Birth, growth, reproduction and death represent the four stages of the life cycle of all animals. Although these stages are common to all animals, they vary significantly among species. For instance, while insects, birds and reptiles are born from an egg, mammals develop as embryos inside the mothers’bodies.
Most animals appear similar to their adult counterparts at birth. Some types of animals, however, including most insects and most amphibians, go through radical transformations during their growing stage. This process is called metamorphosis. The entire life cycle of an animal can last from only several days or weeks, as it happens with many insects, to more than a century, as in the case of some tortoise species.