Copyright © 2007 Dorling Kindersley
Animals reproduce in one of two ways. In asexual reproduction, animals produce young, which are identical to themselves, without mating with another animal. Most creatures that reproduce in this way do not live very long but can reproduce in large numbers rapidly. In sexual reproduction, a female animal’s egg unites with a male’s sperm cell after mating, in a process known as FERTILIZATION. The offspring inherit features, called traits, from both parents. These animals tend to develop more slowly and many have parental care after birth.
Like many animals, including birds, most reptiles, and amphibians, female crocodiles lay their eggs after they have been fertilized. Crocodiles hide them in a nest in the ground away from predators, so that the embryos (growing babies) can develop in safety. The young chirp when they are about to hatch and their mother digs them out. Then she carries the hatchlings in her mouth, and takes them into the water in batches.
In many animals, the male contribution to reproduction ends straight after he has fertilized the female’s eggs. However, with sea horses it is the male who looks after the eggs. The female deposits them in a special pouch on the male’s abdomen, where they are fertilized. He then carries the eggs in his pouch until they hatch 2–6 weeks later.
Simple animals, such as hydra, reproduce asexually by forming new growths from their bodies. This is called budding. Each bud eventually breaks off and forms another animal. Other animals, such as sponges, can reproduce by breaking off parts of their bodies. This is called fragmentation.
Most mammals, some reptiles and amphibians, and several invertebrates give birth to live young. Once the egg is fertilized inside the female it stays there while it develops. This is known as the gestation period. In hippopotamuses, gestation lasts for about 240 days. During this time, the developing calf is protected in a constant environment and obtains nourishment directly from its mother’s body.
During fertilization, a male sex cell (sperm) and a female sex cell (ovum) unite to produce a cell that will grow into a new animal. Sex cells have half the number of chromosomes (chemicals which tell the cell how to grow into another individual) than other body cells. When the sex cells unite, the full amount of chromosomes is restored.
Many animals reproduce through internal fertilization. A male and female pair up and mate, and the female’s eggs are fertilized inside her body. Empid flies mate in this way. The male empid fly is smaller than the female and risks being eaten during mating. To protect himself, he presents the female with a small insect to distract her.
Fertilization outside a female’s body is a random process. Some eggs are not fertilized and sex cells can be easily eaten by predators. Corals simply release eggs and sperm into the water. To increase the chances of fertilization occurring, corals of the same species all spawn at the same time. That way a predator has fewer opportunities to eat the coral’s sex cells.