Animals have been endangered because their natural surroundings have been destroyed by humans.
It is true that zoos have had several success stories with zoo-bred animals.
One was the golden lion tamarin, a species of monkey, which had almost become extinct because humans destroyed its natural habitat and too many were captured for pets and zoos.
Over 100 tamarins were bred in zoos, and when they were released into the wild, only 30 survived.
Some were unable to live life in the wild-they were not able to climb trees, or when they did, they fell off; some did not even move; some were not used to a natural diet.
It is a risky business to re-introduce zoo-bred animals to the wild, because if they have lost their instinct for survival and cannot adapt quickly enough, they will die.