Abstract
Mother-infant vocal interactions play a crucial role in the development of human language. However, comparatively little is known about the maternal role during vocal development in nonhuman primates. Here, we report the first evidence of mother-daughter vocal interactions contributing to vocal development in gibbons, a singing and monogamous ape species. Gibbons are well known for their species-specific duets sung between mates, yet little is known about the role of intergenerational duets in gibbon song development. We observed singing interactions between free-ranging mothers and their sub-adult daughters prior to emigration. Daughters sang simultaneously with their mothers at different rates. First, we observed significant acoustic variation between daughters. Co-singing rates between mother and daughter were negatively correlated with the temporal precision of the song’s synchronization. In addition, songs of daughters who co-sang less with their mothers were acoustically more similar to the maternal song than any other adult female’s song. All variables have been reported to be influenced by social relationships of pairs. Therefore those correlations would be mediated by mother-daughter social relationship, which would be modifiable in daughter’s development. Here we hypothesized that daughters who co-sing less often, well-synchronize, and converge acoustically with the maternal acoustic pattern would be at a more advanced stage of social independence in sub-adult females prior to emigration. Second, we observed acoustic matching between mothers and daughters when co-singing, suggesting short-term vocal flexibility. Third, we found that mothers adjusted songs to a more stereotyped pattern when co-singing than when singing alone. This vocal adjustment was stronger for mothers with daughters who co-sang less. These results indicate the presence of socially mediated vocal flexibility in gibbon sub-adults and adults, and that mother-daughter co-singing interactions may enhance vocal development. More comparative work, notably longitudinal and experimental, is now needed to clarify maternal roles during song development.
Highlights
Recent studies have demonstrated greater complexity and flexibility in the vocal signaling systems of nonhuman primates than was previously assumed [1,2,3,4]
Adult female gibbons are known to adjust their singing pattern to organize duetting with a male [41], we examined vocal flexibility in mothers
Temporal Controllability: Call Onset Time Lags Co-singing rates (CR) were significantly positively correlated with mother-daughter call onset time lags (TL) when the mother initiated the co-singing (GLMM, x21 = 9.9903, p = 0.001, Figure 3b, Table S1), showing that daughters who decreased their co-singing rate better matched the temporal onset of their great call to that of their mothers
Summary
Recent studies have demonstrated greater complexity and flexibility in the vocal signaling systems of nonhuman primates than was previously assumed [1,2,3,4]. The majority of early empirical data on vocal development emphasizes a disparity between humans and other primates. These previous studies revealed that immature monkeys and apes, unlike human infants, possess a fixed vocal repertoire with adult-like acoustic structures [14,15,16]. Experiments using deafening and social isolation have not provided convincing evidence for vocal learning in the production domain [16] some authors have noted that immature nonhuman primates are sensitive to adult models when learning how to use calls in an appropriate context [reviewed in 15]. As older female monkeys are often the central node of the vocal network, it is possible that they may serve as potential models [20]
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