One eye on the telescope and the other on the microscope: from macro-evolution to individual behaviour in vocal communication systems

Biological phenomena can be studied at multiple levels: from individuals to the hundreds or thousands of different species that comprise major taxonomic clades; from experiments that last only a few minutes to investigations of organismal evolution on the vast scale of geological time. In this thesis, I have attempted to investigate animal vocal communication at these two extremes of the biological continuum. In Chapter 2 I studied the tempo and mode of vocal evolution in mammals, birds and frogs, and found that the evolution of hearing is a crucial factor in understanding the evolution of tetrapod vocalizations. In Chapter 3 I examined the match between the auditory capacities of birds and their vocalizations, and found that a mismatch between signallers and perceivers does not necessarily imply that communication systems are deficient. I also showed that the large-scale environment in which communication occurs plays some role in the evolution of avian auditory-vocal matching. In Chapter 4 I zoomed in on the different calling sites used by frogs and their impact on the evolution of body size and vocalizations. We found that aquatic frogs call at lower frequencies than non-aquatic species, likely due to constraints in vocal production (i.e., in the inflation of their vocal sacs). In Chapter 5 I focused on two tropical frog species that call from leaves but have terrestrial and arboreal habits. We showed that the leaves used by these frogs amplify their calls in a frequency-dependent manner: the arboreal, lower-frequency species experienced greater amplification than the terrestrial, higher-frequency one. Finally, in Chapter 6 I studied territorial aggression in one species of tropical frog. We found that males respond to simulated rivals depending on the temporal attributes of the challenger, but also based on the spectral properties of their own calls relative to the playback. Males, therefore, refer to themselves during aggressive decision-making in the wild. The most important take-home message from this thesis is that all levels of analysis are valuable and necessary – none is superior or subordinate to the others. Understanding the rich diversity of signals used by animals (extinct and extant) for communication requires a pluralistic approach that maintains simultaneous focus on both large- and small-scale phenomena.