An animal’s life cycle consists of all the stages between the beginning of one generation and the beginning of the next. In some species — especially insects and other small invertebrates — the entire cycle is completed within a few weeks; in much larger animals, it often takes many years.
Regardless of how long it takes to complete, an animal’s life cycle always involves 2 main steps: a period of growth and development, followed by reproduction. Some animals reproduce once and then die: for them, reproduction marks the end of life as well as the completion of the life cycle. For many, reproduction continues throughout adulthood, giving animals more than one chance to produce young.
The ability to reproduce is the cornerstone of life because it allows living things to multiply, exploit new opportunities around them, and evolve. Animals reproduce in one of 2 ways; asexually (without sex) or sexually. In asexual reproduction a single parent partitions off part of itself, to form a new animal. The partitioning process can happen in a variety of ways. Hydras, for example, produce small buds that grow into complete new animals, while sea anemones literally tear themselves in 2.
Some animals produce eggs that develop without fertilization — a process called parthenogenesis. This is common in aphids and other sap-sucking insects, but it is rare in vertebrates (whip-tail lizards are one of the few examples). Asexual reproduction is relatively quick and simple, but it has one important disadvantage: since only one parent is involved, the offspring are either genetically identical to that parent or very similar to it. As a result, parent and offspring are equally vulnerable to threats such as disease: If one animal dies, the rest will often follow suit.
Sexual reproduction gets around this problem because the involvement of 2 parents means that the offspring are genetically varied. Each one has a unique combination of characteristics, which means the ﬁttest survive and the species slowly evolves. However, the disadvantage of sexual reproduction is that it is much more complicated: the parents must be of the right species and correct sex, and in most cases they must cooperate to breed. In addition, only one parent — the female — actually produces young, so some reproductive potential is lost.
Despite these difficulties, sexual reproduction is widespread throughout the animal world, which demonstrates its long-term value. Even species that normally reproduce without sex periodically include a sexual phase in their life cycle, thereby getting the best of both worlds.
All animals that reproduce sexually with a partner of the opposite sex show dimorphism — that is, the males and females are anatomically different and behave in different ways. In some species, the differences are not obvious, but in others, they are quite distinct. Dimorphism exists because the sexes have developed different roles in reproduction and need different body forms to carry them out.
However, not all sexually reproducing animals are of opposite sexes. Some — earthworms and terrestrial snails, for example — are hermaphrodite (they have both male and female sex organs). This simplifies sexual reproduction because any adult of a species is potentially a suitable mate for another. A further variation is that some species have separate sexes but individuals can change sex during adult life, Parrot fish, for example, often live in small shoals dominated by a single male. If the male dies, a female changes sex and takes his place.
Before an animal can mate, it has to find a partner. This is easy enough for species that live in groups, but for animals that live alone, it poses problems. Solitary animals locate potential mates by sending out signals, such as sounds or airborne scents.Each species has its own characteristic “call sign”, ensuring that it finds others of its own kind. Once the sexes are in contact, one partner – usually the male has to overcome the other’s wariness and demonstrate his suitability as a mate.
This process is known as courtship. It often take the form of ritualized behaviour that displays the males physical fitness or his ownership of a good provision of food. If the female is sufficiently impressed, she will accept him as her mate. While some species form lifelong partnerships, many go their separate ways after mating. In the latter case, the males typically mate with several females but take no part in raising the young.More rarely, things work in the opposite way, with one female mating with several males. Where this happens — for example, in phalaropes the female is more brightly coloured than the male and often takes the lead in courtship. In general, these females take little or no part in rearing young.
Most animals — apart from the ones that use asexual reproduction – start life as a single fertilized egg cell. If the egg has been fertilized externally, it will already be outside the mothers body, perhaps drifting in the sea or glued to sea-bed plants or sand. If the egg has been fertilized internally, it will either be laid, to hatch afterwards, or it will be retained inside the mother while it begins to develop into a young animal.
The degree of development that takes place at any one stage varies from one type of animal to another. Oviparous species. such as birds, lay their fertilized eggs before foetal development begins. In birds. development is often deferred for several more days until the clutch is complete: it begins as soon as the parent starts to incubate the eggs. Ovoviviparous species, which include many reptiles and sharks. incubate their eggs internally, “giving birth” at the moment when the eggs are about to hatch. Viviparous species — which include virtually all mammals, as well as some reptiles, amphibians, and fish — give birth to live young.
All animals change shape as they grow and develop. In some, the changes are gradual and relatively minor, but in others, they are so far-reaching that the animal is completely transformed. This transformation is called metamorphosis. It allows animals to live in different ways — and often in different habitats during their young and adult lives. Although metamorphosis is most common in invertebrates, it does occur in amphibians and some fish. Animals that undergo the metamorphic process spend the early part of their lives as larvae, in the sea.
Larvae often drift near the surface as part of the plankton, and because they are carried far and wide they play an important role in helping their species to spread. In the insect world, metamorphosis occurs in 2 ways. Incomplete metamorphosis. shown by grasshoppers and bugs, involves a series of gradual changes that are made as the young insect. or nymph. matures, Complete metamorphosis shown by butterflies, beetles, and flies – involves more drastic changes, which occur during a resting stage, called pupation, when the body is broken down and rearranged.