The locomotion of Chamaeleo (Reptilia: Sauria) with particular reference to the forelimb

This study examines adaptation for arboreal locomotion in the Old World chamaeleon, Chamaeleo. The locomotor behaviour of three species of Chamaeleo is compared with that of Agama agama , a generalized terrestrial lizard. Cine films of normal locomotion were analysed for differences in gait, lateral undulation, excursion of the shoulder girdle, humeral excursion and forelimb orientation. Chamaeleons are highly specialized arboreal lizards inhabiting the maze of small diameter perches in the tree canopy or bush. This habitat poses two behavioural challenges for the animal‐movement along perches narrower than the body and crossing the open space between perches. Chamaeleons exhibit locomotor specialization for this structural habitat in: (i) the adoption of a slow, stable perch‐walking gait; (ii) increased mobility in the shoulder region which permits them to use slow acrobatic manoeuvres to turn and change perches and (iii) the substitution of limb and girdle excursion in the sagittal plane for lateral undulation and humeral abduction/adduction in the horizontal plane. Lateral undulation in generalized lizards contributes to step length by rotation of the girdle and limb in the horizontal plane. Undulation is responsible for about 24% of the step length in Agama , but only 6% of the step in Chamaeleo. A specialized gait rhythm and the more vertical limb orientation are principally responsible for limiting the contribution of undulation to the step in Chamaeleo. In Chamaeleo excursion of the girdle relative to the body wall is substituted for lateral undulation. The shoulder joint displaces through 14 mm and 33° relative to a point on the dorsal midline, and about 40% of the step length is created by excursion of the shoulder girdle. In Agama the girdle is fixed, and this movement does not exist. The substitution of girdle excursion for lateral undulation in Chamaeleo provides for more excursion in the proximal limb, requires less excursion of the trunk relative to the perch and requires less stereotypy in limb position. The antero‐posterior arc of glenohumeral movement in Chamaeleo can be more than twice as long than that of Agama. In Agama the humerus operates largely behind the glenohumeral joint. In Chamaeleo the humerus is routinely protracted into the quadrant anteroventral to the glenoid, and in acrobatic manoeuvres the humerus can be placed almost anywhere in the anteroventral or lower portion of the anterodorsal quadrant. This extremely long reach contributes to the long step of perch‐walking and is critical in slow acrobatic manoeuvres in which the animal reaches between perches. In perch‐walking the chamaeleon humerus is abducted only 25° from the sagittal plane. In Agama , by contrast, most of the humeral arc occurs within 25° of the horizontal plane through the shoulder joint. The more sagittal limb arc in Chamaeleo permits the animal to: (i) exert forces downward and parallel to a narrow perch and (ii) combine sagittal excursion of the girdle with that of the limb to achieve particularly long antero‐posterior arcs of movement.