Animal growth comprises the stages an animal goes through from conception to the age at which it becomes fit for reproduction.
The stages differ between species, so growth varies according to the type of development of the animal.
Direct development, or growth without metamorphosis, occurs when the newborn animal resembles an adult in miniature format.
This type of development is present in many animals. In this development, the newborn animal has the same shape: there will only be an increase in the size and weight of the animal. It will develop in the same environment as the parents.
In viviparous animals, continuous growth allows the development of the embryo in the womb of the mother. This development, of varying duration depending on the species, allows for the production of a newborn animal with the same characteristics as those of the parents. At birth, nursing ensures that the newborns have an adequate food supply. Afterward, they can then feed themselves and continue to grow.
A cat giving birth to a litter of kittens is an example of direct development in viviparous animals.
For oviparous animals, development occurs inside an egg in which reserves are available for the development of the baby.
A hen laying eggs in which animals resembling the hen will develop is an example of direct development in oviparous animals.
Indirect development, or growth with metamorphosis, occurs when the newborn animal is very different from the adult. It will have to go through various stages of metamorphosis before reaching the adult stage.
These stages of metamorphosis represent all the transformations an animal undergoes before reaching its final form.
There are two types of metamorphosis.
Complete metamorphosis is composed of distinct stages. Consequently, there is a major difference between the initial state and the final state of the living organism and it can be difficult to recognize before it reaches the adult stage. It is usually this type of metamorphosis that is seen in insects.
The stages of metamorphosis are as follows:
- First of all, an egg is produced by the animal.
- This egg transforms into a larva, which is the first stage of the animal's life.
- The larva then transforms to become a nymph.
- The nymph then transforms to reach the adult stage.
The antlion, a dragonfly-like insect, undergoes a complete metamorphosis before reaching adulthood.
In the case of an incomplete metamorphosis, three stages are necessary to reach the final form: the egg, the nymph, and the adult. However, the nymph stage closely resembles the adult stage. Transformation from one stage to another occurs by means of moults.
Moulting is the shedding of an animal's shell or skin that has become too small.
When an animal moults, it gets rid of its layer of skin that has become too small. It is replaced by a new soft shell already formed under the old skin. This envelope hardens when it comes into contact with air.
The reptile Anolus marmoratus sheds its outer skin during a moulting period.
