Allometric escape from acoustic constraints in frog calls is rare

Allometric constraint is a product of natural selection, particularly with respect to body size and traits constrained by physical properties thereof, such as metabolism, longevity, and vocal frequency. Parameters describing allometric relationships are conserved across most lineages, indicating that physical constraints dictate scaling patterns in deep time, despite substantial genetic and ecological divergence among organisms. Acoustic allometry (sound frequency ~ body size) is conserved across frogs, in defiance of massive variation in both body size and frequency. Here, we find four major instances of allometric escape, potentially deriving from ecomorphological adaptations to new signal modalities and invasion of biogeographic regions. In these instances of allometric escape, the optima and strength of the scaling relationship are different than expected for most other frog species, representing new adaptive regimes of body size ~ call frequency. Allometric constraints in frogs are highly conserved and have rarely promoted allometric escape despite frequent invasions of new adaptive regimes and dramatic ecomorphological divergence. Our results highlight instances in which natural and sexual selection combined could overcome physical constraints on sound production.