Adult male Saccopteryx bilineata sing. Territorial songs and courtship songs are composed of distinct syllable types arranged in structured sequences, and males also perform a hovering display that fans scent from the wing sacs towards females.
Pups of both sexes go through a babbling phase lasting several weeks before weaning. Ahana Fernandez and colleagues recorded pups producing long bouts of syllables that included precursors of adult song syllables, mixed with sounds that do not appear in the adult repertoire. The bouts met criteria used in the human infant babbling literature: they are repetitive, rhythmic, universal across pups, produced by both sexes, and not directed at any social goal.
The comparison should not be pushed too far. Babbling here is a motor practice phase for vocal production, and demonstrating full vocal production learning requires showing that a pup's adult song depends on what it heard. Evidence points that way, since pups exposed to different tutor males develop correspondingly different songs, but the mechanism is still being characterised.