Variation of Acoustic Courtship Signals in Insects and Amphibians: No Evidence for Bimodality, but Identical Dependence on Duration

In an influential review, acoustic courtship traits of frogs were classified into the two categories, static and dynamic traits, based on their coefficients of variation and female preference functions. As stabilizing selection can be expected to diminish phenotypic variation, static traits showing comparatively low variation were assumed to be mainly influenced by stabilizing selection through female choice. The more variable dynamic traits on the other hand were proposed to be under directional or less severe selection. Such bimodality in variation would be very valuable, as the pattern of selection could be predicted from signal variability. To examine whether a bimodal pattern of signal variation is generally found in acoustic advertisement signals, I reviewed the literature for available and reliable data for insects and anurans. I found no evidence of bimodality, but detected that coefficients of variation increased with signal duration in accordance with a power law. This relationship between duration and variability seems to be virtually identical in both insects and amphibians. Moreover, such a dependence of the extent of variation on the examined time window seems to be a general pattern, since temporal variation in action potential density, stock market prices and time perception also show a similar pattern.