Pulp Fiction: why some populations of ripe-fruit specialists Ateles chamek and A. marginatus prefer insect-infested foods

Fruit pulp is an easily handled energy source for many frugivorous species but generally has little protein. Accordingly, ripe-fruit specialist primate species with diets dominated by fruit pulp risk protein deficiency. While some species use leaf and flower buds, young leaves, and arthropods as an alternative protein supplement, highly frugivorous spider monkeys (Ateles spp.) use protein-rich young leaves and/or fig fruits. However, not all spider monkey populations have access to abundantly available figs. Comparing infestation frequencies of fruits on trees with those eaten by spider monkeys, we tested the hypothesis that, under such circumstances, spider monkeys preferentially choose those nonfig fruits with pulp infested by insect larvae (a highly protein-rich resource). We predicted that: (i) a large proportion of plant species eaten by Ateles would have insect larvae-infested fruits; and (ii) Ateles would actively select infested fruits. We tested these predictions with Ateles chamek and Ateles marginatus on the banks of the Tapajós River, Brazil. Across a 13-month sampling period, we recorded 27 plant species in the diet of the 2 Ateles species. Of these, 23 (85%) had larvae-infested fruits when sampled; 11 species (40%) had high levels of individual fruits infested (35-78%). We used Ivlev Values to quantify selectivity for infested/uninfested fruits in 20 plant species. Infested fruits were positively selected in 12 species (60%), while aversion to infested fruits occurred in 4 species (20%). This covert carnivory/faunivory in spider monkeys is a largely overlooked aspect of their feeding ecology. This situation would be nearly impossible to ascertain from behavioral observations alone, showing the value of integrated, multimethod approaches. The strategy used by Ateles spp. on the banks of the Tapajós highlights the flexibility of primate foraging choices and the importance of indirect source of protein to ripe-fruit specialist primates.