Chimpanzee culture extends beyond matrilineal family units

The ‘grooming handclasp’ is one of the most well-established cultural traditions in chimpanzees. A recent study by Wrangham et al.[1] reduced the cultural scope of grooming-handclasp behavior by showing that grooming-handclasp style convergence is “explained by matrilineal relationship rather than conformity” [1]. Given that we previously reported cultural differences in grooming-handclasp style preferences in captive chimpanzees [2], we tested the alternative view posed by Wrangham et al.[1] in the chimpanzee populations that our original results were based on. Using the same outcome variable as Wrangham et al.[1] — the proportion of high-arm grooming featuring palm-to-palm clasping — we found that matrilineal relationships explained neither within-group homogeneity nor between-group heterogeneity, thereby corroborating our original conclusion that grooming-handclasp behavior can represent a group-level cultural tradition in chimpanzees. The style of high-arm grooming, or ‘grooming handclasp’, in the Kanyawara chimpanzees has recently been shown to be consistent within matrilineal family units, but not beyond. Van Leeuwen et al. show that in other populations, style homogeneity extends beyond families, thus revealing chimpanzee handclasping to be a group-level cultural phenomenon.