Syntax-like Structures in Maternal Contact Calls of Chestnut-Crowned Babblers (Pomatostomus ruficeps)

The combination of meaning-bearing units (e.g., words) into higher-order structures (e.g., compound words and phrases) is integral to human language. Despite this central role of syntax in language, little is known about its evolutionary progression. Comparative data using animal communication systems offer potential insights, but only a handful of species have been identified to combine meaningful calls together into larger signals. We investigated a candidate for syntax-like structure in the highly social chestnut-crowned babbler (Pomatostomus ruficeps). Using a combination of behavioral observations, acoustic analyses, and playback experiments, we test whether the form and function of maternal contact calls is modified by combining the core "piping" elements of such calls with at least one other call element or call. Results from the acoustic analyses (236 analysed calls from 10 individuals) suggested that piping call elements can be flexibly initiated with either "peow" elements from middle-distance contact calls or adult "begging" calls to form "peow-pipe" and "beg-pipe" calls. Behavioral responses to playbacks (20 trials to 7 groups) of natural peow-pipe and beg-pipe calls were comparable to those of artificially generated versions of each call using peow elements and begging calls from other contexts. Furthermore, responses to playbacks (34 trials to 7 groups) of the three forms of maternal contact calls (piping alone, peow-pipe, beg-pipe) differed. Together these data suggest that meaning encoded in piping calls is modified by combining such calls with begging calls or peow elements used in other contexts and so provide rare empirical evidence for syntactic-like structuring in a nonhuman animal.