Where the tadpole meets the world – Observations and speculations on biomechanical and biochemical factors that influence metamorphosis in anurans

SYNOPSIS. A diversity of issues related to metamorphosis are discussed in this paper, ranging from biomechanical constraints on behavior in premeta-morphic 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 pres-sure 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 tad-poles. 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, bu