Animal testing, also known as animal experimentation and animal research is the use of animals for experiments. Escherichia coli, Fruit flies, and mice are often used for animal testing. About 50 to 100 million vertebrate animals and many more invertebrate animals are used around the world every year. The source of the animals are different according to the country and species. Most animals used for experiments are bred for this purpose. But others may be caught from the wild or bought from people who buy them from animal shelters.