An evolutionary model of rhythmic accelerando in animal vocal signalling

Animal acoustic communication contains many structural features. Among these, temporal structure, or rhythmicity, is increasingly tested empirically and modelled quantitatively. Accelerando is a rhythmic structure which consists of temporal intervals increasing in rate over a sequence. Why this particular vocal behaviour is widespread in many different animal lineages, and how it evolved, is so far unknown. Here, we use evolutionary game theory and computer simulations to link two rhythmic aspects of animal communication, acceleration and overlap: We test whether rhythmic accelerando could evolve under a pressure for acoustic overlap in time. Our models show that higher acceleration values result in a higher payoff, driven by the higher relative overlap between sequences. The addition of a cost to the payoff matrix models a physiological disadvantage to high acceleration rates and introduces a divergence between an individual’s incentive and the overall payoff of the population. Analysis of the invasion dynamics of acceleration strategies shows a stable, non-invadable range of strategies for moderate acceleration levels. Our computational simulations confirm these results: A simple selective pressure to maximise the expected overlap, while minimising the associated physiological cost, causes an initially isochronous population to evolve towards producing increasingly accelerating sequences until a population-wide equilibrium of rhythmic accelerando is reached. These results are robust to a broad range of parameter values. Overall, our analyses show that if overlap is beneficial, emergent evolutionary dynamics allow a population to gradually start producing accelerating sequences and reach a stable state of moderate acceleration. Finally, our modelling results closely match empirical data recorded from an avian species showing rhythmic accelerando, the African penguin. This shows the productive interplay between theoretical and empirical biology.Animal acoustic communication is often structured in time; i.e., some animal sounds are rhythmic. Among all rhythms, accelerando occurs when the time between sounds shortens as a sequence progresses. In humans, we see accelerando for example in music. In other animals, we find accelerando in multiple species, including African penguins. Why is this particular vocal behaviour present in diverse animal lineages? How did it evolve? We use quantitative tools, namely game theory and computer simulations, to link accelerando to a specific rhythmic feature of groups, namely acoustic overlap. In particular, we test whether rhythmic accelerando could evolve under a pressure for acoustic overlap in time. We show that, when acoustic overlap is evolutionarily advantageous, simulated individuals will produce vocalisations with accelerando. To achieve overlap, this strategy to accelerate can be stable for a range of parameter values. Our computational simulations show that a population of individuals vocalising isochronously, i.e., very regularly like metronomes, can evolve to produce increasingly accelerating sequences until all individuals show accelerando. In brief, species for which vocal overlap is beneficial should evolve towards producing accelerating sequences. This, we speculate, is what may have happened to an ancestor of African penguins, resulting in the accelerando shown in this species today.