Tradeoffs in the evolution of frog calls
All biological characteristics are subject to conflicting selection pressures. This is particularly true of those characteristics that are subject to sexual selection. The classic example is the peacock’s tail. Others are the calls used by male frogs and toads to attract their mates. The forces which have acted in the evolution of these calls are varied and the calls that we hear made by these animals are diverse.
Two kinds of factors can be recognized: constraints and forces. Constraints on the kind of a call that a frog might evolve include its phylogeny, the energy required to produce different kinds of calls, the risks incurred from attracting predators. Also important is the morphology of the frog: both the structures used by the males to make the calls and the apparatus with which the females hear the calls. For example, frog size has an important influence both on the frequencies of the sounds that a frog produces and the acuity with which they are heard. Both passive and active selective forces can be identified. Passive forces include the distances that environments transmit sounds of different frequencies and the interference from other sounds that calls encounter. Active forces include the reactions of conspecific males and females to the calls. Males interact acoustically in a variety of ways to organize their choruses in both space and time. They position themselves, and time their calls so that they minimize the interference from other males while maximizing their chances of securing a mate. Female choice has been studied in test arenas. Females choose louder calls, calls that are most easily located, and the calls of their own rather than other species. In choosing among males of their own species, females have been shown to pick the males controlling the best resources, sometimes using calls to do so. They should also be expected to choose those males who can contribute the best genes to their offspring. The extent to which they do this and the role of calls in choosing is actively being argued.
Sexual selection, both interactions between males and female choice, have undoubtedly been important in the evolution of frog calls but only within the constraints imposed by a variety of other factors.