Death from old age, predation, disease, injury, or sudden environmental change is an everpresent threat to animals. A species can survive and adapt, however, because its members reproduce themselves. When animals reach adult size, they become sexually mature and able to reproduce. They do so in one of two basic ways—by asexual or sexual reproduction. Asexual reproduction involves only one parent, whereas sexual reproduction usually involves two adult individuals.
In many animals chemicals called hormones and pheromones cause the body to develop reproductive cells. The production of hormones also influences courtship and mating behavior. In certain climates, environmental conditions, such as the longer days of spring and early summer may activate hormones and pheromones.
A sexual reproduction
Only the more primitive animals reproduce asexually. It is a less complicated means of reproduction and has the disadvantage of producing offspring that are identical to their parents. There is no shuffling of genetic material between generations and therefore less variation within the population.
The simplest method of asexual reproduction-binary fission—is found in unicellular organisms such as amebas. These organisms simply split their nucleus in two by mitosis (cell division) and the cytoplasm separates to surround each new nucleus. The parent no longer exists as a single unit but has become two “daughter” cells.
Some animals that are capable of sexual reproduction may also reproduce by means of two other methods of asexual reproduction-fragmentation (or regeneration) and budding. Regeneration is the process by which a new adult individual grows from a body part that has broken off. Hydras and sponges reproduce through regeneration, in budding, an outgrowth, or bud, develops on the body. The bud then grows into a smaller copy of the parent and eventually detaches itself. In certain corals, for example, buds appear on the body of an adult. These buds grow larger, separate from the parent, and begin to deposit their own limestone in the colony. In this way, thousands of individual animals, all derived from a single parent, may form great colonies of coral. Members of some animal groups, such as cnidarians, produce young both sexually and asexually, alternating generation by generation. Usually one stage is the dominant state for a species, while the other is a temporary phase.
The two parent organisms involved in sexual reproduction each produce special sex cells called gametes. Female gametes, called ova or eggs, are formed in an ovary. They are usually much larger than the male gametes, called sperm, and contain nutrients that feed the embryo. Ova are the largest single cells and have no means of locomotion. Sperm are formed in the testicles and move by means of a whiplike tail called a flagellum.
Gametes contain half the number of chromosomes that every other adult cell in the body possesses and are called haploid, whereas the body cells contain paired sets of chromosomes (homologous chromosomes) and are called diploid. The reduced number of chromosomes in gametes results from a special kind of cell division called meiosis.
When the nuclei of two gametes join during fertilization, the resulting cell, or zygote, has the full number of chromosomes and two sets of hereditary information, one from each parent. The offspring exhibit features of both parents but are not exact copies of either.
Many animals exhibit specific courtship behavior to help them find suitable mates. This behavior tends to follow a particular pattern according to species, ensuring that animals mate only with members of their own species. Animal mates find each other in a number of ways. Female birds are attracted to the songs and brightly colored feathers of male birds. Many male insects and amphibians attract females with their loud calls. Some animals give off scented chemicals called pheromones to attract the opposite sex.
Other animals perform ritualized movements to attract mates. Male fence lizards, for example, bob their heads rhythmically when a female approaches. Still others offer food to possible mates. A male tern catches a fish and places it into the mouth of the female he wants to mate.
Mating is dangerous for some male spiders and insects. Male black widow spiders and male praying mantises, for example, are sometimes eaten by females after mating.
In most animals the male and female sex organs are found in different individuals. However, in some animals, such as hydras, flatworms, earthworms, and snails, are hermaphrodites, in which both ovaries and testes are present in each individual. Tapeworms fertilize themselves, but most hermaphrodites cross-fertilize with other members of their species.
For fertilization to occur, it is important that the sperm and eggs are deposited close to one another. The mobile sperm swim to an egg, attracted by chemicals in the surrounding fluid; but because sperm consist mostly of genetic material, they have a very small energy store and usually cannot survive for more than 24 hours under normal conditions. For this reason, fertilization can take place only in liquid. Animals that live in water, such as fishes and frogs, can reproduce by means of spawning because their reproductive cells are already surrounded by water. In spawning, the female usually releases her eggs first, and the male then fertilizes them by covering them with sperm. Spawning requires exact timing of the release of gametes from both sexes, or the sperm will wash away and the eggs will not be fertilized.
