Interplay of olfaction and vision in fruit foraging of spider monkeys

It is not well understood how primates combine olfactory and visual cues in their natural behaviour, especially during feeding. In this study we conducted field observations of a group of wild, frugivorous black-handed spider monkeys, Ateles geoffroyi (Platyrrhini), consisting of both dichromats (N = 11) and trichromats (N = 9) in Santa Rosa National Park, Costa Rica. We focused on the fruit foraging behaviour, for which involvement of vision has been well studied. We examined how often the monkeys inspected fruits by sniffing them during their fruit feeding attempts (i.e. sniffing index). We found that both dichromats and trichromats sniffed the visually cryptic fruit species more often than the conspicuous species, with the sniffing index being negatively correlated with the luminance and blue-yellow contrasts of fruits to background leaves. Furthermore, the sniffing index was negatively correlated with the proportion of fruits eaten (versus rejected) following a foraging attempt in both dichromats and trichromats. These results suggest that monkeys use olfaction for discrimination between edible and inedible fruits when vision alone is insufficient to evaluate the quality of fruits, showing the first documentation of interplay between vision and olfaction in primate feeding behaviour under natural conditions.