In December of 1938, a primatologist named Clarence Ray Carpenter released several hundred rhesus macaques onto Cayo Santiago, a small island off the eastern coast of Puerto Rico. He had bought them in the Lucknow region of India and shipped them across the world because he was worried, correctly as it turned out, that the coming war would cut American researchers off from the monkeys they depended on, and he wanted a breeding colony on this side of the planet that could sustain itself. It did. The colony has been living on that island, free-ranging and largely left to its own devices, for the better part of a century, and it has become one of the most closely watched primate populations in the world (researchers have tracked its members and their relationships for generations).
On September 20, 2017, Hurricane Maria made landfall on Puerto Rico as a category 4 storm, and it destroyed roughly 63% of the vegetation on Cayo Santiago. Temperatures on the island regularly climb past 40°C, so what the storm really took from the macaques was shade, which went from something taken for granted to one of the scarcest resources on the island.
What happened next is the part I keep coming back to. The monkeys did not simply compete harder for what was left; they became more tolerant of one another, less aggressive, and sought out new social relationships rather than doubling down on the ones they already had, expanding the number of individuals they would allow to share a patch of shade with them. Years of follow-up showed that this tolerance was not a pleasant side effect. After the hurricane, and not before it, social tolerance predicted which individuals survived.
The macaques got through a catastrophe by being better at politics.
I think that’s a very human story, and not because of anything to do with raw intelligence. It’s the story of a species that survives by reorganizing its society when its environment changes out from under it, which is more or less the story of us.
And it’s the reason I’ve always thought the macaque gets undersold in the question of which animal is closest to humans, a question we almost always answer with genetics. By that measure the answer is settled: the great apes are our closest genetic and evolutionary relatives, they have been paramount in evolutionary genetic research, and our lineage split from the monkeys roughly 25 million years ago. I don’t dispute any of it. What I want to argue is that genetic distance is the easiest metric we have for closeness, not the most meaningful one, and that by the measure that matters most (the strategy that actually made our species successful) the macaque has a better claim on us than the apes that overshadow it.
It’s worth being honest first about why apes dominate this conversation, because a lot of it has less to do with apes than with us. The macaque is the famous nonhuman primate model, the workhorse of the laboratory, valued because its small size makes it convenient to keep and study in captivity. But that same small size, along with its quadrupedal locomotion and its life in the trees, makes it more of a pain to study in the field, particularly for non-primatologists looking to validate their theories of human behaviour and evolution. Great apes are large, slow moving, and spend much of their time on the ground, so we can follow them through a forest, study them intimately, and then compare what we see there with what we see in the lab; we get a much fuller, almost meta-level picture of them as a result.
Monkeys require complex arboreal environments that can’t be recreated in any enclosure, so a great deal of the nuance in their behaviour, their social networks, and their adaptations has simply gone unrecorded, given how limited our methods have been in and out of the lab. The literature reflects this. There is an overwhelmingly large disparity in the depth of research on apes compared to monkeys, and it only widens when the goal is comparing nonhuman primates to humans.
It’s the streetlight effect (looking for your keys under the streetlight because that’s where the light is, not because that’s where you dropped them), with an aesthetic bias layered on top: we see ourselves in the upright, terrestrial ape far more readily than in the quadrupedal monkey with a tail. None of this is evidence about closeness; it’s evidence about what has been convenient to study, and my suspicion is that as the methods for studying monkeys become more sophisticated we’ll find more similarities than differences, enough to force at least a moderate paradigm shift in how we talk about primate models of ourselves.
It also depends heavily on who is doing the asking. There are far more monkey species than ape species, with high variability across nearly every feature, so which aspects we count as “more human” depends wholly on how we weigh them; one species can be more human-like than the rest in a single respect (marmosets, say, in certain aspects of cognition) without being a good model of humans holistically, which is the role chimpanzees usually play. If you put a geneticist, a psychologist, and an ecologist in the same room and asked which nonhuman primate is closest to us, each would probably give you a different answer.
When I first made this argument I was a neuropsychology student writing for a classical primatologist, and what each of us considered the single most defining feature of Homo sapiens was not the same. My definition of human-like centers on social behaviour, cognition, and the brain, while for an ecologist it might be adaptive behaviour, migration, and physiology. So I’d rather not pretend there’s a neutral answer. The question I think is actually worth asking is narrower and more interesting: what made humans succeed, and who else succeeded the same way?
