Monkeys Keep Adopting Animals of Other Species, New Data Shows Why

Humans have been keeping pets for thousands of years, but the reasons why have never been entirely clear. A new study suggests the answer may be partly rooted in biology, and that our closest animal relatives have, in rare documented cases, done something surprisingly similar. Researchers compiled what they describe as the first systematic collection of records, to their knowledge, on how primates interact socially with other species. The pattern that emerged was hard to ignore: in unusual but documented instances, monkeys, apes, and their relatives play with, groom, and sometimes form extended bonds with members of completely different species. These tendencies, the authors propose, could point to behavioral roots that predate humanity’s long history of keeping pets, though they note that most such primate interactions are brief and fall well short of true companionship.

For the study, published in the journal Primates, a large international team of researchers gathered 427 documented cases of primates engaging in social behavior with other species. These interactions spanned 88 different primate species and 127 partner species, many of them other primates, along with non-primate animals ranging from dogs and deer to birds, reptiles, and even insects. Cases came from the wild, from zoos, and from a global survey of primate researchers. Together, the records show primates to be unexpectedly sociable animals, ones that sometimes reach across species lines in ways that look familiar to anyone who has ever watched a child befriend a stray cat.

Playing and grooming were the two most common behaviors recorded, with 139 play interactions and 136 grooming interactions in the dataset. Primates started the interaction in roughly two-thirds of cases where the direction was known. In some instances, relationships went well beyond a brief encounter, with a small number of documented cases in which one species carried or cared for an infant from another.

Monkeys, Deer, and the Roots of Pet-Keeping

Most of the interactions in the dataset happened between primates and other mammals, which tracks with what scientists might expect. More closely related animals tend to share similar body language and social signals, making it easier to communicate without things going wrong. White-faced capuchin monkeys interacting with spider monkeys represented the most frequently observed pairing in the wild, with 19 recorded cases. Japanese macaques interacting with Sika deer came in second with 12 cases, followed by chimpanzees with red-tailed monkeys at 11 cases, and rhesus macaques with domestic dogs, also at 11 cases.

That last pairing, macaques cozying up to dogs, might be the most eyebrow-raising on the list. Dogs are domesticated animals that primates would typically encounter near human settlements rather than in more remote habitats. Yet the dataset recorded 26 interactions between primates and dogs, suggesting that when wild primates come into contact with human-associated animals, they don’t always flee.

Interactions weren’t always friendly, however. Researchers also documented 26 cases of abusive behavior, most of it directed at non-primate species. Among those cases, 13 resulted in confirmed deaths, including seven birds, two pangolins, one domestic cat, and three primates. In some troubling examples, what began as apparent caregiving, a primate carrying an infant of another species, turned fatal through rough handling.

Young Primates Play, Adult Females Nurture

One of the clearest patterns in the data involved age and sex. Young primates were far more likely to play with other species, while adult females were far more likely to groom across species lines. Adult males were significantly less likely than adult females to engage in affiliative interactions with other species.

This matches what scientists already know about primate social life within a single species: youngsters play, and adult females do most of the caregiving. What stands out here is that those same tendencies appear to extend beyond the boundaries of a single species.

Adult females showing more cross-species nurturing also lines up with patterns seen in human cultures. Records from hunter-gatherer societies describe women caring for, and in some cases nursing, animals from a range of species, an echo of what this study found among non-human primates.

Contrary to what researchers initially predicted, primates did not prefer interacting with young or infant individuals from other species over adults. When cases where the recipient’s age was known were examined, 44 interactions were directed at adults and 37 at younger animals, a difference that wasn’t statistically meaningful. When the analysis was limited to cases where adult female primates were doing the interacting and the recipient’s age was known, they were more likely to direct their attention toward adults of other species: 25 interactions with adults compared to 9 with immatures.

What Monkey Friendships Reveal About Pet-Keeping

Some of the most memorable examples in the study come from individual animals that formed what look like true cross-species bonds. Perhaps the most famous is Koko, the western gorilla who lived in human care and became closely attached to a kitten named All Ball. The study describes Koko displaying maternal behaviors such as cradling, grooming, and protective posturing toward the kitten.

Another documented case involved a wild infant common marmoset adopted by a group of wild black-striped capuchin monkeys in Brazil. The marmoset stayed with the capuchin group for at least ten months, was cared for by two successive adoptive mothers, and was broadly tolerated by other group members.

In a more recent and somewhat disturbing example, a male Hamadryas baboon in Saudi Arabia was documented apparently kidnapping a domestic puppy and carrying it around for extended periods. Separately, a crested macaque at an Israeli zoo reportedly adopted a chicken that had wandered into its enclosure.

These cases, varied as they are, share one thing: primates forming sustained relationships with animals outside their own species, sometimes without an obvious immediate payoff.

Researchers are careful not to overstate the comparison. “Most heterospecific social interactions involving primates appear to be rather ephemeral in nature, in contrast to pet-keeping in humans, which typically entails long-term companionship,” the authors write.

Still, the study argues that several key building blocks are present in primate behavior: caregiving across species lines, exploratory play with unfamiliar animals, tolerance of other species, and selective friendly responses toward particular individuals of a different kind. These tendencies, the researchers suggest, “may be more widespread among primates than previously recognized.”

Humanity’s habit of bringing animals into the home and treating them as family is often framed as a cultural invention, something that emerged with civilization, or at least with domestication. But if capuchin monkeys groom spider monkeys, if macaques play with deer, and if gorillas cradle kittens, the roots of interspecies bonding may reach far wider across the primate family than previously recognized. The study stops well short of saying pet-keeping is hardwired, but it raises the possibility that some of its underlying tendencies may have been present long before humans ever brought an animal indoors.

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