Examining combinatoriality within the pūkeko vocal repertoire

Most animals use various vocalizations to communicate with others and coordinate activities. However, animals are limited in the number of sounds they can produce. In humans, language allows for the unrestricted communication of information by generating new meaning from the finite set of sounds. For nonhuman animals, some level of combinatoriality has been observed such that segments of sound can be combined at either one of two levels, within calls or between calls. It is rare to discover evidence that animals combine calls on more than one level. This requires a comprehensive analysis of the combinatorial features that characterize a species’ vocal system. Here we studied combinatoriality in the acoustic signals produced by pūkeko, Porphyrio melanotus melanotus. We identified 13 sound elements and verified their distinctiveness using uniform manifold approximation and projection. We next assessed the combinatorial abilities of pūkeko using a two-tiered combinatorial system. We first analysed how different sound elements are combined to form calls and found clear structural patterns, where specific sound elements typically serve either as prefixes or suffixes, whereas others serve as connecting (middle) elements. Second, we examined how calls themselves were combined to generate longer call sequences. At the level of call sequences, we specifically focused on yelling-type call sequences, mostly produced in aggressive contexts and found dynamic structural patterns, with calls increasing in duration with the progression of a call sequence. As these sequences unfold, calls undergo systematic transformation through the addition of new terminal elements and modification of existing ones. This hierarchical combinatorial capacity showed how a relatively limited set of acoustic elements can generate an extensive repertoire of calls. Our findings provide empirical evidence for combinatorial processes operating at multiple levels within a single species' communication system and the first evidence of such in a nonvocal learner.