MCGOWAN, C.P.*; O’REILLY, J.C.; NISHIKAWA, K.C.: Scaling of the Feeding Mechanism in the Colorado River Toad

There are several potential structural and functional consequences of an animal changing size. As the whole animal grows, individual components that make up complex functional systems, such as the feeding mechanism, may grow at varying rates. To explore this issue, we examined the feeding system of the Colorado River Toad, Bufo alvarius, modeled as a simple mechanical lever system. We evaluated several aspects of the feeding mechanism, including skull measurements, linear jaw measurements, mechanical advantage of jaw opening and closing, jaw opening and closing musculature and in vivo bite forces. Components of the feeding system were scaled against body length in order to test for isometry and establish relative growth rates. The sample size included 30 animals ranging from 15 to 130 mm in body length as well as a separate group of 12 live toads in this size range, which were used to acquire bite forces. The results of the changing skull and jaw morphology were related to existing feeding kinematics for Bufo alvarius of a similar size range. Mechanical advantage remained constant for both jaw opening and closing, though the lever arms measured to determine mechanical advantage were not isometric with respect to body size. The predicted jaw closing force was very similar to that observed in vivo, but also did not scale isometrically with size. The results of the predicted jaw opening forces however did not match that observed through kinematics, suggesting that some other component not explored in this study is active in the ballistic tongue protraction mechanism of this species. Supported by NSF 9809942 and a Hooper Undergraduate Research Award from Northern Arizona University.