What primates tell us about the evolutionary origins of keeping pets
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- Cyril Grueter and colleagues at the University of Oxford published a synthesis study in the journal Primates on July 21, compiling 427 cases of primates interacting with heterospecifics drawn from academic literature, YouTube videos, and a survey of 37 primatologists.
- The study documented 311 wild and 111 captive interactions, with play (139 cases) and grooming (136) the most reported behaviors, and Old World monkeys pairing with other Old World monkeys (89) and with dogs (24) the most frequent species combinations.
- The team confirmed 13 deaths of heterospecifics during these interactions—seven birds, three primates, two pangolins, and one domestic cat—with males handling heterospecifics more roughly and juveniles and females more likely to engage socially.
- Most interactions lasted minutes to a couple of days, but Grueter flagged a 2006 report of a marmoset adopted by a wild capuchin troupe for at least 10 months and integrated into the group, which he called a "rudimentary form of pet-keeping behaviour."
- Grueter's interest traces back roughly 20 years to a Shanghai zoo where he watched a gibbon caressing a mouse, followed by fieldwork in China where black-and-white snub-nosed monkey juveniles jumped on domestic pigs trying to play with them.
- The findings frame pet-keeping as evolutionary rather than uniquely modern, since the earliest domestications—dogs perhaps 40,000 years ago—are minuscule on the millions-of-years timescale of human evolution, suggesting our ancestors likely engaged in similar cross-species bonding before formal domestication.
Why it matters: By documenting 427 cases of primates from 88 species bonding with other species—including 311 in the wild—the study reframes pet-keeping not as a modern human invention but as the upper end of an evolutionary continuum. A 2006 case of a marmoset living with capuchin monkeys for at least 10 months blurs the line between brief affiliative behavior and true pet-keeping, challenging the idea that this is a uniquely human trait.
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