SYNOPSIS. A diversity of issues related to metamorphosis are discussed in this paper, ranging from biomechanical constraints on behavior in premetamorphic anurans, to environmental factors that regulate metamorphosis. I begin by reviewing key behavioral features of tadpoles that have implications to their form and locomotion. Tadpoles lack all visceral behaviors, such as emesis, coughing, and egg-laying, that require major elevations of pressure in the pleuroperitoneal cavity. I suggest that the inability to forcefully elevate pleuroperitoneal pressure is a consequence of the tadpoles' short and inflexible spine. Although postmetamorphic frogs have a similarly short vertebral column, they can compress viscera by movement at sacroiliac joints, which are absent in tadpoles. I argue that the short torso of tadpoles is a necessary consequence of the need to innervate the frog's limbs early in development so that they can be fully functional by the end of metamorphosis. The short torso is a good mechanical design for the saltatory frog, but afflicts the tadpoles with a distinctly globous shape that intuitively appears inefficient for aquatic locomotion. Computational fluid dynamics (CFD) models, however, reveal that tadpoles are, in fact, efficient undulatory swimmers compared to subcarangiform fishes and that their swimming kinematics work in concert with their shape to help generate thrust. Furthermore the CFD models show that the crease in the dorsal profile of tadpoles, where the body abruptly meets the tail, produces a “dead water” zone immediately behind the body of the tadpole. As a result, tadpoles can grow hindlimbs in this region without those limbs seriously impeding flow. Thus the shape of tadpoles nicely matches their need to transform into limbed tetrapods. In the latter half of this paper I review ideas on how environmental factors could influence metamorphosis in anurans. One speculation is that tadpoles may secrete a metamorphic inhibitor in their oral mucus, which they then ingest with their food, with the result that metamorphosis is delayed when food is abundant. An epidermal growth factor-like peptide has been suggested as a possible metamorphic inhibitor along the pathway just proposed. Strengths and weaknesses of this hypothesized agent and pathway are reviewed. An experiment to test the hypothesis that orally ingested EGF inhibits metamorphosis yielded inconclusive results. Orally secreted EGF-like peptide may not inhibit metamorphosis of the tadpole gut, but other yet-to-be-identified factors in tadpole oral mucus could still play a role in regulating metamorphosis of the alimentary tract in anurans. The essay ends with some broad speculations on the role that peptides in frogs' skin may play in the metamorphic process as well as several unanswered questions about how other organisms and the physical environment influence tadpole metamorphosis.
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Richard J. Wassersug (1997) studied this question.