Chameleons (family Chamaeleonidae) are a distinctive and highly specialized clade of Old World lizards with 200 species described as of June 2015. The members of this family are best known for their distinct range of colours, being capable of colour-shifting camouflage. The large number of species in the family exhibit considerable variability in their capacity to change colour. For some, it is more of a shift of brightness (shades of brown); for others, a plethora of colour-combinations (reds, yellows, greens, blues) can be seen.
Chameleons are also distinguished by their zygodactylous feet, their prehensile tail, their laterally compressed bodies, their head casques, their projectile tongues used for catching prey, their swaying gait, and in some species crests or horns on their brow and snout. Chameleons' eyes are independently mobile, and because of this the chameleon's brain is constantly analyzing two separate, individual images of its environment. When hunting prey, the eyes focus forward in coordination, affording stereoscopic vision.
Chameleons are diurnal and adapted for visual hunting of invertebrates, mostly insects, although the large species also can catch small vertebrates. Chameleons typically are arboreal, but there are also many species that live on the ground. The arboreal species use their prehensile tail as an extra anchor point when they are moving or resting in trees or bushes; because of this, their tail is often referred to as a "fifth limb". Depending on species, they range from rainforest to desert conditions and from lowlands to highlands, with the vast majority occurring in Africa (about half of the species are restricted to Madagascar), but with a single species in southern Europe, and a few across southern Asia as far east as India and Sri Lanka. They have been introduced to Hawaii and Florida.
Etymology
The English word chameleon ( , or ) is a simplified spelling of Latin , a borrowing of the Greek (khamailéōn),. a compound of (khamaí) "on the ground". and (léōn) "lion"..
Classification In 1986, the family Chamaeleonidae was divided into two subfamilies, Brookesiinae and Chamaeleoninae. Under this classification, Brookesiinae included the genera Brookesia and Rhampholeon, as well as the genera later split off from them (Palleon and Rieppeleon), while Chamaeleoninae included the genera Bradypodion, Calumma, Chamaeleo, Furcifer and Trioceros, as well as the genera later split off from them (Archaius, Nadzikambia and Kinyongia). Since that time, however, the validity of this subfamily designation has been the subject of much debate, although most phylogenetic studies support the notion that the pygmy chameleons of the subfamily Brookesiinae are not a monophyletic group.
While some authorities have previously preferred to use this subfamilial classification on the basis of the absence of evidence principle, these authorities later abandoned this subfamilial division, no longer recognizing any subfamilies with the family Chamaeleonidae.
In 2015, however, Glaw reworked the subfamilial division by placing only the genera Brookesia and Palleon within the Brookesiinae subfamily, with all other genera being placed in Chamaeleoninae.
Change of colour
Some chameleon species are able to change their skin colouration. Chameleon skin contains a layered arrangement of specialised cells known as the dermal chromatophores.<ref name="Bagnara1968"></ref> Starting from the surface, it comprises "xanthophores" and "erythrophores" (yellow and red), as well as an intermediate layer of iridophores whose transparent lattice-shaped guanine nanocrystals generate structural colour through optical interference, and basal "melanophores" containing dark melanin. The colour at a given time is the result of light interacting with all these layers simultaneously.<ref name="Teyssier"></ref>
The colour change in chameleons is controlled by two superimposed layers of iridophores. The superficial layer (S-iridophores) contains small guanine crystals in a triangular photonic crystal lattice, and the deeper layer (D-iridophores) contains larger crystals that broadly reflect light, especially in the near-infrared range.
The skin colour shifts as the chameleon is able to actively "excite" these crystals, changing the spacing between them, and this then affects which wavelengths of light are reflected and which are absorbed. In a resting state, the crystals are much more closely spaced than in an excited state, causing the lattice to reflect blue light, which, filtered through the overlying yellow xanthophores, produces the ordinary green. When the spacing of crystals changes in the excited state, the wavelengths reflected by the iridophore layer can quickly shift to yellow, orange, red, or back to blue. When combined with the normal pigment contained in the other skin layers, this forms the final colour visible to the eye.
The exact mechanism by which chameleons can adjust the spacing of their iridophore crystals is not fully determined. Nevertheless, Iridophores share the neural crest of all other chromatophores, which are regulated through neural or hormonal signals, as such some uncertain analgous signal is thought to be how the lattice adjustments are triggered.
