Meeting Abstract
Alligator mississippiensis undergoes an ontogenetic niche shift from a wetland to a riverine habitat and an ontogenetic locomotor shift from terrestriality to swimming. A number of studies have examined ontogenetic scaling of A. mississippiensis limbs (using traditional linear morphometrics, scaling of muscle masses, with gait analyses). We further these studies of limb ontogeny by performing a geometric morphometric study of the scapulae, humeri, ilia, and femora of a range of sizes of A. mississippiensis (n = 62) in order to examine within-bone allometry, shape disparity between juveniles and adults, and integration between bones. We show that allometry plays a significant role in shape determination in all four of these bones, potentially relating to ontogenetic locomotory transitions. Allometry accounts for a larger component of shape variation in the humeri and femora (>40%) than the scapulae and ilia (<25%) showing that the limb bones are more relevant to the niche transitions than the girdle bones. In all bones, the morphological disparity was higher in the adults than in the juveniles, reflecting broader niche occupation in adults. Our study shows that the scapulae and humeri are more highly integrated than the ilia and femora, contrary to previous studies. Further, the humeri and femora are more integrated with each other than either is with its respective girdle element. The girdles evolved independently relative to the limb bones perhaps because the limb bones are changing in unison through ontogeny in response to the ontogenetic niche shift. This study demonstrates the power of geometric morphometric analyses for providing new insight into a well-understood taxon.
