An oviparous animal is one that lays eggs that then develop into offspring. Of the millions of members of the animal kingdom, many species reproduce in this manner. Animals that do not lay eggs tend to give birth to live young.
Birds are the only group of animal that reproduce entirely by laying eggs. Fertilisation takes place internally within the female after mating with the male. With the exception of some birds such as the cuckoo, most birds take care of their offspring until they are grown enough to look after themselves. Cuckoos and other birds lay their eggs in other species' nests, leading the nest owner to act as a surrogate parent.
Most reptile species reproduce by laying eggs. This includes snakes, lizards , crocodiles and turtles. As a general rule, care after birth is not common in reptiles; baby snakes are able to defend themselves from birth. Crocodiles and alligators do show some level of care for their offspring, and they protect both eggs and newborns. Some reptile species do not lay their eggs; instead their eggs mature and hatch internally, and the mother gives birth to live young.
Most species of amphibian lay eggs to reproduce, but unlike reptiles and birds, fertilisation tends to occur externally, with a male releasing sperm onto eggs that have already been laid. Many species leave the eggs to hatch unguarded in ponds or streams. Some species that lay their eggs on land stay around until hatching and transfer the young to a safe pond. Live birth also occurs in some amphibians, with eggs being incubated internally.
Fish have three main ways of producing offspring. First is the laying of unfertilised eggs, which are then fertilised by a male. Second is internal fertilisation via sexual contact, with the subsequent laying of eggs. The final is internal fertilisation and the hatching of eggs internally for live birth. Sharks and a few other fish tend to produce live young, and the rest lay eggs.
The wide range of arthropod species includes insects, crabs, scorpions and spiders. Most reproduce by laying eggs that have been internally fertilised, but some use external fertilisation. A select few species, such as scorpions hatch the eggs internally and carry the young around on their back.
Oviparous animals are common in most groups except mammals, of which only five living species lay eggs. The platypus, which lives only in Australia, and the echidna, which is native only to Australia and New Guinea, lay eggs. This sets them apart from other species, but they share the main traits that classify them as mammals. The presence of middle ear bones, hair and the production of milk to feed the young are traits that only mammals possess.