Chromatophores are found in chameleons' skin. (Illustration by Joseph E. Trumpey)
because its hearing is very poor. It scans the surrounding environment with telescopic vision that enables it to plan and execute a defense (usually concealment or flight) well in advance of the predator's approach. Phenomenal eyesight also facilitates locating prey from a great distance. As chameleons target prey, two separate images merge into one to gauge distance. Then chameleons engage their most fascinating feature—the tongue.
In 2000 a group of researchers published the results of a study on the mechanics of prey prehension in chameleons that unraveled the mysteries of how a chameleon's tongue really works. The hyoid bone is a piece of cartilage that extends into the mouth from the throat bones (called the hyolingual apparatus) and is attached to a chameleon's long tongue. This is where the tongue rests when it is not in use. The tongue is launched from the hyoid bone with the use of ringed muscles in the tongue. This highly complex structure, composed of cartilage, muscles, nerves, glands, and tissue, is used this way in prey capture:
• The central cylindrical accelerator muscle is responsible primarily for projecting the tongue for prey capture.
• The tip of the tongue, or tongue pad, sits atop the accelerator muscle, connected by several pairs of muscles. As the chameleon launches the tongue pad at prey, it turns inside out (evaginates) and actively reverses (invaginates) to form a pouch immediately before prey contact.
• The pouch engulfs the prey, and wet adhesion and interlocking maintain grip while suction created by the largest paired muscles, the pouch retractor, transfers prey deeper into the pouch.
• The tongue retractors attached to the accelerator muscle return the tongue to the resting position on the hyoid bone, and the prey is in the chameleon's mouth to be crushed and swallowed.
Before this study, the capture of prey often was attributed only to adhesion to the tongue pad. The withdrawal of the tongue pad to form a pouch not only creates suction forces on the prey but also increases the adhesive properties of the tongue. Suction accounts for more than two-thirds of the total force generated by a chameleon's tongue. This permits capture of larger prey, such as lizards and birds, than is possible using just adhesion. Chameleons also employ the tongue pad to lap drinking water from leaves or other surfaces.
The skeletal structure of chameleons is remarkable for it's flexibility. They can compress their bodies to bask in sunlight or inflate their lungs and expand the rib cage to bluff potential predators. Chameleon feet are designed to grasp, with five toes on each foot fused in bundles of two and three toes to form a pincer. On the front feet the bundle of three toes is on the inside of the foot, and the bundle of two toes is on the outside. This is reversed on the rear foot, giving them a secure and strong grasp and allowing them to navigate horizontally or vertically on a wide variety of vegetation or structures. Sharp claws on each toe help them climb and grip surfaces that they cannot grasp tightly, such as tree trunks, and are used by females to excavate tunnels to lay eggs.
In the genera Bradypodion, Calumma, Chamaeleo, and Furcifer, tail length is roughly equal to or slightly longer than body length. These species can use their prehensile tail as a fifth limb and to anchor themselves while launching their long tongue at prey. Some species, such as Parson's chameleon (C. parsonii parsonii), use their tails to communicate. Males engaged in ritualistic threat displays repeatedly coil the tips of their long tails tightly, curl them up and over the back, and whip them forward. Sleeping chameleons often roll their long tails into a perfect coil like a watch spring, and chameleons may intertwine tails during copulation. Members of the genera Brookesia and Rhampholeon have much shorter, less prehensile tails that nevertheless can be used as a grasping hook in some species.
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