Juggling options: Manipulation ease determines primate optimal fruit‐size choice

Abstract

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.

Activity budget and seasonal activity shifts in sympatric lemurs: Increased lean season effort in a cathemeral frugivore contrasts with energy conservation in a diurnal folivore

“…Ways forward in these areas will require two main directions. The first is more detailed chemical and physical characterization of foods; this will allow a more informed discussion of what actually makes certain species high‐payoff (preferred) foods, low‐payoff (fallback) foods or even non‐foods (Dias da Silva et al, 2020). This context will help researchers understand animals’ behavioral responses.…”

“…In contrast, lean season fruits are lower-quality, in part to due small size, and not worth traveling long distances for; they maintain high frugivory but include more leaves and flowers in the diet. We note that the role of fruit size in primate food selection remains poorly explored relative to chemical characteristics (Dias da Silva et al, 2020;Mopán-Chilito et al, 2022), so this idea requires further testing. Regardless, this scenario is consistent with the positive effect of frugivory on travel time in linear mixed models, and the fact that activity and feeding are negatively correlated with day length and temperature, while travel shows the opposite.…”