Length‐tension and histochemical properties of select shoulder muscles of the savannah monitor lizard (Varanus exanthematicus): Implications for function and evolution

Abstract

AbstractWe explored the mechanical properties of four shoulder muscles of the Savannah Monitor Lizard (Varanus exanthematicus) and related these to locomotion. We studied the biceps brachii, supracoracoideus, two parts of the latissimus dorsi, and two parts of the pectoralis muscles by measuring twitch contraction time, peak tetanic tension, and contractile fatigue resistance to repetitive stimulation. In addition, we generated length‐passive and length‐active tension curves at those muscle lengths used during slow and moderate speed walking. Finally, we used conventional histochemistry to identify anatomical compartments within these muscles. Major findings include (1) the length‐active tension of the biceps brachii, supracoracoideus, both parts of the latissimus dorsi, and the pectoralis remains near peak value over a range of muscle lengths associated with normal limb movement, (2) all muscles possess a mean twitch contraction time of between 65 and 70 msec (24°C), (3) all muscles are relatively fatiguable, and (4) evidence for anatomical compartments exists for the latissimus dorsi, pectoralis, and supracoracoideus.We compared the data for the pectoralis of V. exanthematicus to that for the domestic pigeon (Columba livia) in an effort to identify features of musculoskeletal design which may have accompanied the evolution of flight. We conclude that at least two factors were important: (1) the development of a capacity for sustained, aerobic contraction, and (2) a rearrangement of some of the anterior pectoralis fascicles so that they arise from the furcula. The latter provides for a distinct protractive component to the humerus, which is important during downstroke in some forms of flapping flight.