Birth, growth, reproduction and death are the four stages of the life cycle of all animals. Although common to all animals, such stages happen in different ways in distinct animal species. For instance, while insects, birds and reptiles are born from an egg, mammals have embryos that develop inside the mother's body. Also, most animals show appearance similar to adults at birth, but most insects and some amphibians go through radical transformations during their growing stage, a process called metamorphosis. The entire life cycle of an animal can last for only some days or weeks, as it happens with many insects, to more than a century, as in the case of some tortoise species.
Animals are called viviparous, when born from the mother's womb or oviparous when developed inside an egg. However, some reptiles are ovoviviparous, when their embryos develop inside eggs kept inside the female's body until they hatch. The embryonic development is similar in most vertebrates, but it can last from 19 days, in mice to more than a year in big mammals, such as the giraffe, the whale and the elephant.
Before reaching the sexual maturity or adult age, animals pass through a growing stage. Some species, especially among invertebrates and amphibians, go through metamorphosis during the growing period. Butterflies, grasshoppers, mosquitoes, frogs and salamanders are some examples of animals that undergo metamorphosis, which has a larvae and a pupa stages. Hormones, such as somatotropin, induce growth in animals.
Animals reproduce in a sexual or asexual way. While the sexual reproduction involves male and female gametes, the spermatozoid and the ovule, the asexual reproduction depends on a single individual to generate new life. Hydras, sponges, starfish and flatworms of the class Turbellaria, know as planarians, can reproduce in an asexual way, but most animals depend on the sexual cells to reproduce.
After aging, animals end their life cycle by dying. Loss of hearing and sight, lack of energy, body weakness and illnesses are some signs of aging and often precede the natural death of an animal in the wild, which is often more common among the predators. Different species have distinct lifespans. Among the birds, swans and parrots have the longest lifespans, with up to 100 and 80 years respectively, while humming birds generally die before they reach 10 years of age.