Cyril Grueter's Review Reveals Hundreds of Cross‑Species Bonds Among Primates

A new review paper led by evolutionary anthropologist Cyril Grueter of the University of Oxford compiled 427 documented cases of primates forming affiliative relationships with other animal species. The analysis, published in the journal Primates, covers 88 primate species and 127 partner species from sites across Brazil, Thailand, Sri Lanka, China, Madagascar and beyond. Notable examples include black‑striped capuchin monkeys in Brazil adopting a common marmoset infant, stump‑tailed macaques grooming domestic dogs at a Thai temple, tufted gray langurs stroking giant squirrels in Sri Lanka, snub‑nosed monkeys playing with pigs in China, and ring‑tailed lemurs grooming bamboo lemurs in Madagascar. The interactions are not driven by survival needs such as food or protection; they appear to stem from primate social wiring that extends beyond species boundaries. Grueter notes that similar cross‑species caring may underlie the human tendency to keep pets, citing observations like a northern white‑cheeked gibbon gently stroking a mouse at the Shanghai Zoo.