A comprehensive review of 303 cases, primatologist surveys, and media accounts published in the journal Primates reveals that cross-species social bonds among monkeys and apes are far more common than previously thought, offering new insight into the deep evolutionary roots of human pet-keeping behaviors.
For generations, humanity has viewed its deep penchant for keeping companion animals as a uniquely our-species trait. From the family dog to the house cat, our impulse to form affectionate, long-term bonds with entirely different creatures felt like a cultural milestone that separated us from the rest of the animal kingdom. But a massive new review paper Popular Science challenges that assumption by documenting hundreds of instances where our closest biological relatives engage in very similar behaviors.
Inside the 427 Documented Cross-Species Interactions
When an international team of researchers set out to investigate how often primates interact with other animals, they expected to uncover a modest collection of interesting anomalies. Instead, the team found 427 cases involving 88 primate species and 127 other animal species by combing through scientific literature, survey responses from 66 primatologists, and documented media accounts.
These unexpected animal friendships span the globe and involve a surprising variety of species. Documented encounters range from a northern white-cheeked gibbon gently stroking a mouse inside its enclosure at the Shanghai Zoo to tufted gray langurs petting grizzled giant squirrels in Sri Lanka. In China, wild snub-nosed monkeys have been observed repeatedly attempting to climb onto the backs of domestic pigs. In the wild woodlands of Brazil, a group of adult capuchin monkeys took in an infant marmoset named Fortunata, raising the tiny outsider as part of their troop for months.
While many inter-species encounters in the wild occur strictly for practical survival—such as mixed-species fish schools or zebras and ostriches grazing together to avoid predators—the behaviors recorded in this review offered no immediate, tangible rewards. Primates napping beside, grooming, or playing with members of other species were engaging in acts driven by curiosity and social flexibility rather than foraging efficiency or predator defense.
Age, Gender, and the Patterns of Primate Play
The comprehensive review set out to answer three core questions regarding these cross-species relationships: whether primates favor closely related species, whether females exhibit higher rates of nurturing behavior, and if juveniles treat other animals as playmates. The data yielded clear patterns across all three areas.
Regarding taxonomy, primates were most likely to interact with other species belonging to the same family, sharing communicative signals and behavioral repertoires that reduced the risk of misunderstandings. When it came to behavioral roles, demographics dictated the nature of the interaction. Juvenile primates were significantly more likely to engage in playful antics with other creatures, mirroring how immature animals behave within their own species before transitioning toward adult social structures.
Adult females, meanwhile, drove most of the grooming behavior across species boundaries, reflecting their primary evolutionary roles as caregivers. Not all cross-species interactions remained gentle, however. The dataset also captured abusive encounters, which researchers noted were predominantly directed at non-primate species like rodents, birds, insects, and swine.
Evolutionary Building Blocks of Human Companionship
Researchers are careful to draw a firm line between these remarkable wild encounters and full-fledged domestic pet ownership. As Cyril Grueter noted, these observations do not demonstrate that nonhuman primates maintain pets in the human sense. Instead, the interactions point to deeper evolutionary foundations that long preceded the arrival of modern humans.
“Rather, they suggest that some of the behavioral capacities that make pet-keeping possible (curiosity, tolerance, caregiving and maintaining long-term relationships with another species) did not suddenly appear in humans but have deeper evolutionary roots.”
Cyril Grueter, School of Anthropology and Museum Ethnography, University of Oxford, via Defector
Understanding these cross-species dynamics carries practical applications beyond evolutionary theory. According to the research team, mapping why certain species readily form positive relationships can directly improve the management of mixed-species animal exhibits within zoos and wildlife sanctuaries.