Land animals, on the other hand, do not have a watery environment in which fertilization can take place, and so have developed special reproductive fluids to carry the reproductive cells. They also have special organs to effect the transfer. Unlike many aquatic animals, mammals and such land animals as reptiles and birds rely mostly on internal fertilization, which occurs inside the female’s body. The males of these animals have special organs that can deposit sperm inside the female during mating. The sperm then swim to the eggs.
Oviparity and viviparity
Although they fertilize internally, most reptiles and birds lay their hard-shelled eggs outside their bodies, and the eggs develop outside the mother. This egg-laying process is known as oviparity. Most higher animals, including most mammals and a few cold-blooded animals, use the form of reproduction called viviparity. In viviparity the fertilized egg implants inside the mother and is nourished by her bloodstream until birth, when it has reached an advanced stage of development and looks like a smaller version of the adult Some fish, amphibians, and reptiles produce eggs that remain inside the female until the young hatch, but are not nourished directly by her. This process is called ovoviviparity.
Most insects reproduce sexually and produce yolk-filled eggs, which follow three different patterns of growth before the insect becomes an adult.
The most highly evolved insects, such as butterflies and bees, develop by complete metamorphosis. They hatch from their eggs as larvae that do not resemble their parents. The larvae spend their lives eating and growing. This growth is normally achieved by molting, or the shedding of skin. When the insect is full-grown, it stops eating, and the bloated larva becomes a pupa such as the chrysalis of a butterfly. The adult insect develops inside the pupal case.
Another pattern of development, incomplete metamorphosis, is exhibited by the group of insects that includes dragonflies and cockroaches. The insects hatch from their eggs as nymphs, or naiads, which resemble the parents, but are wingless, smaller, and sexually immature. Like larvae, nymphs eat all the time. They do not enter a pupal stage, however, but grow and shed their hard outer skin until they reach adult size, when they become sexually mature.
The most primitive kinds of insects, such as the wingless bristletails, hatch as miniature replicas of their parents and no metamorphosis is apparent.
Fishes and amphibians
The reproductive behavior of fishes varies considerably between species. A herring, for example, may shed tens of thousands of eggs and relies on external fertilization, whereas a dogfish reproduces by internal fertilization and produces very few eggs, which are protected inside an egg purse while they develop.
Amphibians are considered to be more advanced than fishes, but they are not fully adapted to life on land and must always return to water for breeding. Most amphibians fertilize externally. Frogs and toads, for example, mate after they have exchanged specific courtship signals. During the amphibian mating process, called amplexus, the male seizes the female and sits on her. This act induces both the male and the female to release gametes into the water simultaneously. The fertilized eggs hatch as tadpoles, going through a larval stage and gradually metamorphosing into adult amphibians. In some salamanders, however, the young are born live and do not go through a larval stage.
All birds reproduce by internal fertilization. Unlike amphibians, they do not rely directly on water for breeding. Instead, the hard-shelled eggs they lay each contain a “private pond” in which the embryo floats and develops. The temperature inside the egg is maintained by incubation, in which one or both of the parents, but usually the female, keeps the egg warm. Incubation can take two to three weeks or more before the young chicks hatch. In the case of some birds, the female continues to care for the young in the nest for long periods because the chicks are born blind, helpless, unable to feed themselves, and without feathers to keep them warm.
Most mammals fertilize internally and the young develop inside the mother’s uterus. However, a few species of mammals, called monotremes, are egg-layers and have a uterus structure similar to that of reptiles. In other mammals, the structure of the uterus varies greatly—female rabbits and kangaroos, for example, have a double uterus and vagina, cows have a two-horned (bicornuate) uterus, and human females have a single, triangular uterus. While the embryo grows inside the mother’s uterus, it is supplied with oxygen and nourishment from the mother’s bloodstream via the placenta and is kept at a constant warm temperature. After birth, the young mammal is entirely dependent on the mother for milk until it can eat solid foods.