Do Monkeys Keep Pets? Study Finds Widespread Primate Interactions With Other Animal Species, From Mice to Deer

When a mouse wandered into the white-cheeked gibbons’ enclosure in the Shanghai Zoo, the monkeys didn’t try to eat it—or even chase it away. One female primate scooped up the rodent and gently held and caressed it. University of Oxford evolutionary biologist Cyril Grueter watched the interaction, and he was mystified.

I couldn’t stop wondering why.” Working with an international group of researchers, Grueter then uncovered more examples of primates interacting closely with other species. A young male bonobo carefully cradled a mongoose pup. Some monkeys climbed onto the backs of pigs; others mounted deer.

A tufted gray langur stroked a squirrel. A group of stump-tailed macaques worked together to groom a stray dog. In all, the team tallied 427 cases from scientific papers, media reports and a global survey of 37 primatologists.

The interactions, most of which included play or grooming, occurred both in the wild and in captivity. They involved 88 primate species alongside 127 other species. The researchers published the findings in July in the journal Primates.

The compilation reveals that “this is quite a common behavior,” Grueter tells New Scientist’s Michael Marshall. The new work is only a first step, but it’s starting to fill a void on the subject in the mainstream scientific literature, he adds. But it doesn’t stop there: “Macaques groom deer and eat their parasites, play with them and ride them.” They even try to mate with the deer, a behavior that might be spread through social transmission among the primates.

Sueur has published studies on the macaques and deer, but he was not involved in the new research. With the team’s assembled anecdotes, he says, “scattered curiosities become a dataset one can interrogate.” But he emphasizes that the observations were not a designed experiment. Not all interactions documented by the team seemed friendly.