Comparative forelimb kinematics of the “pygmy” chameleon Brookesia superciliaris

Abstract

Understanding the locomotor characteristics of early diverging “pygmy” or “leaf” chameleons can help to explain how their unique morphology is adapted to fit their environment and mode of life. However, nearly all quantitative studies of chameleon locomotion thus far have focused on the larger, highly arboreal “true” chameleons. Unlike “true chameleons, “pygmy” chameleons spend most of their time on the ground and are considered terrestrial. We investigated forearm kinematics in the Brown Leaf Chameleon Brookesia superciliaris to quantify and compare the locomotor characteristics of B. superciliaris relative to “true” chameleons such as Trioceros and Bradypodion. We predicted that elbow joint angles would be smaller in B. superciliaris, corresponding to a less upright posture, and that wrist range of motion would be more limited because of their simplified wrist structure. Kinematic data were collected from B. superciliaris and Trioceros jacksonii, a three horned chameleon native to east Africa. Markers were painted on the skin, and locomotion was recorded using four high speed cameras at 120 Hz while the chameleons traversed a horizontal dowel. Five events (recordings) for B. superciliaris and one event for T. jacksonii were analyzed. Markers on the elbow, forearm, wrist, and manual digits were tracked throughout one stride in each event. Outcome measures were range of motion at the wrist, elbow angle, and footfall patterns. Elbow joint angles in B. superciliaris at mid-stance and touch-down were similar to values reported in the literature forBradypodion pumilim, a larger arboreal chameleon found in parts of south Africa. The elbow joint angle of B. superciliaris at lift off was smaller when compared to B. pumilim. T. jacksonii had a much smaller elbow range of motion than B. superciliaris and B. pumilim. Wrist range of motion was similar between B. superciliaris and T. jacksonii. B. superciliaris used a lateral sequence walk with a lower step frequency and higher duty factor than T. jacksonii, which used a walking trot. Our results do not support the hypothesis that “pygmy” chameleons have more restricted ranges of motion at the wrist than “true” chameleons, but they indicate that the elbow is slightly more flexed and has a smaller range of motion. However, the greatest difference between the two groups was that B. superciliaris moved much more slowly and kept at least three feet in contact with the substrate, trading speed for increased stability. Thus, the transition to a fully arboreal way of life in “true” chameleons may have involved changes in limb kinematics as well as morphology.