Meeting Abstract

83.1  Friday, Jan. 7  Evolution of body posture with body size within Varanid lizards (Squamata: Varanidae: Varanus). CLEMENTE, C.J.*; WITHERS, P.C.; THOMPSON, G.; LLOYD, D; Harvard University; University of Western Australia; Edith Cowan University; University of Western Australia clemente@rowland.harvard.edu

Geometric scaling predicts that stresses on limb bones and muscles should increase with body size. Mammals counter this size related increase in stress partially through changes in bone geometry, but largely through changes in posture, with a more erect stance being characteristic of larger species. However, the ability to counter size related stresses in this fashion may be limited to taxa which employ a parasagital gait, where legs are swung underneath the body. We examined kinematics of 11 species of varanid lizard ranging in body mass from 0.04 kg to 8kg, which employ sprawling kinematics where legs are held laterally, and the femur is moved in the horizontal plane, and can undergo significant torsion. Posture, as indicated by hip height, joint angles and ground reaction force (GRF) joint moment estimates, did not change significantly with body size beyond that expected from geometrical scaling. Instead it appears that taxa which employ a sprawling posture mitigate size related increases in stress by an increase in duty factor and a reduction in femur rotation. We show that incorporating these factors in biomechanical models predicts that both bending stresses (α M^0.016) and torsional stresses (α M^-0.049) should be nearly independent of body size over the size range examined. However, increasing duty factor and reducing femur rotation probably has deleterious effects on speed, and this difference in kinematics with size probably explains why speeds scales much lower for lizards than for terrestrial mammals (α M^0.17 and α M^0.24 respectively). Further, these findings suggests that evolution from sprawling to upright posture in archosaurs did not occur as a response to larger size, rather that they became upright first and larger later.