An international review documents 427 cases of primates forming cross-species friendships.
Why do humans keep pets? A new study led by researchers at the University of Oxford and collaborators worldwide suggests the behavioral foundations of human-animal companionship may not be unique to humans. The team compiled the first comprehensive review of friendly interactions between nonhuman primates and other species, documenting 427 cases involving 88 primate species and 127 partner species, 55 of them non-primates.
Published in the journal Primates, the study draws together evidence spanning several decades, from the 1970s through 2025, combining 303 cases from scientific literature, 58 cases from screened photographs and videos, and 66 cases collected through a global survey of primatologists. Together, these sources produced 204 distinct species pairings, making this the most comprehensive overview to date of cross-species social behavior among monkeys and apes.
The diversity documented among primates forming cross-species friendships proved remarkable. Partner species included sheep, tapirs, deer, domestic pigs, cats, and dogs, as well as owls, lizards, toads, and even insects. Play and grooming emerged as the most common behaviors, recorded in 139 and 136 cases, respectively, but researchers also found primates carrying, huddling, sharing food with, and occasionally adopting individuals from entirely different species.
One of the most striking and now well-documented examples of primates forming cross-species friendships involves Japanese macaques in Yakushima, Japan, who groom and ride on the backs of sika deer, repeatedly interacting peacefully over long periods. In China’s Yunnan Golden Monkey National Park, researchers observed a young black-and-white snub-nosed monkey playing on the back of a domestic pig. Elsewhere, a langur was seen attentively stroking a squirrel, a silverback gorilla carefully cradled a tiny wild bushbaby, and a bonobo gently held a mongoose pup.
Domestic dogs appeared repeatedly throughout the review, with researchers recording 26 wild or free-ranging encounters involving dogs. Rhesus macaques and dogs formed one of the most frequently reported species pairings in the entire dataset, suggesting these interactions represent more than isolated curiosities.
Most interactions among primates forming cross-species friendships occurred between closely related species with similar bodies, gestures, and social habits, animals that may find it easier to interpret one another without conflict. Yet evolutionary distance proved far from an insurmountable barrier. Age and sex patterns emerged clearly across the dataset: juvenile and infant monkeys were significantly more likely to initiate play than grooming, while adult females showed the opposite pattern, favoring grooming behaviors instead.
Researchers suggest play may offer young primates a flexible, low-stakes way to test an unfamiliar partner’s temperament. A chase, a leap, or a playful climb can quickly reveal whether another animal responds with tolerance, enthusiasm, or aggression, information valuable before investing in a longer social bond.
Notably, unlike many mixed-species groupings found in nature, these friendships did not appear to provide obvious survival advantages such as improved access to food or protection from predators. Instead, the animals frequently played together, groomed one another, carried each other, or simply rested side by side, behaviors suggesting that primates forming cross-species friendships reflect genuine curiosity and social flexibility extending well beyond mere practical necessity.
Primates forming cross-species friendships suggest curiosity and social flexibility rather than a survival advantage, with juveniles more likely to initiate play while adult females favored grooming behaviors instead. Photos by (a) Nicholas Chapoy, (b) Sabit Hasan, (c) Senthi Aathavan Senthilverl, and (d) Neil Challis, courtesy of Primates.
Not every interaction ended positively. Curiosity and caregiving occasionally tipped into rough handling, and researchers recorded 13 cases in which a partner animal died, with victims including birds, pangolins, a domestic cat, and other primates. In one striking example, a male baboon in Saudi Arabia appeared to have kidnapped a domestic puppy, carrying it around for extended periods.
Beyond illuminating the possible origins of human-animal companionship, researchers believe these findings carry meaningful implications for conservation and animal welfare. Understanding why certain species readily form positive relationships with others could help improve management of mixed-species environments, from wildlife sanctuaries to zoos housing multiple species in shared enclosures.
The sheer range of documented behavior suggests that primates forming cross-species friendships is far more widespread than scientists had previously realized, indicating that social lives among monkeys and apes extend well beyond their own species boundaries. While these animals are not keeping pets in the deliberate, sustained way humans do today, their behaviors may represent early evolutionary building blocks eventually giving rise to the caregiving instincts and cross-species companionship that define humanity’s relationship with domesticated animals.
