Prehension was examined in tufted capuchins (Cebus upellu) and squirrel monkeys (Sairniri 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.

Consistent individual differences in long-term dominance are a basic underlying assumption of hypotheses linking dominance and reproductive success. Long-term and temporary dominance of a colony group of sturnptailed macaques was studied for 20 years. There were two variously constituted groups for the first 4 years and a single group for the last 16.Stumptails displayed the matrilineal dominance organization found for several other cercopithecine species. A method was devised to standardize ranks so they could be compared over the years across groups of varying size and composition. No animal maintained the same dominance rank over the entire period of the research or over the last 16 years, but there was considerable consistency over long periods. Although occupants of the male and female alpha positions changed several times, one female was dominant for 18 of the 20 years. She was dominant in 1968, a t the start of the study, and at its end in 1988 a t which time her 18-year-old son was the dominant male. Variation in dominance ranks was greatest among members of mid-ranking matrilines and least for the lowest ranking. The same female or her son were the lowest ranking animals of their groups in all samples taken over the entire 20 years.