Prehension in Cebus and Saimiri: I. Grip type and hand preference.

Prehension was examined in tufted capuchins (Cebus apella) and squirrel monkeys (Saimiri sciureus). Individual subjects were videotaped from frontal and sagittal planes while they grasped small objects presented in several ways (in view, out of view, embedded, moving). Capuchins used a precision grip in 30% of trials and in more than half of trials with stationary objects. Most (54%) of the precision grips used were opposition of the thumb to the index finger; however, eight other forms were also observed. Squirrel monkeys never used a precision grip. Data on hand preference, preliminary data on movement velocity, and preliminary observations of movement trajectories (up to the time of hand shaping prior to contact with the object) do not indicate significant differences across genera in these aspects of prehension. The presence of varied precision grips in capuchins and no form of precision grip in squirrel monkeys leads to two conclusions. First, a thumb classification of "opposable" (vs. "pseudo-opposable") is not essential for precision gripping. Capuchins, with pseudo-opposable thumbs, use precision grips routinely. Second, the fundamental difference between these genera, which accommodates precision gripping in capuchins, is the capacity in capuchins (but not in squirrel monkeys) to produce lateral pressure between opposing digits. The anatomy of the spinal pyramidal tract and neuromuscular interfacing in the hands may be more sensitive gauges of manual dexterity than the anatomy of the thumb.