Across forests, islands, and villages, monkeys have been observed reaching beyond the boundaries of their own kind — grooming dogs, adopting infants of foreign species, riding deer, and playing with animals they might otherwise eat. A 2026 review of 427 such encounters, spanning 127 species, offers the first systematic portrait of primates as creatures whose social instincts do not stop at the edge of their own genus. What these moments mean — whether they reflect friendship, curiosity, instinct, or something else entirely — is a question that resists easy answers, and perhaps that is precisel