Monkeys' Cross‑Species Friendships Offer Clues to Pet‑Keeping Evolution

Researchers have documented monkeys forming social bonds with a range of other animals, including dogs, deer, pigs, squirrels and other primates. A survey of scientific literature, media reports and primate‑researcher questionnaires identified 427 instances involving 88 monkey species and 127 partner species, with 55 partners being non‑primate.

The observed interactions—most commonly play (139 cases) and grooming (136 cases)—often occur without obvious survival benefits, prompting scientists to consider them as possible precursors to the human tendency to keep pets. Notable examples include a bonnet macaque grooming a pet dog in Thailand, a langur gently stroking a large owl in Sri Lanka, a young macaque playing with a domesticated pig in China, and a Brazilian howler monkey group caring for a marmoset infant for over ten months. Researchers are now analyzing whether these interspecies social behaviors correlate with cognitive traits required for long‑term animal domestication.