The scaling of ground reaction forces and duty factor in monitor lizards: implications for locomotion in sprawling tetrapods

Abstract

Geometric scaling predicts a major challenge for legged, terrestrial locomotion. Locomotor support requirements scale identically with body mass ( α M 1 ), while force-generation capacity should scale α M 2/3 as it depends on muscle cross-sectional area. Mammals compensate with more upright limb postures at larger sizes, but it remains unknown how sprawling tetrapods deal with this challenge. Varanid lizards are an ideal group to address this question because they cover an enormous body size range while maintaining a similar bent-limb posture and body proportions. This study reports the scaling of ground reaction forces and duty factor for varanid lizards ranging from 7 g to 37 kg. Impulses (force×time) ( α M 0.99−1.34 ) and peak forces ( α M 0.73−1.00 ) scaled higher than expected. Duty factor scaled α M 0.04 and was higher for the hindlimb than the forelimb. The proportion of vertical impulse to total impulse increased with body size, and impulses decreased while peak forces increased with speed.

Musculoskeletal modelling of the Nile crocodile (Crocodylus niloticus) hindlimb: Effects of limb posture on leverage during terrestrial locomotion

“…This postural shift is in contrast to varanids—somewhat comparably sized monitor lizards. Muscle masses and cross‐sectional areas, rather than a change in limb posture, have been found to scale with size to mitigate increased stress in varanids (Cieri et al, 2021 ; Clemente et al, 2011 ; Dick & Clemente, 2016 ), although it is unclear if varanids have greater limb adduction in certain behaviours, as found here for Nile crocodiles (also see Gatesy, 1991 ). Varanid muscles thus scale differently from Crocodylia, in which muscles in general scale closer to isometry (Allen et al, 2014 ).…”

“…Specifically, how can a group that can vary so dramatically in body size and postural kinematics support its body weight during locomotion (cf. Cieri et al, 2021 ; Clemente et al, 2011 )? And how do the pelvic and hindlimb muscle functions relate to limb orientation and anti‐gravity support during terrestrial locomotion (e.g.…”

Scaling of fibre area and fibre glycogen concentration in the hindlimb musculature of monitor lizards: implications for locomotor performance with increasing body size

“…Thus, larger animals must become relatively stronger or change shape to locomote similarly to smaller animals. In varanid lizards, which do not change substantially in overall shape or posture (Clemente et al, 2011), peak ground reaction forces scale ∝M b 0.89-0.99 (Cieri et al, 2021) and while PCSA and mass of the forelimb and hindlimb muscles scales with positive allometry, this is not sufficient to fully explain the scaling of ground reaction forces. An increased fCSA means that muscle fibres should generate more total force because of a greater number of sarcomeres in parallel (Bruce et al, 1997;Krivickas et al, 2011).…”

“…Among geometrically similar animals, this biomechanical challenge suggests that larger animals will be proportionally weaker than smaller animals. As a result, larger animals compensate with allometric shifts in limb posture or musculoskeletal architecture and suffer reduced maximum sprint speed and increased duty factor (Cieri et al, 2021;Clemente et al, 2013;Dick and Clemente, 2017). For example, many mammals compensate for predicted decreases in proportional strength by adopting a more erect posture, thereby increasing the effective mechanical advantage of the extensor muscles and allowing animals to locomote effectively without substantial changes in muscle architecture (Biewener, 1989(Biewener, , 2005.…”

“…Our previous work has shown allometric increases in muscle mass and PCSA with body size in varanid forelimb and hindlimb muscles, however muscle force-generating capacity could also be increased by means of an increased proportion of faster fibre types. We predicted that, given the constraints imposed by the geometric scaling of muscle force: (1) both individual fCSA and fibre number would increase with body size in order to increase the forcegenerating capacity of locomotor muscles, and (2) muscle glycogen concentration (as an index of muscle fibre type) would increase with body size, reflecting a greater capacity for the higher peak forces with increased body size involved in varanid locomotion (Cieri et al, 2021). fCSA and glycogen content may also scale more clearly against muscle mass rather than body mass, given that muscle mass has been shown to scale with positive allometry in these animals (Cieri et al, 2020;Dick and Clemente, 2016).…”