We usually answer the first half with intelligence, because we like to believe we’re successful by virtue of being the smartest animal on Earth. But the thing that truly distinguishes humans isn’t cleverness in the abstract; it’s that we spread into nearly every environment on the planet and held on, and that’s an ecological fact before it’s a cognitive one.
The old Darwinian way of thinking about evolutionary diversification revolves around mutability, such that natural selection is treated as the driving force behind what is ultimately a tendency to change. A more recent school of thought treats robustness and spread as the better indicator of a successful species: the stability and preserved state of a species through the continued maintenance of its developmental and functional systems, despite an environment that keeps changing around it. I find this a fascinating shift of lens, because it describes systems that are dynamic enough to withstand disruption without deviating in nature, and it defines success as persistence rather than transformation.
Cayo Santiago is, in a way, a small experiment in exactly that.
By this measure the macaque looks remarkably like us. Macaques have been described as a genus of “profound unity and great diversity”, and second only to Homo, the genus Macaca has spread across more of the world than any other primate: tropical, temperate, and subalpine forests, tidal areas, arid land, farmland, and cities. They do it while maintaining broadly similar physiologies, which means they have to be ethologically pliable enough to find the nutrients they need, meet their housing needs, and keep their social structures intact in every one of those very different communities of plants and animals. Humans got there through cultural diversification; macaques got there through evolutionary diversification. Many ape species, by contrast, have stayed within fairly narrow ranges without much colonization or expansion, and whether apes are capable of the kind of cultural diversification humans have undergone has yet to be seen.
The cities are where the comparison stops being abstract. Rhesus macaques have colonized the heart of New Delhi, including the grounds around parliament and key ministries, snatching food and phones, breaking into offices and homes, and generally treating the seat of government as prime habitat. For years officials dealt with them by bringing in langurs, the larger monkeys macaques are afraid of, and when captive langurs were banned after pressure from animal rights activists, the authorities hired men to disguise themselves as langurs and imitate their calls instead, many of them from the Madari community, which has worked with monkeys for generations.
I think about that often; it’s hard to imagine a clearer admission that another primate has moved into our niche than a government paying people to impersonate that primate’s predator.
Robustness alone isn’t the whole story, though, and it would be a weak argument if it were. Rats are robust colonizers too, and nobody is proposing the rat as our closest relative.
What sets macaques apart is that their spread is carried by a genuinely complex society, and this is the heart of the argument for me. Macaques are something like a proof of concept for normalized social structure in an intelligent animal. Their social infrastructure is consistent across the genus despite evident plasticity between individuals, and while structures differ between species, they sort into a limited number of distinct social styles. That consistency, carried across climates and diets, is what makes them so useful for asking whether a predictable social structure can be biologically conserved in a way that resists environmental influence, and whether that extends to us. They are consistent at the level of overall organization while holding an enormous variety and richness of relationships inside it, a combination we otherwise really only see in ourselves, and one conspicuously absent in the rat.
That richness is political. Macaque societies run on dominance and strong kin bias (nepotism, in plainer terms), but the more telling detail is that conflict is almost never one directional; most aggression is met with some form of protest or retaliation, and sometimes followed by reconciliation. Within the overarching dominance order there are countless embedded microstructures, alliances and grudges that each trace a given monkey’s personal history with the troop. A mother will shape her infant’s social world by being selective about who she allows to approach it, so political alignments are managed and passed down across generations, which is perhaps the best single example of macaque social opportunism.
Troops commonly hold 20 to 40 individuals and have been reported as large as 100 to 200, at which point the networks become so complicated that they can’t feasibly be studied without tracking each individual across its lifespan, and home ranges that shift with ecological resources add yet another layer once you account for many troops interacting. Primatologists call this Machiavellian intelligence, and Dario Maestripieri made the case for it in rhesus macaques at book length.
The standard socioecological model explains primate social organization mostly through resource allocation and feeding competition, and I don’t think it can account for this. Something more is going on than competition over food, and Cayo Santiago is a good illustration of what: when the resource disappeared, the monkeys that did best were not the ones that fought hardest for it but the ones that had the widest circle of others willing to tolerate them.
Even their appearance carries social information. Colouring varies between species, within species, and across sex and age in ways that signal status and identity at a glance; great apes are relatively uniform within a species, while humans do exactly what macaques do (we just use clothing and jewellery). All of it assumes persistent social memory and the ability to recognize and reason about a great many individuals over a lifetime, which is very human and something that isn’t really demanded of apes, whose groups are far smaller. Macaques have even been shown to transmit parental behaviours across generations through experience. This flexibility, social and ecological at once, runs through human life, and I can’t find a real equivalent in the great apes.
