Plasticity in the Auditory System across Metamorphosis

Abstract

Many species of anuran amphibians undergo a developmental process called metamorphosis during which free-living, herbivorous, nonreproductive larvae (tadpoles) transform into partly terrestrial, carnivorous, reproductively active adults. Metamorphosis in anurans is a period of rapid morphological and physiological change affecting all sensory, motor, and vegetative systems. The pattern of larval development and the extent of change during metamorphosis vary (McDiarmid and Altig 1999); some species (e.g., Eleutherodactylus) undergo direct development, whereas others (such as the bullfrog, Rana catesbeiana) have relatively lengthy tadpole periods. Pipids, such as the African clawed frog, Xenopus laevis, remain aquatic as adults, and metamorphic change is not as extensive as that observed in anuran species which become partly terrestrial. Development in anurans thus encompasses diverse patterns of change and is a rich resource for analysis of theories and mechanisms of plasticity, offering a vertebrate system in which neural and anatomical development can be examined in nonfetal animals. Metamorphosis features regression of structures important only in larval forms, transformation of larval into adult structures, and development of new structures necessary for the adult (Fritzsch et al. 1988). The metamorphic transition involves considerable alteration in external morphology (Gosner 1960; Nieuwkoop and Faber 1994); in behaviors such as respiration, locomotion, and feeding (Etkin 1964; Stehouwer 1988; Burggren and Infantino 1994); and in peripheral and central nervous system structure and functioning (Fritzsch et al. 1988; Lannoo 1999). Metamorphosis also involves changes in neural and hormonal foundations allowing later emergence of reproductive behaviors. All sensory systems undergo some sort of modification or reorganization during the larval period, and at different time courses (Spaeti 1978). In particular, the metamorphic shift from aquatic tadpole to amphibious frog imposes substantial changes in the type of auditory input available to the organism, based on the different acoustic properties of underwater and terrestrial environments (Bass and Clark 2002). Adult anurans rely on vocalizations for mate attraction and territorial defense (Gerhardt