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By Heather Heying
Chameleons are well known to most people, easily recognizable by their body shape, independently moving eyes, paw-like hands and feet, and ability to change color rapidly. Most researchers identify two subfamilies of chameleons, containing 4-6 genera, and more than 150 species. Chameleons have experienced two distinct radiations in Madagascar and Africa. A few species are also found in southern Spain, the Arabian peninsula, India and Sri Lanka.
Chameleons cannot be mistaken for any other lizards. Easily recognizable features include their diagnostic zygodactyl feet, in which adjacent digits are fused on each hand and foot, forming opposable grasping pads, and the presence of casques, horns, and crests on the heads of most species. They change color easily and quickly, and their eyes, which are on protruding cones, can move independently of one another. Arboreal species have prehensile tails, and most species have laterally flattened bodies. Among the many synapomorphies of this group are extremely extensile tongues (due to modifications of the hyoid apparatus), failure of the pterygoid to meet the quadrate, a reduction in the number of sternal ribs, the loss of both the gular fold and femoral pores, and the V-condition of the ulnar nerve pathway. Like the agamids, but unlike other iguanians, chameleons have acrodont teeth. Chameleons range in size from tiny Brookesia (2.5 cm) to some very large Chamaeleo species (up to 68 cm total length).
Chameleons are diurnal and primarily insectivorous, although birds have been recorded in the diets of some larger species. They can extend their tongues almost the entire length of their body, making their hunting behavior an impressive display. Chameleons exhibit a distinct locomotion in which they slowly rock back and forth between each step taken. Most species are oviparous. In some species (e.g. Chamaeleo pardalis), females dig holes in sandy soils into which they deposit their fertilized eggs, afterwards covering their tracks, presumably to confuse would-be egg predators. Several of the larger species show distinct sexual dimorphism, with males having larger body size, and elaborate horns and casques, with which they fight. Chameleons are not social animals, and when two animals have visual contact, they can change color within seconds. Depending on species, bright coloration may be restricted to males, found in both sexes, displayed only during the breeding season, or exhibited during aggressive encounters. Male displays may also include head jerking, the spreading of occipital lobes (if present), and gular sac extension. Sexual dimorphism in the dwarf chameleons (Brookesia in Madagascar, Rhampholeon in Africa) tends to run in the opposite direction, with females being the larger sex. Chameleons are extremely popular in the pet trade. In at least part of their range (e.g. Madagascar), local people are quite scared of them.
Chamaeleonids are unambiguously placed in the Iguania, a group that is sister to all other squamates (lizards and snakes). Within Iguania, however, relationships are in dispute. Frost and Etheridge (1989) recently split the large family Iguanidae into eight families (including Iguanidae sensu stricto). Most researchers agree that the only iguanian families that were not previously members of Iguanidae -- Chamaeleonidae and Agamidae -- form the monophyletic group Acrodonta, which is sister to the remaining families (equivalent to Iguanidae sensu lato). The monophyly of Chamaeleonidae has never been called into question. Frost and Etheridge's (1989) analysis did find Agamidae paraphyletic with respect to Chamaeleonidae, however, and they argue that the two agamid subfamilies should be subsumed within a larger Chamaeleonidae. Despite this, the literature persists in reflecting the traditional groupings. Within Chamaeleonidae, two hypotheses of relationship include Brookesia sister to the remaining chameleons, and Brookesia + Rhampholeon sister to the remaining taxa.
Fossil Chamaeleo are known from the Miocene in Europe.
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Contributors
Heather Heying (author).
