The Ontogeny of "Twitter" Calls in White-Faced Capuchins (Cebus imitator): Usage, Context, and Acoustic Structure.

In stark contrast to our own highly plastic communicative abilities, nonhuman primate vocalizations were historically considered fixed and innate, with very little ability to learn or modify vocal signals. However, recent studies indicate that primate vocalizations do show evidence of developmental plasticity, most notably in their context and usage. We build on these studies by investigating developmental changes in one of the most common calls of white-faced capuchins (Cebus imitator), the twitter. Specifically, we examined the behavioral context around twitter vocalizations in a wild population of white-faced capuchins in the Taboga Forest Reserve, Costa Rica. We analyzed the use of twitters in relation to behavioral state (social, foraging, travel, or resting), immediate context (approaching and leaving others), and specific social behaviors (grooming and aggression). Immatures (infants and juveniles) twitter primarily in a social state, while adults twitter primarily in a foraging state. The twitters produced by immatures were more closely associated with approaching other group members within 1 m, compared to adults. This contextual shift with development from social to foraging prompted us to also look for corresponding acoustic changes. However, the acoustic properties of twitters were not related to context or age, with the one exception that adult twitters were longer than those from immatures. Overall, our results suggest that the twitter is a call with multiple functions, with a shift in usage from social to foraging contexts across development. This adds to the growing evidence of flexibility and learning in primate vocal communication.