Monkeys Are Neither Simple nor Fools: A Primatologist Speaks Out Against Experiments on Monkeys

By Lisa Jones-Engel, Ph.D.

Scientists design and use models to understand and make predictions about complex systems. Effective models should be simple and well characterized, which means that they have been adequately evaluated for their responsiveness and ability to ensure consistent results. Rhesus macaques are not simple; rather, they are as behaviorally, physiologically, and immunologically complex as humans. And they certainly are not well characterized in laboratories, where their biology, research, husbandry, and rearing histories are diverse and ill-defined. So we must ask ourselves what the biomedical behavioral relevance is when highly complex, poorly understood monkeys are used as translational models for self-injurious/anxious behaviors in humans.

PETA obtained 40 hours of videos showing laboratory-reared macaques being exposed to the “human intruder test”—a set of personality experiments using hundreds of caged monkey “models” purportedly to gain insight into human anxiety disorders. In the test, a supposedly unfamiliar human approaches the cage in which the monkey is being held, and off-camera the experimenters evaluate the monkey’s fear and anxiety by, for example, measuring how long the monkey freezes in response to the intruder. These videos reveal an astonishing lack of scientific rigor. Experimental conditions vary widely and exhibit a clear lack of standardization of research variables, such as housing/testing conditions, age, sex, and familiarity with the tester. The scientists themselves acknowledge as much in their publications, citing a lack of consistency in their study subjects and design, their use of convenience sampling, and variation in testing protocols across the different centers.
The macaques used in this research had already been severely damaged—both mentally and physically—by their captivity. The primate biomedical community has known for decades that the standard practices of rearing monkeys in cages no bigger than a dorm-room refrigerator and separating mothers and infants lead to “behavioral pathologies.” Hundreds of publications and millions of research dollars have shown time and again that monkeys in tiny cages are physically and emotionally traumatized and that the only thing more emotionally destructive to them than living under these conditions would be to separate the infants from their mothers at birth. So let’s be honest: The laboratory-reared macaques used in these studies are not simple, well-characterized models for humans suffering from anxiety. They are unnatural, broken, defeated, defenseless monkeys.
The premise behind the human intruder test is that an experimenter “unfamiliar to the monkey” stands in front of the monkey’s cage. This is perhaps the most fundamental flaw in these studies because it is obvious to anyone who knows macaques that they understand us better than we do them. Scientists may have been studying macaques in the laboratory for decades, but for millennia these highly social, intelligent, ecologically adaptable monkeys have been observing and living with humans. They pay attention. They watch and remember every single living being who has ever moved through their environment. You don’t get to be the most widely distributed nonhuman primate in the world without developing keen observational skills. They know us! Nobody is fooling these macaques. These experiments are useless.


Primatologist Lisa Jones-Engel, Ph.D., is a Fulbright scholar who has studied the human-primate interface for 35 years. Her scientific career has spanned the field, the research laboratory, and the undergraduate classroom. Dr. Jones-Engel serves as senior science adviser on primate experimentation with PETA’s Laboratory Investigations Department.

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