Human language is a recombinant system that achieves its productivity through the combination of a limited set of sounds. Research investigating the evolutionary origin of this generative capacity has generally focused on the capacity of non-human animals to combine different types of discrete sounds to encode new meaning, with less emphasis on meaning-differentiating mechanisms achieved through potentially simpler temporal modifications within a sequence of repeated sounds. Here we show that pied babblers (Turdoides bicolor) generate two functionally distinct vocalisations composed of the same sound type, which can only be distinguished by the number of repeated elements. Specifically, babblers produce extended 'purrs' composed of, on average, around 17 element repetitions when drawing young offspring to a food source and truncated 'clucks' composed of a fixed number of 2-3 elements when collectively mediating imminent changes in foraging site. We propose that meaning-differentiating temporal structuring might be a much more widespread combinatorial mechanism than currently recognised and is likely of particular value for species with limited vocal repertoires in order to increase their communicative output.
Keywords: Animal communication; Combinatoriality; Element repetition; Language evolution; Temporal structure; Turdoides bicolor.