From simple to complex: vocal development of the trill call in grey mouse lemurs

During language acquisition, human infants undergo a developmental phase of high vocal plasticity. Infant calling bouts of high vocal plasticity occur also in a strepsirrhine primate species. This study aims to investigate how these variable calling bouts contribute to the development of the complex advertisement call (Trill), not present at birth, in the grey mouse lemur. We used 180 infant calling bouts (N=9 families) recorded during four age classes representing different infant developmental stages and 50 adult Trills (N=42) 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. Transition networks were established to visualise the sequential order of the syllable types within a bout 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. The sequential order of syllable types reflected the adult Trills when infants start to leave the nest box on their own. This suggests that adult Trills develop during infancy by increasing frequency modulation of syllables and by combining specific syllable types into a complex sequence. Further studies will address whether this is due to maturation or vocal learning.