The study covered 88 primate species and 127 partner species — not only mammals but birds, reptiles, amphibians, malacostracans and even insects. Observations were gathered both in the wild and in captivity.

The paper "Heterospecific social behavior in primates" was published in the journal Primates; the findings were announced by the University of Oxford.

Three sources instead of one

The authors deliberately did not rely on scientific literature alone. They took 303 cases from published work, found 58 more in media reports and obtained 66 by surveying primatologists directly. Together that makes 427 episodes.

This approach closes a familiar gap: behaviour a researcher observed once in the field usually does not become a separate publication and is lost. Surveying colleagues retrieves precisely those observations.

Macaques on deer and a gibbon with a mouse

Two forms of interaction were most common: play, with 139 cases, and grooming, with 136. Stump-tailed macaques in Thailand groomed domestic dogs, ring-tailed lemurs in Madagascar groomed bamboo lemurs, and grey langurs in Sri Lanka gently stroked giant squirrels.

Young snub-nosed monkeys in China played with domestic pigs. Japanese macaques on Yakushima Island rode on the backs of sika deer. At Shanghai Zoo a white-cheeked gibbon kept a mouse with it — the case that most closely resembles pet-keeping as we know it.

The capuchins that raised another species' infant

The longest documented episode occurred in Brazil in 2006: a group of capuchins took in an infant marmoset — a monkey of a different genus — and cared for it for several months.

The researchers do not romanticise the picture: in 13 cases the interaction ended in the death of the partner animal. In primates, curiosity and care coexist with aggression, and the line between them is thin.

"Evolutionary building blocks"

"Humans have long been considered unique in forming close social relationships with other species," notes lead author Cyril Grueter, Associate Professor at the School of Anthropology and Museum Ethnography, University of Oxford.

According to him, many primates also display remarkable curiosity, tolerance and even caregiving towards animals that are not their own species. Such behavioural patterns, the researcher believes, may be the evolutionary building blocks from which human pet-keeping later grew.

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