In a forest in Brazil, a group of capuchin monkeys spent months caring for an infant marmoset — feeding it, carrying it, keeping it close. The marmoset wasn’t their kin. It wasn’t even their species.
And it turns out this kind of bond is far from rare. A new comprehensive review has compiled hundreds of documented cases of monkeys and apes grooming, playing with, and sometimes adopting other animals entirely. The question researchers are now asking: is this behavior a curiosity of the wild — or something that runs much deeper in the primate lineage?
A study unlike any before it
Researchers at the University of Oxford set out to do something no one had done before: compile a sweeping, systematic review of every documented case of primates forming bonds with other species. The result, published in the journal Primates, draws on 303 cases from scientific literature, 66 surveys completed by primatologists, and 58 media accounts — together painting a picture far broader than anyone anticipated.
The behaviors documented range widely: carrying, grooming, playing, and in some cases outright adopting animals from entirely different species. Even the researchers described the scale as surprising. These weren’t isolated curiosities buried in obscure field notes. They were widespread, recurring, and distributed across multiple continents and dozens of primate species.
From squirrels in Sri Lanka to pigs in China
The individual cases are striking on their own terms. In Sri Lanka, tufted gray langurs were observed petting squirrels — not chasing them, not ignoring them, but engaging with them in a way that looked remarkably gentle. In China, snub-nosed monkeys were documented playing with domesticated pigs. These aren’t species you’d expect to find sharing anything beyond the same patch of ground.
Then there’s the capuchin case from Brazil. A group of capuchin monkeys raised an infant marmoset for several months — carrying it, keeping it close, integrating it into their social group. The marmoset wasn’t a capuchin. It offered no obvious benefit to the group. Yet they cared for it anyway.
These examples span South America, Asia, and South Asia, involving wild primates, semi-wild populations, and animals living near human settlements. That breadth alone makes the phenomenon hard to dismiss as coincidence.
Not mutualism — something more
Cross-species relationships aren’t unusual in nature. Different fish species school together to reduce predation risk. Ostriches and zebras share territory because each compensates for the other’s sensory limitations — zebras smell what ostriches can’t see, and vice versa. Practical arrangements, both. Each party gains something measurable.
The primate behaviors reviewed in this study don’t fit that model at all. A capuchin monkey carrying a marmoset gains no obvious food, no protection, no survival advantage. A langur petting a squirrel isn’t building a coalition. There’s no clear transaction.
That distinction matters. When a behavior persists without a practical payoff, it suggests something more internally driven — rooted in social instinct rather than strategic calculation. The researchers point toward innate curiosity and tolerance as the more likely explanation, traits deeply embedded in primate social life rather than responses to external pressure.
Evolutionary building blocks of human-animal companionship
Study co-author Cyril Grueter was careful not to overstate the parallel. These bonds aren’t pet ownership. The primates involved aren’t choosing companions the way a person picks a dog from a shelter, and the cognitive processes involved are almost certainly different.
But Grueter didn’t dismiss the connection either. These behaviors, he suggested, “may represent some of the evolutionary building blocks from which human-animal companionship eventually emerged.” That’s a significant claim — one implying that the capacity for cross-species bonding didn’t appear suddenly with Homo sapiens, but may have been developing across millions of years of primate evolution.
The implications reach well beyond companionship. The findings offer new angles for understanding the evolution of empathy, social flexibility, and the capacity to extend care outside one’s own group. Humans have long been seen as uniquely capable of forming close bonds with other species. This study challenges that assumption directly.
Practical implications beyond the wild
The research doesn’t stop at evolutionary theory. The authors note that a deeper understanding of positive inter-species relationships among primates could have real applications in animal care settings. Wildlife sanctuaries and zoos housing multiple species together — so-called mixed-species enclosures — could benefit from knowing more about when and why these bonds form naturally.
If primates are predisposed toward curiosity and tolerance across species lines, that changes how caretakers might approach introductions, housing decisions, and behavioral enrichment. It opens a practical door, not just a theoretical one, and points toward further research into the conditions that encourage or suppress these relationships in captivity.
There’s something quietly unsettling about the capuchin and the marmoset — not because it’s alarming, but because it’s familiar. We recognize something in it. A creature extending care to another that isn’t its own, for no reason that survival logic can fully explain.
If this study is right, that impulse didn’t begin with us. It was already there, running through the primate lineage long before humans arrived to name it affection, or companionship, or love. What we do with our pets may be less a human invention than an ancient inheritance — one we share, it turns out, with a lot of our cousins.
You can check the full study here: Grueter, C.C., Roncin, F., Nekaris, K.AI. et al. Heterospecific social behavior in primates: insights into the evolutionary roots of pet-keeping. Primates (2026). https://doi.org/10.1007/s10329-026-01281-0
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