Life Cycles

Written by A-Z Animals Staff
Updated: February 16, 2021
Image Credit © Greg Willis from Denver, CO, usa / CC BY-SA 2.0


What Are Life Cycles?

The life-cycle of an animal, includes all of the stages from the end of the last generation to the beginning of the next. Life-cycles vary in time depending on the species of animal, and can be as short as just a few weeks for insects, to up to 200 years for sea urchins. Despite the time differences, all animal cycles begin with the growth and development process, which is then followed by reproduction. The reproduction stage in the cycle of life marks the end of the cycle and many animals often die after they have reproduced just once.

Starting Life

Most animal species begin life as a single egg cell that is then fertilised by a sperm cell. The growth process varies in speed depending on the species as some animals are born in less than a week of conception, where others can be born nearly two years afterwards. Animals are born in different ways depending on the group as mammals give birth to live young, where birds, fish and reptiles lay eggs which their young hatch out of. In some egg-laying animals such as sharks, the eggs are laid and incubated inside the body of the mother shark, who therefore gives birth to live young.

Living Life

Depending on the animal species, some animals live very solitary lives and only meet up with other animals from the same species to mate, where other animal species such as termites and monkeys are very sociable animals and spend their lives in a group. Animals spend their lives hunting for food and water, staying safe and looking for a mate to reproduce with.

Animal Sexes

All animals that need another animal to reproduce are split into males and females, both of which have very different reproductive organs that need to be combined in order for those animals to produce offspring. A small number of animals are asexual meaning that they have both male and female reproductive organs and are therefore able to self-fertilise. Slugs, snails and barnacles are all asexual animals.

Mating and Courtship

Before an animal can reproduce it must first find a partner. Animals attract their mates by courting them, a process which varies dramatically between different species. Some species of male bird have brightly coloured feathers and do elaborate dances in order to try and get the female to notice them. Other animals, such as certain birds and fish, are known to almost dance together, often copying one another. The male animals of many species often have features to attract females such as long noses or large manes. Courtship leads to mating between animals in order to reproduce, and these relationships can last for life or just momentarily.


The ability to reproduce is the most fundamental part of life for any living species as it allows that species to multiply and evolve. Reproduction in animals either occurs asexually (just one animal that has both male and female reproductive organs and can therefore self-fertilise), or using sexual reproduction which involves both the male and female from that species. Female animals produce egg cells, which need to meet a male sperm cell in order for them to start developing into a new animal. Depending on where fertilisation occurred (either internally or externally), the young will either be looked after by both parents (such as birds) or just by the mother in mammals.


Fertilisation is the beginning of new life and occurs when the female egg cell meets the male’s sperm. Some animal species are fertilised outside of the body of the female, and some animal species are fertilised inside the body of the female. External fertilisation occurs most commonly in the water as the cells are able to float through the water and become fertilised in the process. External fertilisation does not occur so commonly on land as it is too dry, so it is more effective for land animals to fertilise their eggs when they are still inside the body of the female.

Life Spans

Generally, the amount of time an animal will live for is greatly dependent on the size of it’s body. Blood temperature also plays an enormous part in an animal’s life-span as the slowly moving, cold-blooded animals such as reptiles tend to live longer than the more frantic warm-blooded animals such as birds. Animals that are successful and therefore healthy, also live for longer than those animals which are weak and vulnerable in their environment.

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AZ Animals is a growing team of animals experts, researchers, farmers, conservationists, writers, editors, and -- of course -- pet owners who have come together to help you better understand the animal kingdom and how we interact.