Monkey friendships with other animals offer clues to the evolutionary origins of pet-keeping

Why do humans keep pets? A new international study suggests that some of the behavioural foundations of human-animal companionship may have much deeper evolutionary roots than previously thought.

Researchers from around the world, led by Dr Cyril Grueter, Associate Professor in Evolutionary Anthropology at the School of Anthropology and Museum Ethnography, have compiled the first comprehensive review of friendly interactions between non-human primates and other species. The study documents 427 cases involving 88 primate species and 127 partner species, 55 of them non-primates , revealing that monkeys and apes frequently play, groom, hold, carry and even adopt individuals from other species.

Published in Primates, the study combines evidence from the scientific literature (303 cases), media reports (58) and a survey of primatologists (66) to provide the most comprehensive overview to date of cross-species social behaviour in primates.

Among the remarkable examples are stump-tailed macaques grooming domestic dogs in Thailand, tufted gray langurs gently stroking giant squirrels in Sri Lanka, young black-and-white snub-nosed monkeys playing with domestic pigs in China, and ring-tailed lemurs grooming bamboo lemurs in Madagascar. In one extraordinary case, first documented in Brazil in 2006 and compiled in this review, wild capuchin monkeys adopted an infant marmoset and cared for it over many months.

The researchers found that play (139 cases) and grooming (136 cases) were by far the most common forms of interaction. Juveniles were much more likely to play with members of other species, while adult females were more likely to groom them. Adult males were significantly less likely to engage in friendly interspecific interactions.

The study also found that primates most often formed social relationships with closely related species, although many interactions also involved more distantly related animals, including dogs, birds, squirrels and reptiles. Most interactions were initiated by the primates themselves.

When animals of different species spend time together, it is usually for practical reasons: easier foraging, or safety in numbers. But these primates went further, playing with, grooming, carrying and curling up alongside other animals for no obvious reward. It hints at a social curiosity, and a readiness to make friends across the species line, that looks strikingly familiar. 

Not every encounter ended happily. Curiosity and caregiving could tip into rough handling, and the team recorded 13 cases in which a partner animal died. In one striking example, a male baboon in Saudi Arabia appeared to have kidnapped a domestic puppy, carrying it around for long periods. 

Beyond shedding light on the origins of human-animal relationships, the authors believe the findings also have implications for conservation and animal welfare. Dr Grueter said:
"Understanding why some species readily form positive relationships with others could help improve the management of mixed-species groups in zoos and sanctuaries. More broadly, these findings provide new insights into the evolution of empathy, social flexibility and the remarkable capacity of primates to engage with animals beyond their own species."

The authors hope their review will encourage primatologists to record these often-overlooked interactions more systematically during long-term field studies. 

The study, ‘Heterospecific social behavior in primates: insights into the evolutionary roots of pet-keeping’, is published in Primates.