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 90–95% 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. (See A Revolutionary Model)
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 pain and 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.
Researchers increasingly rely on species such as zebrafish, fruit flies, and worms for basic research about gene function and biological processes.
Species such as dogs, cats, and non-human primates account for less than 1% of all mammals in research. Although not used widely, these animals have characteristics that make them vitally important for the study of heart disease, neurological disorders, and diseases such as HIV/AIDS. They are also needed for research to benefit others of their own species, including studies of feline leukemia or the Ebola virus. (See also Why do researchers study animals?)