Kinematics, maximal metabolic rate, sprint and endurance for a slow-moving lizard, the thorny devil (Moloch horridus)

Abstract

Metabolic physiology, morphology, activity patterns, performance traits and movement kinematics are thought to have coevolved in lizards. We examined links between these parameters for the thorny devil (Moloch horridus), a morphologically and ecologically specialised agamid lizard (body mass ~30 g). It has a maximum sustainable metabolic rate (VO_2max) of 0.99 mL O₂ g^–1 h^–1 while running at a velocity of 0.11 m sec^–1 at 35°C. This VO₂ is typical of that for other lizards (except varanids), but its burst speed (1.21 m sec^–1) is slower than for a typical agamid (e.g. Ctenophorus ornatus at 3.59 m sec^–1) and its endurance is appreciably higher. The kinematic pattern of hind-limb movement for M. horridus is different to that of a 'typical' similar-sized agamid, Ctenophorus ornatus, which is a fast-moving lizard that shelters in rock crevices. It is also different to the ecologically equivalent Phrynosoma platyrhinos. The slow and erratic ventilation of M. horridus (2.3 breaths min^–1) at its maximum sustainable aerobic running speed occurs when it stops running. This might be a consequence of the hypaxial muscles being used for both lung ventilation and locomotion, which might be impairing pulmonary ventilation when running, but might also contribute to its high endurance. M. horridus is metabolically typical of agamids, but its body shape, movement patterns and locomotory performance traits are different, and might have coevolved with its specialisation for eating ants.