If macaques share the strategy, you’d expect to find some of the same machinery supporting it, and here the evidence is encouraging and humbling in about equal measure. Rhesus macaques have been the focus of neuroanatomists and molecular neuroscientists for the better part of seventy years, with comparatively little equivalent work done on great apes. The prefrontal cortex is a relatively recent development in evolutionary time and accounts for much of what separates humans from other primates, and the medial prefrontal cortex in particular is implicated in high-level social cognition (theory of mind, and by extension introspection), which is precisely what macaque politics demands.
Comparative work has found strong similarities between humans and macaques in how the frontal pole, medial prefrontal cortex, and dorsal prefrontal convexity are organized, regions that are highly developed in humans and more extensively studied in the macaque than in any other primate.
The differences are real, though, and large enough to challenge how far the macaque model can be pushed. The human brain is several times larger, and its proportions differ: the prefrontal cortex makes up 28.5% of the human neocortex and only 11.3% of the macaque’s. Humans have far more specialized subregions, sensory areas that are more sophisticated both in isolation and in how they communicate with the rest of the brain, roughly 70% greater dendritic spine density in layer III of the prefrontal cortex, more complex pyramidal neurons, and spindle cells.
Nobody who works with macaques presumes their brains are equal to ours; they’re the gold standard of neuroscientific models because of practicality, and they’re invaluable for exactly that reason. And because so little comparable work has been done on great ape brains, we don’t know nearly enough about comparative neuroanatomy to say definitively which model is most analogous to humans, or by what margin, so the default assumption that apes win this comparison is, again, partly a product of what’s been studied. My own reading is that the social brain of the macaque may be organized very much like ours, and that what separates us intellectually is scale and elaboration rather than design.
There’s one more way macaques have stood in for us that I’ve never been able to shake. When the United States started sending animals up on captured V-2 rockets after the war, the passengers were rhesus macaques, all named Albert. Albert I was launched from White Sands, New Mexico, in June of 1948 and died on the flight; Albert II reached space the following year, the first primate to do so, and died when his capsule’s parachute failed on the way down. The Alberts after him mostly met the same fate.
It wasn’t until May of 1959 that a rhesus macaque named Able, riding a Jupiter missile out of Cape Canaveral alongside a squirrel monkey named Baker, made it to space and back alive, splashing down in the sea off Puerto Rico, about twenty years after Carpenter’s colony had been released on its little island off the same coast. That’s no small feat, and if anything it speaks to the macaque’s resilience in adapting to nonarboreal and, frankly, thoroughly depressing conditions on our behalf.
And in the mountains of Nagano, Japanese macaques spend the winter soaking in the Jigokudani hot springs to cope with the cold, and the same species has been an important model for studying homosexual behaviour in ways that extend to humans. Individually these read like curiosities. Taken together, I think they’re part of a growing body of evidence that the monkey-human parallel runs deeper than we’ve let it.
So, are humans closer to macaques than to apes? It depends on whom you ask, and that dependence is the point. Comparative research leans so heavily on basic science, genetic similarity above all, that we’ve stopped weighing much else. Evolutionarily the apes are our closest relatives, and they’ve taught us an enormous amount about terrestrial primate living, social organization, mating systems, genetics, cognition, and behaviour. But they shouldn’t overshadow what actually made humans so distinct, which is our near total domination of the Earth; part of that can be attributed to intellect, but much of it came from robustness and colonization, achieved through political prowess.
The near extinction of the great apes makes the point uncomfortably clear: intelligence, dictated by things like neuroanatomy, isn’t enough on its own to ensure the survival of a species. Macaques are one of the very few species besides us that operate something like an army, in large numbers, across continents, held together by alliances, memory, and maneuvering, which may never develop in the great apes given their limited group sizes and populations.
When the island went dark and hot and the shade ran out, the macaques of Cayo Santiago survived by widening the circle of who they’d tolerate beside them. If what makes us human is how we came to succeed rather than who we happened to descend alongside, then I don’t think we can keep defining “close” by the family tree alone.
References
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- Maestripieri, D. (2007). Macachiavellian Intelligence: How Rhesus Macaques and Humans Have Conquered the World. University of Chicago Press.
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