Ground reaction forces and muscle allometry in monitor lizards (Varanidae) with implications for the scaling of locomotion in sprawling tetrapods

Abstract

Geometric scaling predicts a major constraint for legged, terrestrial locomotion. Locomotor support requirements scale isometrically with body mass (α M1), while force generation capacity should scale α M2/3 as it depends on cross-sectional area. Mammals compensate with more erect postures at larger sizes, but it remains unknown how sprawling tetrapods deal with this constraint. Varanid lizards are an ideal group to address this question because they cover an enormous body size range while maintaining similar posture and body proportions. This study reports the scaling of ground reaction forces and duty factor from varanid lizards ranging from 7-37,000 g. Impulses (α M0.96-1.34) and peak forces (α M0.72-0.98) scaled higher than expected. Duty factor scaled α M0.04 and was higher in the hindlimb than in the forelimb. The proportion of vertical impulse to total impulse increased with body size, and impulses decreased while peak forces increased with speed. Muscle parameters (fascicle length, muscle mass, and physiological cross-sectional area) were also found to scale with positive allometry in varanids, suggesting that varanid lizards respond to predicted biomechanical demands of increased body size with both anatomical and kinematic adjustments. Peak forces scale (> M2/3), suggesting that muscle and skeletal strength increases with positive allometry, but (< M1), suggesting that larger varanids also increase duty factor to spread body support over a longer period. These results provide insight into the biomechanics of extinct, sprawling megafauna.