Vocal plasticity during infancy: ontogeny of the trill call in grey mouse lemurs

During language acquisition, human infants undergo a developmental phase of high auditory-feedback-dependent vocal plasticity also reported for marmosets and gibbons. Vocal streams with high vocal plasticity during infancy occur also in grey mouse lemurs. Therefore, this study aims to investigate how these variable calling bouts contribute to the development of the complex advertisement call not present at birth, the trill call, in grey mouse lemurs. We used 290 infant calling series (N = 15 families) recorded during four age classes representing different infant developmental stages and 50 adult trill calls (N = 39 individuals) as reference. Temporal and spectral acoustic parameters were measured for each syllable of a bout. To define syllable types, dimensionality reduction techniques combined with unsupervised cluster analysis were performed. Based on the obtained results, transition networks were established to visualise the sequential order of the syllable types for each age class. Several syllable types were obtained, which differed in their occurrence across age classes. Syllables with almost no frequency modulation occurred mainly around birth, whereas frequency-modulated syllables occurred in the later age classes. With increasing age, the sequential order of the syllable types bore more resemblance to the adult trill call. This suggests that adult trill calls develop during infancy by increasing the frequency modulation of syllables and by combining specific syllable types into a complex sequence. Further studies will address whether this is based on maturation or whether auditory templates of the parents are imitated.