It is difficult to calculate the number of animals used in research because most countries do not keep records. However, it is estimated that more than 115 million animals—including mice, rats, birds, fish, rabbits, guinea pigs, farm animals, dogs, cats, and non-human primates—are used globally in laboratory experiments each year. They are used as models to study human disease, injury, development, psychology, anatomy and physiology. Animals often suffer greatly in these studies, as they are inflicted with diseases, traumas, and pain in order to mimic human conditions. Using animal models for biomedical research is criticized because non-human animals do not accurately mimic human diseases, because the diseases do not naturally occur in other species.