A mechanism for punctuating equilibria during mammalian vocal development

Evolution and development are typically characterized as the outcomes of
gradual changes, but sometimes such changes are abrupt: States of
equilibrium can be punctuated by sudden change. Here, we studied the early
vocal development of three different mammals: common marmoset monkeys,
Egyptian fruit bats, and humans. Consistent with the notion of punctuated
equilibria, we found that all three species undergo at least one sudden
transition in the acoustics of their developing vocalizations. This was
expected. To understand the mechanism, we modeled different developmental
landscapes. We found that the transition was best described as a shift in
the balance of two vocalization landscapes. We show that the natural
dynamics of these two landscapes are consistent with the dynamics of
energy expenditure and information transmission. By using them as
constraints for each species, we predicted the differences in transition
timing from immature to mature vocalizations. Using marmoset monkeys, we
were able to manipulate both infant energy expenditure (vocalizing in a
heliox environment) and information transmission (closed-loop contingent
parental vocal playback). These experiments supported our predictions
regarding the role of energy and information in leading to punctuating
equilibrium states of vocal development.