are limited to just five animal species. These species are the duck-billed platypus and four species of spiny anteaters. Unlike other egg-laying creatures, however, monotremes produce milk for their young and have high metabolic rates. As of 2011, monotremes are only found in Australia and New Zealand.