The functions and evolution of graded complex calls in a treefrog

Variation in signal complexity is common among different species. Understanding why some species have evolved extensive call complexity can offer insight into the evolution of complex signals. In this study, we investigated the functions of different call types in a treefrog using call network analysis, male playback experiments, and correlation analysis between call properties and body size. Our results show that the male treefrogs can produce three kinds of notes that can be combined to produce single-note call types as well as composite calls. We also identified an intermediate note type between the advertisement call and suppress call, indicating that male suppress calls may have evolved from advertisement calls. The call network revealed functional correlation and distinction between different calls. Additionally, our study revealed that the dominant frequencies of the advertisement calls and suppress calls can signal the size of calling males. Male frogs can combine different notes in different sequences to form graded complex calls, which may perform different communication functions. Our results indicate that different communication needs may drive the evolution of signal complexity, which provides new evidence to the social complexity hypothesis and deeper insights into the function and complexity of animal acoustic communication.