Behavioural context shapes vocal sequences in two anuran species with different repertoire sizes

Abstract

Acoustic signals in animals serve to convey context-dependent information to receivers. Birds and mammals combine diverse sounds into complex sequences to communicate, but the role of temporal sequencing of signals remains understudied in other taxa. Anuran vocalizations are a prominent feature of their life history, and function in defence of territories and to attract mates. However, there are few data on whether anurans pattern their calls into sequences, and whether temporal sequences convey information about context. Here, we investigated the context-dependent vocal repertoire and the use of vocal sequences by two anuran species belonging to different lineages, comparing frogs vocalizing alone and in the presence of a territorial rival. Using a robust analytical framework, we present evidence that both species modify their vocal sequence structure according to context. Specifically, one species (with a smaller repertoire, from a more basal lineage) appends notes to generate more complex sequences, whereas the other (more recently diverged and with a larger repertoire) shifts to different note types, resulting in different sequences for different contexts. Thus, despite differences in repertoire size, both frog species are capable of adjusting the temporal sequence of vocalizations to communicate in different contexts. Vocal sequences and context-dependent ‘syntax’ may be more common in anurans than previously thought, and our methodology presents a paradigm to study the evolution and function of these complex vocal patterns.