Ethological study of manual laterality in the chimpanzees of the Mahale Mountains, Tanzania

Laterality of function in hand use by the chimpanzee (Pan troglodytes) is unresolved. Ethological studies of groups observed in nature find mostly ambilaterality, while experimental studies of individuals in laboratories and zoos show evidence of rightside bias. These disparate data can be compared systematically using a 5-level scheme, ranging from Level 1, comprehensive ambilaterality, to Level 5, exclusive use of left or right hand across tasks. We sought to clarify this discrepancy with an ethological study of wild chimpanzees in the Mahale Mountains of Tanzania, East Africa. We observed 44 subjects of all ages using focal-subject sampling, showing 35 patterns of manual behaviour (excluding tool use) over 31/2 months. For the 15 most frequent patterns, which included subsistence, social, communicatory, and self-maintenance patterns, we found mostly Level-1 results, whether pooled across subjects or patterns, or analysed individually or pattern-by-pattern. These findings closely replicate earlier findings from another population in Tanzania, Gombe. We hypothesise that chimpanzees in confined settings may show right side preferences due to (inadvertent) human influence; these can be tested empirically.