Like all living things, animals have limited life spans. Although individual animals eventually die, reproduction ensures that they hand on their characteristics to future generations. Animals reproduce at markedly different rates, but all have the potential to increase their numbers if resources allow it. In practice, sharp increases are rare, kept in check by predators and food shortages. AForms of Reproduction Animal reproduction takes two overall forms. In the first form, called asexual reproduction, animals produce offspring without needing a partner. Asexual reproduction is most common in simple animals such as flatworms and coelenterates. In flatworms, the parent often develops a constriction in its body, and the rear part eventually tears itself free. The rear part grows a new head, while the front part grows a new tail. Some coelenterates can also divide in two, but many reproduce by a different process, called budding. During budding, a small outgrowth of the body slowly develops into a complete new animal, which eventually takes up life on its own. Asexual reproduction also occurs in insects such as aphids and in a few unusual vertebrates, such as whiptail lizards. However, in general, it is rarely used as an animal's sole method of reproduction. This is because asexual reproduction produces offspring that are genetically identical to their parent. They inherit all their parent's weak points and are equally vulnerable if a disease or other changes in the environment threaten the group's survival. A second and much more common form of reproduction, sexual reproduction, involves two parents. The parents produce sperm and egg cells , which are brought together to form a fertilized cell with a new and unique combination of genes. In this genetic lottery, offspring inherit unique combinations of characteristics that increase the likelihood that at least some individuals in the population can survive changes in the environment. Sexual reproduction is used by the vast majority of the world's animals. However, a significant number of species, particularly in the world of insects, use both forms of reproduction at different stages of their life cycles. They reproduce asexually when food is abundant, but turn to sexual reproduction when conditions become more severe. BReproductive Strategies Asexual reproduction is relatively easy to achieve because it involves only a single animal. Sexual reproduction is much more complex because the partners often have to find each other and precisely coordinate their reproductive behavior. In most cases, each partner is either male or female, but in some animals-such as earthworms, slugs, and snails-each one is a hermaphrodite, an animal that has both male and female organs. Hermaphrodites usually fertilize each other, with both partners producing young . Most aquatic animals shed their eggs and sperm into the water, where external fertilization takes place. In corals and many other sessile species, the moment of spawning is often triggered by the tides, maximizing the chances that the egg and sperm will meet. In a minority of marine animals, fertilization is internal, meaning that the male mates with the female, inserting his sperm into her body. For this to work, the male needs special adaptations to make the transfer. Male sharks and rays use special claspers that are attached to their pelvic fins, while barnacles, which are often hermaphrodites, use a thread-like penis that can be almost as long as their bodies. On land, external fertilization is rare because egg and sperm cells cannot survive for long in the open. As a result, almost all land animals must mate to trigger internal fertilization in order to reproduce. Different groups of animals have evolved a wide variety of mechanisms to make sure that males and females manage to locate suitable partners. Some female insects emit chemicals called pheromones, which guide males towards them, while others use sound signals or biochemically produced light . In birds, elaborate plumage and courtship displays help to attract females towards the males .The males of many insects and virtually all mammals use a penis to transfer sperm to the female, who harbors the eggs, in a process known as copulation. The penis ensures that sperm is transferred successfully without being carried away by wind, water, or other environmental elements. Most birds and reptiles mate using a cloaca, a single opening located on the lower abdomen. During mating, these animals align their cloacas for transfer of sperm. Some birds, such as bald eagles, can perform this feat in mid-air. Once a female has mated, egg development can proceed in two different ways. In oviparous species, which include the majority of vertebrates except mammals, and also most insects, the fertilized eggs are laid and develop outside the mother's body. In viviparous animals, which include nearly all mammals together with some reptiles and sharks, the young develop inside the mother and are born live. Most animals that are born live look similar to their parents, although they are not fully developed. By contrast, many egg-laying invertebrates look completely different from their parents when they hatch and often live in a completely different way. Known as larvae, these young change shapes as they grow up, during a process called metamorphosis. Larvae are also found in some fish and most amphibians.
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