In a landmark decision, the National Institutes of Health announced today that it will drastically scale back its research on chimpanzees, humans’ closest living relatives. The agency expects to retire about 310 chimps out of a population of 360 that it owns and has available for research.

The decision follows recommendations from an NIH advisory panel that said in January that the agency should curb research on chimps and instead “emphasize the development and refinement of other approaches, especially alternative animal models,” and move all but approximately 50 of the chimps to sanctuaries. The NIH today accepted almost all of the recommendations from that independent panel after studying them along with more than 12,500 comments from the public. “Chimpanzees have already taught us a great deal…but deserve special respect,” NIH Director Francis Collins says. The decision is a “major milestone ushering in a new era, a compassionate era.”

The NIH has not funded new research involving chimps since December 2011, when the Institute of Medicine, a nonprofit that advises the government on health policy, issued a report establishing strict criteria for the use of chimps in biomedical and behavioral research. At the time, the IOM said, "...that research use of animals so closely related to humans should not proceed unless it offers insights not possible with other models and unless it is of sufficient scientific or health value to offset moral costs. We found very few cases that satisfy these criteria."

The IOM recommended that chimp research satisfy three principles: the knowledge gained would be necessary to advance the public’s health; there was no other research model available to obtain that information; and that the animals would be kept in their natural habitat or in environments that closely mimic it. The NIH today accepted those principles as the criteria for any future research on chimps that it owns or supports.

A number of medical breakthroughs have come about, thanks to chimp research, including vaccines against polio and hepatitis B. But recent developments, including computer modeling and the ability to experiment on isolated cells, have equipped scientists with viable alternatives to primate research. Under today’s decision, NIH will still keep up to 50 chimpanzees—drawing from the pool of those it owns and supports—for future research purposes. Those are expected to include prophylactic vaccines for hepatitis C as well as behavioral studies.

The remaining 50 chimps will not be bred in captivity, so over time the number of chimps available for research will dwindle. Observational or behavioral studies on chimps that occur in zoos or sanctuaries are considered noninvasive and will proceed. Current research that does not meet the IOM principles will be allowed to wind down, and the implementation of the decision will take place over several years. “Everybody should understand this is not something that will happen quickly,” Collins says.

The challenge now will be trying to find money to pay for the maintenance and care of the chimps slated for retirement. Under the Chimpanzee Health Improvement, Maintenance and Protection (CHIMP) Act of 2000, a $30-million cap was put on spending for chimpanzees in sanctuaries. Collins today said that his agency is very close to hitting the $30-million limit and will probably hit it in the “next few months.” The agency is asking Congress to amend the law to free up NIH funding so it will be able to support the more than 300 chimpanzees that are expected to eventually be placed into retirement. The NIH does not have an estimate for how much it will cost to maintain the chimps in sanctuaries or exact plans on where the chimps will retire. Existing sanctuaries can hold only 150 chimpanzees, according to the NIH. The agency will be taking care of the retirement of only those chimpanzees it owns.

The Texas Biomedical Research Institute, which owns 91 chimps that are supported by NIH funds, said it was “disappointed” by today’s announcement and felt that this move would slow research. “As the number of chimpanzees eligible for research decreases below 50 as a result of death from natural causes, the pace of research will be slowed even more, and human and chimpanzee lives will be lost unnecessarily due to delays in bringing new drugs and vaccines to market,” it said in a statement.

Although the NIH accepted most of the recommendations from its panel, it did not accept the recommendation that chimpanzees each be housed in spaces up to 1,000 square feet (93 square meters). James Anderson, NIH deputy director for program coordination, planning, and strategic initiatives, says there is currently “scant” science that suggests that those space recommendations are needed in order to ensure chimps have normal behavior, moreover, creating spaces that size would be costly. “What we’re going to do is take a step back and review all the published literature.” Current guidelines for the care for animals in research call for about 25 square feet (2.3 square meters) for each animal, but current cages are closer to the 1,000-square-foot size, Anderson says.

Today’s NIH decision comes on the heels of a proposed rule from the U.S. Fish and Wildlife Service that would place all chimps—including those in captivity—under the protection of the Endangered Species Act. Currently, only chimps in the wild are classified as “endangered” whereas those in captivity are “threatened.” If the FWS rule is enacted as proposed, that would mean permits would be required to do research on the chimps, and they would only be awarded in instances where research benefits the animals directly. “Research to benefit human beings would not be considered research to benefit chimpanzees,” says Kathy Hudson, NIH deputy director of science, outreach and policy. Still, she says that the NIH is confident it will be able to make arrangements to allow the agency to continue research essential for human public health.

The NIH owns or supports the maintenance of 670 chimpanzees, but only 451 (including those it funds at the Texas facility) are available for research. The agency expects approximately 310 to be designated for retirement to the federal sanctuary system.