Turns Out, We’re Not the Only Ones
Who Keep Pets
A landmark Oxford-led review of 427 documented cases finds that monkeys and apes groom, cradle, play with, and sometimes adopt animals from entirely different species — and the pattern looks a lot like the evolutionary rough draft of pet-keeping.
For as long as anyone’s been paying attention, “pet-keeping” has been filed under uniquely human behavior — a strange, resource-costly habit of caring for another species for no practical return. A sweeping new review, published in the peer-reviewed journal Primates and led by evolutionary anthropologist Cyril Grueter of the University of Oxford, suggests that filing was premature.1,2 The research team — drawing on collaborators across the UK, mainland Europe, Asia, and the Americas — spent years assembling the largest catalogue ever built of primates forming affectionate bonds with other species. What they found were 427 separate, verified instances of monkeys and apes grooming, playing with, carrying, huddling with, and in a handful of cases outright adopting animals that were not their own kind.3
The review combined three streams of evidence: 303 cases pulled from decades of scientific literature, 66 cases reported directly by primatologists via a global survey, and 58 carefully vetted media records — the kind of “monkey befriends duck” clip you’ve probably scrolled past a dozen times, now finally taken seriously by science.3,4 Altogether, the primates studied formed relationships with 127 different partner species, spanning mammals, birds, reptiles, amphibians, crustaceans, and insects.3
The Anatomy of a Primate Friendship
The most common behaviors were the same two things you’d recognize from any household with a cat curled on the sofa: play and grooming. Of the 427 cases, 139 involved play and 136 involved grooming — together accounting for the clear majority of interactions.4 But who did what depended heavily on age and sex. Juveniles and infants overwhelmingly favored play, while adult females were significantly more likely to groom their interspecies companions. Adult males, notably, were the least affiliative of any group across the board.3,4
“Humans have long been considered unique in forming close social relationships with other species. Our study shows that many primates also display remarkable curiosity, tolerance and even caregiving towards animals that are not their own species.” — Cyril Grueter, University of Oxford, lead author
Most relationships formed between closely related species — two kinds of monkey are more likely to bond than a monkey and, say, a beetle — likely because shared body language and social signals make it easier to read one another’s intentions.2,4 But the review also documented plenty of exceptions that break the pattern entirely, including bonds stretching across taxonomic classes to reptiles, birds, and even insects.3
Meet the Odd Couples
The single most frequently recorded wild pairing in the entire dataset. Macaques groom and ride on the backs of the deer in what researchers call a remarkably stable, long-running interspecies relationship.5,6
A wild infant marmoset, nicknamed “Fortunata” by researchers, was adopted by a group of capuchins and stayed with them for roughly a year — traveling, playing, eating, and responding to the troop’s alarm calls as one of their own.7,8
A female white-cheeked gibbon was observed gently holding and stroking a mouse that had wandered into her enclosure — the encounter that first set lead researcher Cyril Grueter down this entire line of study.2,8
Juvenile wild monkeys were repeatedly observed clambering onto the backs of domestic pigs during fieldwork — playful, persistent, and entirely one-sided.2,9
One relationship looms largest in the public imagination, even though it falls outside the review’s core dataset of wild and zoo-documented cases: Koko, the western lowland gorilla famous for her sign-language vocabulary, and All Ball, the kitten she cradled, groomed, and treated as a surrogate infant.2 Researchers cite it as one of the clearest examples of a primate treating another species almost exactly the way humans treat a pet.
Not Every Story Ends Sweetly
Curiosity and caregiving don’t always land gently. Among the interactions researchers were able to track to an outcome, they confirmed 13 cases in which a partner animal died from rough handling — among the victims were birds, pangolins, a domestic cat, and even other primates. Most of these harder interactions were directed at smaller, more distantly related species rather than close primate relatives.4,9
Researchers are careful not to romanticize the data. Most of these interspecific bonds are brief — a few minutes of grooming, a short play bout — and vanish without a trace. The genuinely sustained, months-long relationships, like Fortunata’s year with the capuchin troop, remain rare standouts rather than the norm.8
Why It Matters
The team’s working theory is that pet-keeping in humans didn’t spring from nowhere — it may be built on older behavioral scaffolding already present in our primate relatives: curiosity toward the unfamiliar, tolerance of other species at close range, and a caregiving instinct that doesn’t always stay confined to one’s own kind.9,10 Notably, many of these bonds offered the animal forming them no obvious survival benefit at all, which is exactly what makes pet ownership such a strange, singular behavior to explain evolutionarily — and exactly why finding its echoes in other primates matters so much.1
The researchers stop short of calling any of this “pet ownership” in the full human sense — there’s no leash, no food bowl, no conscious choice of companion. But as one of the study’s authors put it in his own account of the fieldwork, these interactions may represent some of the evolutionary building blocks from which human-animal companionship eventually emerged.7,9
Sources & Notes
- University of Oxford, “Monkey friendships with dogs, deer and even pigs offer clues to the evolutionary origins of pet-keeping,” Oxford University News (UK), July 2026. ox.ac.uk
- Cyril Grueter, “Monkeys keep pets too — animal specialist on the world of interspecies friendship,” The Conversation (UK/Australia, nonprofit), August 2026. theconversation.com
- Grueter, C.C., Roncin, F., Nekaris, K.A.I. et al., “Heterospecific social behavior in primates: insights into the evolutionary roots of pet-keeping,” Primates, Springer Nature, DOI: 10.1007/s10329-026-01281-0, July 2026 (open access, peer-reviewed). link.springer.com
- PubMed record for the above study, U.S. National Library of Medicine indexing (scientific database, not media). pubmed.ncbi.nlm.nih.gov
- Phys.org, “Monkey friendships with dogs, deer and even pigs offer clues to the evolutionary origins of pet-keeping,” Phys.org (Switzerland-based science network), July 2026. phys.org
- Defector, “Can A Monkey Have A Pet?” — interview-based coverage citing lead author Cyril Grueter directly, 2026.
- ZME Science, “Monkeys Make Friends With Dogs, Deer and Pigs. They Might Even Keep Pets,” ZME Science (Romania), 2026. zmescience.com
- Izar, P. et al. (2006), cross-genus adoption case report, American Journal of Primatology, cited within the 2026 Primates review.
- Primatology.net, “When a Monkey Adopts a Marmoset,” independent primatology research newsletter, 2026. primatology.net
- Ecoportal, “Capuchin monkeys raising infant marmosets, langurs petting squirrels,” Ecoportal (independent environmental outlet), 2026. ecoportal.net
Every species on this list — deer, marmosets, mice, pangolins, pigs, and yes, cats — deserves a relationship built on curiosity and care rather than harm. The Cat House Foundation supports sanctuary and welfare work for animals across species lines, echoing the very instinct this research suggests may be older than humanity itself.