Monkeys keep pets too: cross-species bonding among primates and other animals revealed by a large study

The Conversation reports on a study that compiled 427 documented primate interactions with 127 other species, spanning several decades from the 1970s to 2025. The findings show that cross-species encounters are more common than previously thought, with primates engaging in play, grooming, sharing food, carrying infants, and in some cases adopting animals from other species. Notable examples include langurs stroking a squirrel, Japanese macaques grooming sika deer, and macaques adopting a chicken in an Israeli zoo. The work discusses how these interspecies relationships can illuminate social flexibility and tolerance, while also raising questions about welfare, disease risk, and conservation. Author: The Conversation.

• Across 88 primate species and 127 other species studied
• Common behaviors are play and grooming, with caregiving and adoption observed
• Young primates tend to play with other species; adult females more often groom
• Implications span conservation, animal welfare and public health

Introduction

The article synthesizes a new study that aggregates 427 documented interspecies interactions involving primates and 127 other animal species, drawing on decades of observations from the 1970s to 2025. The scope is broad, encompassing a wide range of primate lineages and a diverse set of non-primate species including mammals, birds, reptiles, and even insects. The central finding is that cross-species social interactions among primates are far more widespread than previously appreciated, extending beyond rare anecdotes to a pattern visible across many taxa and contexts.

Study scope and methods

The researchers compiled reports from scientific journals, photographs, videos and field notes from primatologists around the world. The dataset covers 88 primate species and 127 other animal species, representing a remarkable variety of ecological settings from wild forests to zoological facilities. The interactions fall mainly into play and grooming, but also include carrying, sharing food, huddling, adoption-like caregiving, and occasional negative outcomes. This breadth highlights a previously underappreciated facet of primate behavior: flexibility in social relationships that extend beyond their own species.

Notable examples and patterns

Among the most striking cases cited are: a langur in Yala National Park stroking a Grizzled giant squirrel; a northern white-cheeked gibbon in Shanghai Zoo caressing a mouse that wandered into its enclosure; Japanese macaques grooming and riding on the backs of sika deer; and capuchin monkeys adopting a marmoset. These examples are not isolated curiosities but part of a broader, more persistent pattern of cross-species interactions. The study also notes long-term bonds in captivity, such as macaques forming stable relationships with dogs or other domestic animals, suggesting that these interactions can endure when social familiarity is established.

Mechanisms and potential benefits

The most common behaviors—play and grooming—likely confer benefits similar to within-species sociality, including parasite removal, hygiene, and strengthened social bonds. Play supports motor coordination, cognitive flexibility, and social learning, while grooming can reduce parasites and improve hygiene. Shared foraging opportunities or predator protection can also arise when different species associate, and repeated contact may foster social familiarity that gives rise to emergent cross-species tolerance and cooperation. Importantly, the authors propose that these interactions reveal deeper evolutionary roots of sociality that extend beyond kin and close associates.

Developmental and demographic patterns

Across the observed cases, young primates were more likely to engage in play with individuals of other species, while adult females were more frequently involved in grooming. This aligns with established patterns in many primate societies, where juveniles explore social spaces to acquire skills, and females invest in caregiving and social bonding. These findings suggest that cross-species sociality is structured by age and sex in ways that resemble intra-species social dynamics, pointing to a broad social flexibility rather than a collection of isolated anecdotes.

Complexity and cautions

While many encounters appear benign or beneficial, not all interactions are positive. Some involve infant carrying that resembles caregiving, but others include neglect or injury, and a few cases hint at infant abduction. The study emphasizes that cross-species interactions can be mixed in outcome and duration, with some relationships lasting weeks or longer and others being ephemeral. The authors caution against over-idealizing these encounters as friendship, noting the potential for harm in certain contexts.

Zoos, welfare and public health implications

In human-care settings, cross-species bonds are observed and sometimes encouraged, raising questions about welfare and management. The study notes examples such as a macaque adopting a chicken in an Israeli zoo, illustrating how human care environments can foster long-term cross-species relationships. From a welfare perspective, managers may consider which species interact compatibly and how to design housing to support positive interspecies relationships, while remaining mindful of disease transmission risks in fragmented habitats where wildlife encounter livestock and domestic animals more frequently.

Wider significance and conclusions

The authors argue that these unusual encounters carry implications for both conservation and public health as landscapes become increasingly fragmented and human-wildlife contact increases. Understanding when and why different species tolerate one another could inform strategies to reduce disease spillover and improve welfare in captive settings, while also offering insight into the deep evolutionary roots of social tolerance and caregiving across species. The overarching message is that cross-species sociality is a meaningful dimension of primate behavior with relevance to how we think about animal welfare, ecosystem management and the broader human-animal connection.

Source: The Conversation.