The hand that cradles the rock.

Separate research teams, one working in western Africa and the other in New Guinea, have reported in the past 5 years that right- or left-handedness does not occur in groups of wild chimpanzees, with one exception. When using a stone to crack open nuts, about half the animals hammer exclusively with their right hands, the rest with their left.

Chimpanzees living in Tanzania's Gombe National Park use their hands similarly, hold Linda F. Marchant and William C. McGrew, both of Miami University in Oxford, Ohio. In a variety of daily behaviors, such as grooming others and plucking fruit from trees, no hand preferences emerged in a group of 38 Gombe chimps observed last year for 3 months, Marchant and McGrew contend. However, the animals split about evenly into those who wielded flexible pieces of vegetation only with the right or with the left hand to fish termites out of earthen mounds.

Chimps' neurological makeup may not promote preferential hand use, McGrew theorizes. But individuals may randomly choose to develop one hand as the performer of fine motor skills, such as termite fishing and nut cracking, he argues.

Observations of Gombe chimps' hand use challenge the theory that primates have evolved to favor the right hand for precise manipulation and practiced acts and the left for reaching out and pulling in various objects (SN: 1/7/89, p.10). Several experiments with captive monkeys and apes have supported that theory. But those studies consist of small numbers of animals given tasks unlike any they face in their natural environments, McGrew asserts. Scientific evidence to date for any type of strong right- or left-handedness in nonhuman primates is "very dicey," he maintains.