If a rhesus macaque (Macaca mulatta) has a broken hand, you may notice only if you examine her carefully, because she will just use the other hand for everything from grasping a grape from your hand to climbing around. In the eight years that I’ve been working with these primates as a systems neuroscientist, I’ve noticed that, unlike (most) human patients, they tend to hide rather than show their injuries.
The behavior raises a question: As fellow primates, why would these intelligent animals hide an injury? Wouldn’t showing distress in a group-living species invite help from others, whether from other macaques or from us?
Primatologists have long known the answer: Rhesus macaques have steep despotic social hierarchies, in which subordinate individuals may challenge a visibly injured animal’s status. These social structures are also relevant when we test their cognitive skills. Research long suggested that macaque monkeys do not understand perspective-taking of others—for example, ignoring cooperative cues from humans signaling where to find treats. Yet primatologists in my native Netherlands discovered in 2013 that long-tailed macaques (Macaca fascicularis) do understand; we just hadn’t tested them correctly. In a competitive context, the subordinates actively reach for treats if they know a dominant monkey can’t see them.
As neuroscientists, we need this primatological perspective. Yet interactions between us and primatologists are limited. We operate in parallel worlds: We attend different conferences, use different language to describe the same behavioral phenomena and rarely read each other’s work. The resulting blindness weakens both fields. Neuroscientists may misinterpret the findings from experiments they designed without understanding the evolutionary and social context that shapes how nonhuman animals behave. Primatologists, who observe what animals do, lack the benefit of neural recordings to capture an animal’s internal state when it hides what it knows or feels.
Even so, primatologists can be reluctant to work with systems neuroscientists. The late Jane Goodall, probably the most well-known primatologist to the public, categorically rejected invasive neuroscientific work. Although the field holds a range of perspectives, she and other primatologists who work on animal advocacy have often labeled neuroscience as “vivisection,” translated from Latin as “cutting into the living.” That labeling can, in turn, make neuroscientists reluctant to interact with primatologists.
Mariska Kret, a rising star in primatology, also has ethical concerns over invasive primate research, and she acknowledges important scientific issues: “I have ethical objections to this research and therefore do not like writing about it. Furthermore, I believe if we want to understand emotions, we can best do so without applying coercion, punishment, or injury, because that influences the scientific results,” she wrote in her 2021 book. Her concerns are not something we can ignore as neuroscientists.
And, increasingly, we don’t have to, thanks to technological innovations that are quickly changing neuroscience.
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ike others, I have long studied primates in head-fixed, solitary conditions: the only way to insert the fragile probes electrophysiology requires. Yet I agree that these conditions are coercive and out of context for these social animals.In June, I attended a conference in Erice, Sicily, on primate neuroethology, and I was struck by findings from Luca Bonini and his team, who use wireless chronic recording to study premotor cortex in monkeys that are moving freely, not head-fixed. Michael Platt and his colleagues have also used these wireless recordings to obtain neural signals of several species-specific behaviors, such as grooming reciprocity and empathy in response to an aggressive intruder. Neuroscientists can now explore at single-neuron resolution some of the questions that primatologists have long cared about, no experimental rig or head-fixed animals required.
Technology is also improving welfare conditions, beyond making sure these social animals have group housing that is adequately sized, for example. European primate facilities have started adding touch screens for the animals’ cognitive enrichment. For instance, a behavioral unit at the Deutsches Primatenzentrum in Göttingen features responsive touch screens and facial recognition for individual monkeys, which can be attached to an animal’s home cage. Macaques can enter the unit voluntarily, and most of them do. Such innovations are spreading across primate research, further reducing the need to run experiments in the rig.
Better ethics and better science are going hand in hand—at a key moment in time. Scientific work with primates is societally sensitive. Political decisions almost led to the closure of the Biomedical Primate Research Center (BPRC), one of the largest primate centers in Europe. According to Dutch Minister of Education, Culture and Science Rianne Letschert, the BPRC provides “essential infrastructure” for medical research, an argument that won the fragile reversal. Scientists from the BPRC have argued that primate research is essential for pandemic preparedness and understanding neurodegenerative disease. Although I share this view, I think their defense of the BPRC is incomplete. We need to understand primates as primates. This requires us to go beyond mechanistic biomedical explanations and keep asking fundamental questions about primate evolution. This is the kind of work primatologists at the BPRC did in 2014 with macaque perspective-taking.
The Sicily neuroethology conference offered a place I had long been looking for, one where neuroscientists were asking primatological questions. Yet I noticed who wasn’t there. The technical revolution has laid the foundation for a bridge between neuroscientists and primatologists. But the primatologists have not yet made it across. To use this technology to the fullest and truly understand the primate brain, we need primatologists at the table. Not to endorse current practices but to help shape what comes next. To build a future in which we learn more about both the brain and behavior of our closest relatives. And ourselves.
