Many correlated changes in the structure and function of the skull occur during metamorphosis in salamanders. To separate correlated from causal changes in form and function, we experimentally converted the unidirectional flow feeding system in axolotls (Ambystoma mexicanum) to a bidirectional system by suturing closed the gill slits. This mimics one of the major structural changes in the feeding mechanism that occur at metamorphosis. Eleven variables were measured from the intra-oral pressure traces recorded during feeding. The major changes in buccal pressure produced by suturing the gill slits were in the duration and area of the negative pressure traces. In addition, the ratio of positive to negative pressure areas increased by four times following closure of the gill slits. In contrast to pressure changes during metamorphosis in tiger salamanders, increases in the positive portion of the pressure traces occur in axolotls. These data corroborate the hypothesis that the decreased feeding performance after metamorphosis is a direct consequence of the change from a unidirectional to a bidirectional feeding mechanism, and demonstrate the causes of ontogenetic changes in function.

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