A new study analyzing 427 documented cases of primates interacting with other species has revealed that cross-species social behavior is far more widespread across the primate order than scientists previously realized. The findings, spanning records from the 1970s through 2025 and compiled into one of the largest systematic datasets of its kind for primates, may help illuminate the deep evolutionary roots of the close relationships humans form with other animals.

The study, led by Cyril Grueter, Adjunct Associate Professor at The University of Western Australia and affiliated with the University of Oxford, brought together observations gathered from scientific journals, photographs, videos, and reports shared by primatologists worldwide. The increasing availability of smartphones and digital cameras over recent decades has likely contributed to the volume of documented encounters, making it easier for both researchers and casual observers to capture and share evidence of behaviors that once might have gone unreported. What initially appeared to be isolated, unusual incidents gradually emerged as a surprisingly common pattern of primate behavior.

According to the research, the reviewed cases involved 88 primate species and their interactions with 127 other animal species. The diversity of species involved was extensive, encompassing mammals such as sheep, tapir, deer, domestic pigs, cats, and dogs, as well as owls, lizards, toads, and even insects.

The most commonly documented behaviors were play and grooming. However, researchers also recorded instances of primates carrying, huddling with, sharing food with, and occasionally adopting members of entirely different species. In most cases, the interactions were initiated by the primates themselves.

Well-Known and Rare Observations

One prominent example involves Japanese macaques grooming and riding on the backs of sika deer, a behavior that has become one of the best-known cases of two very different species repeatedly interacting peacefully over extended periods.

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The researchers also documented less familiar but equally striking observations: a langur attentively stroking a squirrel, a silverback gorilla carefully cradling a tiny wild bushbaby, a bonobo gently holding a mongoose pup, and macaques grooming stray dogs.

Many of these interactions may serve functions similar to those found within a single species. Grooming can help remove parasites, improve hygiene, and reinforce social bonds. Play contributes to the development of motor coordination, social skills, and cognitive flexibility.

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Patterns of Interaction

Some cross-species encounters occur naturally where primates and other animals share foraging grounds or benefit from mutual predator protection. For instance, tropical primates sometimes form associations with other primate species to forage jointly in the tree canopy. In such cases, social behaviors like grooming or play may arise simply from repeated contact and growing social familiarity.

However, not all observations fit this utilitarian pattern. The reasons a langur might stroke a squirrel or macaques might groom stray dogs are less immediately obvious, suggesting that primates may possess a broader capacity for social flexibility, curiosity, and tolerance toward other species than researchers had previously appreciated.

The study also identified consistent demographic patterns. Young primates were significantly more likely to engage in play with members of another species, while adult females were more frequently involved in grooming behaviors. This mirrors well-established age and sex differences observed in many primate societies, where juveniles devote considerable time to play and females typically invest more heavily in caregiving.

Not Always Friendly

While it would be tempting to characterize all of these encounters as examples of friendship, the reality is more complicated. Some primates carried infants of another species in ways that resembled adoption or caregiving, yet rough handling or neglect sometimes resulted in injury or death. The researchers also documented cases in which primates appeared to abduct a young animal from its group, and a small number of interactions were clearly abusive, involving violence.

Most interactions were likely of relatively short duration. However, some cases persisted for weeks. One notable example involved the adoption of a marmoset by a capuchin monkey.

In zoos and other human-care settings, some species appear capable of forming long-lasting cross-species relationships. In 2017, a macaque named Niv at an Israeli zoo adopted a chicken and treated it as if it were her own offspring.

The study found that these interactions were not concentrated among great apes. They occurred across all major primate lineages.

Implications for Conservation, Welfare, and Evolutionary Science

The findings carry significance beyond natural history. As forests become increasingly fragmented and wildlife is forced into closer proximity with people, livestock, and domestic animals, opportunities for cross-species interactions are shifting rapidly. Primates now encounter dogs, cats, and other human-associated animals more frequently than in the past.

Some of these interactions may be benign or even beneficial. Others, however, could elevate the risk of diseases spreading between wildlife, domestic animals, and humans — a concern central to the One Health framework, which recognizes that human, animal, and ecosystem health are deeply interconnected. Understanding when and why different species tolerate one another therefore has important implications for both conservation strategies and public health.

The research may also help improve animal welfare practices. Zoos increasingly house compatible species together, and a better understanding of which species are most likely to form positive relationships could help managers make more informed decisions.

Perhaps most notably, the findings suggest that some of the core ingredients underlying cross-species relationships — curiosity, tolerance, caregiving, and play — may have deep evolutionary roots, offering a new perspective on the origins of the bonds humans share with animals.

Cyril Grueter is an Adjunct Associate Professor at The University of Western Australia and the University of Oxford.

This article is republished from The Conversation under a Creative Commons license.

This story was originally featured on Fortune.com.