Published in the journal Primates, the review found primates playing with, grooming, carrying, huddling with and caring for animals outside their own species. Researchers say the behaviour suggests humans are not unique in developing close social bonds with members of other species.

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The review included 88 primate species and 127 partner species. Researchers observed stump-tailed macaques grooming dogs in Thailand, langurs stroking giant squirrels in Sri Lanka and capuchin monkeys in Brazil adopting an infant marmoset. The team says understanding these interactions could help improve the management of mixed-species groups in zoos and sanctuaries.