Anuran Reproductive Modes: Evolving Perspectives

Although most of the unique ways that frogs reproduce were described in the 1800s and 1900s, additional modes are still being discovered in the 21st Century. The concept of reproductive mode has evolved from descriptive natural history to an integration of developmental biology, genetics, systematics, evolution, ecology, behavior, and physiology that frames our thinking about the transition of vertebrates from water to land and about anuran reproductive adaptations to diverse environments today. We have classified reproductive modes, examined quantitative parameters, searched for geographical and ecological patterns, and described variability. We have speculated about selective pressures guiding the evolution of terrestrial reproduction and argued about the usefulness of reproductive mode as a character to construct phylogenies. In the past, researchers assumed that the reproductive modes exhibited by living frogs represented stages in an incomplete, linear sequence of steps toward greater independence from open water, with direct development at the end of the spectrum. Newly proposed phylogenies based on molecular data allow us to re-think the evolution of anuran reproductive modes. On another level, we are increasingly realizing the value of incorporating life history information (aquatic larvae or terrestrial development) in setting priorities to formulate more effective and ecologically relevant conservation strategies. The next decade is certain to witness significant advances in our understanding of anuran reproductive modes.