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Of all the animals used for invasive research, nonhuman primates are perhaps the most controversial. While the total number of animals used in experiments and covered by animal welfare laws has declined in recent years, the number of nonhuman primates used has increased.

Developing models to replace animals in research, a longstanding goal of animal welfare advocates, has become a priority for the National Institutes of Health, the Food and Drug Administration, and the Environmental Protection Agency.

Debates over the use of animals for research often center on the animals’ experience of pain. Animal welfare regulations require researchers to report whether the animals they use are subjected to pain and whether it is relieved by medication. If pain is unrelieved, researchers must give a justification.

The vast majority of animals used for research in the U.S. — 96 percent — are mice, rats, fish, and birds. Two charts show the breakdown of all animals used in research and their quantities.