Juggling options: manipulation ease determines primate optimal fruit size choice

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 golden-backed-uacari (Cacajao ouakary,
Pitheciidae) and single seeded pods of the macucu tree (Aldina latifolia,
Fabaceae). We predicted 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 taxidermised C.
ouakary hands. To test if 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.