How Animals
Reproduce

All types of animals reproduce. Many animals have special organs that are used in reproduction. These organs are called gonads. Some simple animals do not have gonads, but they are still able to reproduce.

There are two general forms of animal reproduction: (1) asexual reproduction and (2) sexual reproduction. In asexual reproduction, only one parent produces the offspring. In sexual reproduction, two parents of opposite sexes are needed to produce the offspring. Many of the simplest animals, including sponges, sea anemones, and some flatworms, reproduce asexually most of the time. Sometimes, they reproduce sexually as well. Most other kinds of animals reproduce only sexually.

Asexual reproduction. Planarians and some other flatworms can reproduce by fragmentation, the division of the body into two or more pieces. When a planarian reproduces asexually, it typically divides into two sections, one with the head and the other with the tail. Each section then grows the parts that are missing and becomes a complete new individual.

Hydras and some sea anemones reproduce by budding. The animal produces small projections, called buds, from its side. These buds develop into miniature copies of the parent. The buds eventually detach from the parent, and the individuals produced by budding grow to be as large as their parents. Then they can put forth buds to create their own offspring.

Sexual reproduction. Most animals that reproduce only sexually do so with special sex cells known as gametes. Female sex cells are called eggs and are produced in the female gonads, the ovaries. The male sex cells are known as sperm and are made in the male gonads, the testes. Sperm are much smaller than eggs and have a tail that enables them to swim toward eggs. When a sperm cell unites with an egg cell, a new animal starts to form. The process in which the sperm unites with the egg is called fertilization.

External fertilization occurs outside an animal's body. Many aquatic animals reproduce sexually without ever meeting. Female sea urchins release millions of egg cells directly into the water. About the same time, the males release their sperm. The sperm swim through the water, and some unite with eggs, leading to fertilization. The fertilized eggs develop into swimming offspring, which are called larvae. The larvae grow and eventually sink to the bottom of the sea, where they become small sea urchins with bodies similar to those of their parents.

Internal fertilization occurs within an animal's body. If gametes are released on land, they dry up and die. Consequently, land-dwelling animals that reproduce sexually have developed ways for fertilization to take place inside their bodies.

Animals mate in many ways. Males of such species as snakes, lizards, birds, and mammals mate by releasing sperm directly into an opening in the female's body. Fertilization occurs in the female's reproductive organs.

Male salamanders do not release sperm directly into the female's body. Instead, they deposit a packet of sperm at the bottom of a stream or pond. When the female passes over the sperm, she draws them into an opening in her body that leads to her reproductive organs. Several other animals, including mites and scorpions, mate in a manner similar to that of salamanders. Males deposit packets of sperm on the ground, which are then picked up by females.

In almost all mammals and some reptiles, the embryo (undeveloped animal) grows inside the female's body after fertilization. However, in birds and some reptiles, the embryo develops outside the body. The female lays an egg in which the embryo develops.

Courtship behavior consists of actions that help animals find and choose suitable mates. This behavior tends to follow a specific pattern according to species. As a result, courtship behavior helps ensure that animals mate with members of their own species. If two different species mate, they may not produce young, or their offspring may be unhealthy or unable to reproduce. Such courtship behaviors as singing and displaying colors help animals recognize their own species.

Animal mates find each other in a number of ways. Female birds are attracted to the beautiful songs and bright feathers of males. Female grasshoppers, cicadas, bullfrogs, and toads also are attracted to the calls made by males of their species. Female silkworm moths release into the air a perfumelike chemical called a pheromone to attract males from as far away as several miles or kilometers. At certain times of the year, female dogs give off a pheromone that attracts male dogs. Female fireflies watch for male fireflies that flash their lights in a certain rhythmic pattern. Male fence lizards bob their heads rhythmically when a female approaches. Siamese fightingfish perform a complicated courtship dance, followed by the release of eggs and sperm into the water.

Some animals choose particular mates. The female anole lizard typically prefers to mate with the largest male. The peacock spreads his fantastic tail feathers, hoping to coax a peahen into becoming his mate. Peahens choose males with many spots on their tail feathers. Male birds of paradise gather in a tree. When a female appears, the brilliantly colored males strut and dance to show off their bright feathers. If a female chooses to watch this display, she will usually mate with the male that has the brightest colors.

Male bowerbirds build chambers or runways, called bowers, made of sticks or other material. They decorate these structures with brightly colored stones, bones, or other objects. The male dances and bows in front of his bower, hoping that a passing female will accept him as a mate. If one does, she enters the bower with him, and they mate there.

Some male animals give food to possible mates. A male tern catches a fish and places it into the mouth of the female he wants for his mate. A male dance fly brings a dead insect to a female. She eats the insect while mating with the male. A male that does not bring a dead insect risks being eaten by the female.

Mating is dangerous for some male spiders and insects. Male black widow spiders are sometimes eaten by females after mating. A female praying mantis may pounce unexpectedly on a male in her vicinity. Sometimes, she mates with a male and then eats him.

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