The U.S. government’s decision to end its support of most medical experiments on chimpanzees came after decades of impassioned, often bitter debate — yet in some ways, it was an easy decision.
After all, chimpanzees and humans famously share 98 percent of our DNA. They’re our closest living relatives. They’re charismatic and enough like us that it’s difficult not to treat them with compassion. But what about monkeys?
Even as the ethics of chimp research grab headlines, the use of monkeys — tens of thousands in the United States alone — receives minimal attention. They arguably share many of the traits that make experiments on chimps and other great apes so ethically troubling, yet elicit barely a whisper of public concern.
That absence of attention says less about the ability of monkeys to think, feel, and suffer, than our willingness to think about it.
“It’s been very hard to show differences between great apes and monkeys in terms of cognitive abilities,” said evolutionary anthropologist Brian Hare of Duke University, who has studied both. “I’m not saying there are not differences — of course there are — but it’s hard to show. Most of what you know about great apes is also true about monkeys.”
Nearly 120,000 nonhuman primates are kept in captivity in the United States, with roughly 70,000 used for research; more than 20,000 are imported every year, either sold as infants by monkey breeders or caught in the wild. Most are monkeys, and most of the monkeys are rhesus macaques, which have become the model primate of choice for medical research.
They’re used to study dozens of diseases and conditions, from neurodegeneration and cancer to depression, diabetes, stroke recovery, and addiction. They’re easier to work with than chimps, and solve what geneticist Vincent Lynch of the University of Chicago calls a biomedical Goldilocks problem.
Monkeys are just different enough from humans “that we are comfortable experimenting on them and just close enough genetically that those experiments are still applicable to human health,” Lynch said. “They would seem to be just right.”
By “just right,” Lynch doesn’t mean that he works with monkeys himself. “They are just too similar to me to justify it to myself,” he said. “Monkeys clearly are valuable as animal models, and I would never do experiments on them.” It’s the sort of sentiment often characterized as animal activist rhetoric, but it’s also where the science is pointing.
Studies of rhesus monkeys have found them capable of empathy, long considered an essential human trait. They think about their own thoughts, which is essential for complex self-awareness. They can recognize themselves in mirrors, experience regret, have a sense of justice and fairness, and possess what cognitive scientists call theory of mind: an understanding of what other individuals think and feel.
Their brains possess anatomical features that, in humans, are central to emotion, and it makes intuitive sense that monkeys would feel deeply. After all, cognition and emotion are intertwined, and emotion is a deeply rooted evolutionary feature intertwined with living in large, social groups — which monkeys certainly do.
Monkeys share with chimpanzees nearly all of the features that a landmark Institute of Medicine report cited in concluding that chimpanzees are worthy of special consideration when assessing their use in research. Yet there’s one crucial difference: Unlike chimps, which are presently useful for studying just one or two diseases, monkeys are useful for many.
“Monkeys are especially valuable for a wide realm of research domains,” said Stuart Zola, director of Yerkes National Primate Research Center. “Their close-to-human brain anatomy and close-to-human genome make them very good models,” especially for brain-related diseases. Human and monkey immune systems “are organized in similar ways, so viruses and infectious agents can be effectively studied safely in monkeys.”
Indeed, monkeys are only becoming more medically useful. The maturation of techniques for inserting disease-related human genes into monkeys “sets the stage for grand possibilities of clarifying the mechanisms of disease in ways heretofore never before accessible to us,” Zola said.
How might this equation of monkey consciousness and medical utility be balanced? One possible response would be to say monkeys should not be used at all; another is to say that, however unfortunate, the possibility of easing human suffering outweighs any suffering in monkeys. Between these two positions is the argument that experiments on monkeys are acceptable, but only if the human benefits are truly significant.
For chimpanzees, the National Institutes of Health says they should only be used in medical experiments when no other alternatives exist. That high standard likely won’t be reached in the near future with monkeys, but researchers might think more carefully about whether they want to use monkeys, as Vincent Lynch did. Institutional committees responsible for approving research proposals might also raise the bar for approving monkey experiments.
After the question of whether and when monkeys should be used comes the issue of how they are used. Tetsuro Matsuzawa, a Kyoto University primatologist and current president of the International Primatological Society, doesn’t personally think monkeys should be used in medical experiments, but he also respects arguments in favor of using them to save human lives. Matsuzawa can accept monkey experiments, he said, if researchers treat them as compassionately as possible.
Hare echoed those words, saying that monkey welfare — even things as simple as improving the lighting in their housing – is often overlooked. “People are always talking about whether we should use them or not, and not about whether we can reduce their suffering,” he said. “We need a culture of compassion, and we don’t have that.”
Federal laws and National Institutes of Health guidelines do technically require that researchers promote the welfare of their monkeys, but those rules aren’t strictly enforced, said Hare. Only the most egregious violations are punished meaningfully, and researchers are given few incentives to improve their monkeys’ lives.
“The question is, ‘Have you demonstrated compassion for the animals in your own lab? Have you, on your own, without anyone patting your back, done something to improve how your monkeys are housed?” Hare said. “It’s a rare researcher who does that.”
Kathleen Conlee, animal research director with the Humane Society of the United States, noted that the NIH has resisted attempts to strengthen the federal Animal Welfare Act, which guides how animals are used in research in the United States. Those rules are often vague and researchers can too easily apply for exemptions, said Conlee.
Many rhesus monkeys are, for example, kept in solitary cages, an experience known to drive these highly social animals crazy. “The Animal Welfare Act was supposed to create environments that addressed the psychological well-being of these animals,” said Conlee. “A lot of the facilities are falling short of meeting those minimal standards.”
One Humane Society analysis of documents from two major U.S. primate research facilities found that their animals spent, on average, 53 percent of their lives in solitary housing, sometimes with nothing in their cage but a piece of metal hung on one side for “enrichment.”
It might sound strange to consider how best to treat monkeys that are consigned to medical sacrifice anyways — but if we’re going to use these intelligent, emotional creatures, we should do it right, said Conlee. “Until the day we ultimately replace them in research,” she said, “we have an obligation to address their welfare.”
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