In both viviparous animals and oviparous animals, fertilization of the mother's egg with the father's sperm takes place inside the mother's body. One of the advantages to giving birth to live young is that the mother protects the fetus inside her body as it develops. The developing fetus derives nutrients from the mother's body, and so is assured of receiving all the nourishment it needs to complete development.
The length of time between fertilization and birth in viviparous animals is called the gestation period. The length of the gestation period varies according to species. The gestation period of mice is 21 days, of rabbits is 30-36 days, and of dogs and cats is 60 days. The largest mammal, the baleen whale, has a gestation period of 12 months—only three months longer than the gestation period of humans. Elephants have one of the longest gestation periods of all animals, 22 months.
Some viviparous animals such as humans, horses, and cows, give birth to only one offspring at a time, although occasionally these animals produce twins or triplets. Other animals give birth to many offspring at a time. Usually, the multiple offspring in a litter are each derived from a separate egg, but the armadillo gives birth to four identical offspring that are derived from the same fertilized egg.
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