Optimal foraging theory predicts that animals will seek simultaneously to minimize food processing time and maximize energetic gain. To test this hypothesis, we evaluated whether a specialist seed-predator primate forages optimally when choosing among variable-sized thick-husked fruits. Our objects of study were the goldenbacked uacari (Cacajao ouakary, Pitheciidae) and single-seeded pods of the macucu tree (Aldina latifolia, Fabaceae). We predict that golden-backed uacari will consume fruits of the size class that requires the least time to obtain, handle, and ingest. We used scan sampling, ad libitum to record feeding observations, and measured fruits, their penetrability, and the size of taxidermized C. ouakary hands. To test whether uacaris selected for optimal characteristics, we compared 8 metrics from 75 eaten and 105 uneaten seeds/fruits collected. Uacaris selected fruits of medium size and weight disproportionately to their abundance. Processing large fruits took six times longer than did medium-sized fruits, but seeds were only four times as large, that is, for energetic yield per unit time, thus choosing medium-sized pods was optimal. Disproportionate selection by C. ouakary of fruits of medium size and mass in relation to their abundance suggests active sub-sampling of the available weight-size continuum. This selectivity probably maximizes trade-offs between the energy derived from a seed, and time and energy expended in processing fruit to access this, so following optimal foraging theory predictions. The greater time spent processing large pods is attributed to difficulties manipulating objects five to seven times the size of the animal's palm and one-sixth its own body weight.