Specially bred rats and mice are the mammals used most often in medical research. Because rats and mice have so many biological similarities to humans, they make up 90 to 95 percent of the mammals in biomedical research. Some strains of rats and mice are susceptible to diseases such as cancer or high blood pressure. In addition, rodents develop diseases over a span of days or weeks instead of months or years. In the 1980s, major research discoveries made it possible to create strains of mice whose genetic make-up has been altered so that they carry specific disease-causing genes.
Other mammals commonly found in research are guinea pigs, rabbits, hamsters, and farm animals such as pigs and sheep. Most of these animals are specifically bred and raised for research. Researchers choose the species that best parallels the biology of what they want to study. For example, sheep provide a model to study osteoarthritis, a breakdown of cartilage that occurs as people age, causing inflammation in the joints. Pigs offer a model for research on skin problems, including what may happen when medicine or a toxic substance is absorbed through the skin. Species such as dogs, cats and non-human primates account for less than one percent of all mammals in research.