How does a tadpole know when to metamorphose? Integrating evolutionary, ecological, and developmental biology with the neuroendocrinology of amphibian metamorphosis

Amphibian metamorphosis is an excellent model for studying how selective pressures, environmental cues, endocrine signals, and molecular mechanisms influence developmental timing. In this minireview, I discuss key questions about the timing of metamorphosis, including: when and how do larvae acquire the ability to initiate metamorphosis; what historical and current ecological factors influence the optimal timing of transformation; and what are the physiological mechanisms that translate environmental information into a developmental response? From an ecological perspective, the timing of amphibian metamorphosis depends on growth opportunities and predation risks in the larval habitat. Nutrition, through its influence on growth and energy storage, plays a pivotal role in determining when a larva can undergo metamorphosis. Several hormones, whose production is controlled by nutrients, regulate growth and energy balance and promote development of the neuroendocrine system. The ability to initiate metamorphosis requires adequate body size and energy levels, expression of thyroid hormone (TH) receptors (TRs), and a functional thyroid gland and neuroendocrine system. Once started, metamorphic transformation is driven by rising plasma TH concentration. Environmental cues modulate thyroid activity via neuroendocrine stress pathways. The stress neuropeptide corticotropin-releasing hormone is the primary neuroregulator of metamorphosis. It stimulates secretion of both TH and corticosteroids, and corticosteroids synergize with TH to enhance TH bioactivity by increasing TRs and TH metabolism. The combination of habitat quality, which affects growth and energy status, environmental stressors, and neuroendocrine signaling shapes the diversity in body size and larval period length among species and the plasticity in metamorphic timing within a species. Furthermore, many of these mechanisms are ancient and evolutionarily conserved.