Endocrinology of Complex Life Cycles: Amphibians

Amphibians exhibit considerable diversity in behavioral, physiological, and life history strategies. They are geographically widespread, occupying a diverse range of habitats. Amphibians that undergo metamorphosis have two very different life stages that are affected differently by environmental factors. Most anuran (frog) larvae are aquatic, and tadpoles are found in a wide variety of habitats, ranging from water-filled crevices in rocks, logs, or leaves to larger ponds or streams. Most then undergo morphological, biochemical, and physiological transformation into adults, which are sensitive to different environmental variables than larvae, due to this shift in habitat (Duellman and Trueb, 1994). Some amphibians have lost the larval form and develop directly into the adult morphology (direct development); others do not metamorphose but reproduce in the aquatic habitat while retaining the larval morphology (paedomorphosis). Amphibians that undergo a metamorphosis exhibit strong variation, both between and within species, in the duration of the larval period (Wilbur and Collins, 1973; Werner, 1986). Larvae encounter diverse ecological conditions during development. Variation in abiotic factors (e.g., water availability, temperature, and photoperiod) as well as biotic factors (e.g., intraand interspecific competition, and predation) can interact in complex ways to influence larval growth and development (Semlitsch, 1987a; Sredl and Collins, 1992; Rowe and Dunson, 1995; Taylor and Scott, 1997). The timing of metamorphosis is a central amphibian life history trait that probably reflects the quality and relative permanence of the larval habitat. Species that breed in predictable habitats (i.e., permanent or semipermanent lakes and ponds) tend to have longer larval periods. Species that breed in unpredictable habitats (i.e., ephemeral pools) generally have much shorter larval periods (see Fig. 1). Fig. 1 Amphibian larvae exhibit plasticity in the timing of metamorphosis and can capitalize on favorable conditions for growth as long as such conditions last (up until a genetically determined upper limit to the length of the larval period; see Newman, 1992). Such plasticity may permit amphibian larvae to match their phenotype (morphology, physiology, and metamorphic timing) to prevailing environmental conditions. Animals capable of phenotypic plasticity may have a higher probability of surviving in unpredictable habitats than those with a genetically fixed phenotype (Stearns, 1989; Newman, 1992). Among the most extreme evolutionary modifications of the ancestral complex life history is paedomorphosis.