Allometry and size evolution in the rattlesnake, with emphasis on predatory strike performance

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Date

2003

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LaDuc, Travis James

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Abstract

Changes in body size have significant implications for an animal’s morphology and physiology, with such ontogenetic or evolutionary changes affecting surface-to-volume ratios, metabolic rates, and kinematics (body movements exclusive of their underlying forces). The majority of comparative studies on feeding in vertebrates have centered on interspecific comparisons between species of similar ecologies with similarly sized animals and descriptive papers on the feeding in a single species of vertebrate typically focus on restricted size ranges of animals to specifically avoid ontogenetic-based variation. Rattlesnakes (genera Crotalus and Sistrurus) are an excellent group to investigate the effects of body size on feeding kinematics because of their substantial body size variation and a feeding system uncomplicated by limb movements. The western diamondback rattlesnake (C. atrox) was selected for examining intraspecific body size effects on feeding because of its large intraspecific size variation and local availability. Using preserved specimens, body size was found in to increase isometrically for six out of seven morphological measurements (head volume with negative allometry). Multiple predatory strike sequences were filmed (1000 fps) and analyzed for 20 individual western diamondback rattlesnakes in the lab, with maximum acceleration found to be size independent, contrary to kinematic values predicted based on isometric growth. Predatory and defensive strikes were compared for a small subset of rattlesnakes: maximum acceleration did not vary between strike types, although distances in defense strikes were twice those of predatory strikes. A phylogenetic estimate was created using a maximum parsimony analysis on 2385 base pairs of sequence data (downloaded from GenBank) and 311 morphological characters (generated for this analysis) for 28 rattlesnake taxa and five outgroup taxa. This estimate served as the backbone for interspecific regression analyses of morphological and feeding variables for six rattlesnake taxa (four Crotalus species and two Sistrurus species) using independent contrasts to account for historical relatedness. Body scaling relationships similar to those found in C. atrox were found in each of the additional five taxa and strike comparisons between adults of the same six species again demonstrated size independence in maximum acceleration of the strike.

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