Callicebus and the pitheciins are closely related; however, differences in their diets and dental morphology suggest that they differ in the use of mechanically protected food. We describe physical traits of fruits consumed by white-handed titi monkeys (Callicebus lugens) and determine their influence on fruit part selection and immediate seed fate after fruit handling. We tested two hypotheses about the effects of mechanical fruit traits on fruit part selection and seed fate: (1) fruits selected for seed consumption are harder than fruits selected for their fleshy parts and (2) consumed seeds are softer than seeds with other fates. In addition, we analyzed the influence of other physical fruit traits on fruit part selection and seed fate. C. lugens included 69 species in its diet, from which it mainly consumed their fleshy parts. It also consumed seeds, alone or with fleshy fruit parts, but most of them ended up close to parent trees after being dropped or spat out. The first hypothesis was supported while the second was rejected, indicating that C. lugens tends to rely on hard fruits for obtaining seeds, while seed hardness had no influence on fruit part selection and seed fate, contrasting with the pattern reported for Pithecia and Chiropotes in other studies. Ripeness was the most influential factor for fruit part and seed fate discrimination. Results suggest a tendency to sclerocarpic foraging in C. lugens when feeding on seeds.
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