Data from: Juggling options: optimal selection of size-weight combinations of Aldina latifolia (Papilionoideae: Fabaceae) pods by Cacajao ouakary (Primates)

1. Optimal foraging theory predicts that animals will seek simultaneously
to minimize food processing time and maximize energetic gain. Overall,
when abundances are equal, this leads to preference for bigger prey items.
However, for species where upper size ranges are such that effective
processing requires protracted processing time, choosing medium-sized
items may be more energetically rewarding, even though larger ones have
more embodied energy. 2. We evaluated whether a specialist seed-predator
primate would forage optimally when choosing between variable sized
thick-husked fruits, and consume the size-class requiring least time to
obtain, handle and ingest, and was easiest to manipulate. 3. We tested
this with the golden-backed uacari (Cacajao ouakary, Pitheciidae), and
single-seeded pods of the macucu tree (Aldina latifolia, Fabaceae). We
used scan-sampling and ad libitum feeding observations; fruit
penetrability measured with a prosthetic uacari canine; measurements of C.
ouakary hands obtained from museum specimens. To test if uacaris selected
for optimal characteristics, we compared 27 metrics from eaten and uneaten
seeds/fruits. 4. Uacaris selected fruits of medium size and weight
disproportionately to their abundance. Large fruit processing times were
over six times greater than for medium-sized fruits, but seeds were only
four times as large, so that, for energetic yield per unit time, choosing
medium-sized pods was optimal. 5. Disproportionate selection by C. ouakary
of fruits of medium-size and mass in relation to their abundance shows a
primate selecting a sub-sample from the available weight-size continuum,
selectivity that presumably 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.