Colour change in chameleons has functions in camouflage, but most commonly in social signalling and reactions to temperature and other conditions. The relative importance of these functions varies with the circumstances, as well as the species. Colour change signals a chameleon's physiological condition and intentions to other chameleons. Because chameleons are ectothermic, another reason why they change colour is to regulate their body temperatures, either to a darker colour to absorb light and heat to raise their temperature, or to a lighter colour to reflect light and heat, thereby either stabilizing or lowering their body temperature. Chameleons tend to show brighter colours when displaying aggression to other chameleons, and darker colours when they submit or "give up". Most chameleon genera (exceptions are Chamaeleo, Rhampholeon and Rieppeleon) have blue fluorescence in a species-specific pattern in their skull tubercles, and in Brookesia there is also some in tubercles on the body. The fluorescence is derived from bones that only are covered in very thin skin and possibly serves a signaling role, especially in shaded habitats.
Some species, such as Smith's dwarf chameleon and several others in the genus Bradypodion, adjust their colours for camouflage depending on the vision of the specific predator species (for example, bird or snake) by which they are being threatened.Young, Emma (2008) Chameleons fine-tune camouflage to predator's vision. New Scientist In the introduced Hawaiian population of Jackson's chameleon, conspicuous colour changes that are used for communication between chameleons have increased, whereas anti-predator camouflage colour changes have decreased relative to the native source population in Kenya, where there are more predators.
Chameleon colour palettes have evolved through their endemic environment. Chameleons living in the forest have a more defined and colourful palette compared to those living in the desert or savanna, which have more of a basic, brown, and charred palette.
Evolution
The oldest described chameleon is Anqingosaurus brevicephalus from the Middle Paleocene (about 58.7–61.7 mya) of China. Other chameleon fossils include Chamaeleo caroliquarti from the Lower Miocene (about 13–23 mya) of the Czech Republic and Germany, and Chamaeleo intermedius from the Upper Miocene (about 5–13 mya) of Kenya.
The chameleons are probably far older than that, perhaps sharing a common ancestor with iguanids and agamids more than 100 mya (agamids being more closely related). Since fossils have been found in Africa, Europe, and Asia, chameleons were certainly once more widespread than they are today.
Although nearly half of all chameleon species today live in Madagascar, this offers no basis for speculation that chameleons might originate from there. In fact, it has recently been shown that chameleons most likely originated in mainland Africa. It appears there were two distinct oceanic migrations from the mainland to Madagascar. The diverse speciation of chameleons has been theorized to have directly reflected the increase in open habitats (savannah, grassland, and heathland) that accompanied the Oligocene period. Monophyly of the family is supported by several studies.
Daza et al. (2016) described a small (10.6 mm in snout-vent length), probably neonatal lizard preserved in the Cretaceous (Albian-Cenomanian boundary) amber from Myanmar. The authors noted that the lizard has "short and wide skull, large orbits, elongated and robust lingual process, frontal with parallel margins, incipient prefrontal boss, reduced vomers, absent retroarticular process, low presacral vertebral count (between 15 and 17) and extremely short, curled tail"; the authors considered these traits to be indicative of the lizard's affiliation with Chamaeleonidae. The phylogenetic analysis conducted by the authors indicated that the lizard was a stem-chamaeleonid. However, Matsumoto & Evans (2018) reinterpreted this specimen as an albanerpetontid amphibian. This specimen was given the name Yaksha perettii in 2020, and was noted to have several convergently chameleon-like features, including adaptations for ballistic feeding.
While the exact evolutionary history of colour change in chameleons is still unknown, there is one aspect of the evolutionary history of chameleon colour change that has already been conclusively studied: the effects of signal efficacy. Signal efficacy, or how well the signal can be seen against its background, has been shown to correlate directly to the spectral qualities of chameleon displays. Dwarf chameleons, the chameleon of study, occupy a wide variety of habitats from forests to grasslands to shrubbery. It was demonstrated that chameleons in brighter areas tended to present brighter signals, but chameleons in darker areas tended to present relatively more contrasting signals to their backgrounds. This finding suggests that signal efficacy (and thus habitat) has affected the evolution of chameleon signaling. Stuart-Fox et al. note that it makes sense that selection for crypsis is not seen to be as important as selection for signal efficacy, because the signals are only shown briefly; chameleons are almost always muted cryptic colours.
Viviparity evolved twice in chamaeleonids, and it is believed that both of these developments of viviparity occurred by at least the Early Oligocene, some 30 to 29 Ma.
Description
Chameleons vary greatly in size and body structure, with maximum total lengths varying from in male Brookesia nana (one of the world's smallest reptiles) to in the male Furcifer oustaleti. Many have head or facial ornamentation, such as nasal protrusions, or horn-like projections in the case of Trioceros jacksonii, or large crests on top of their heads, like Chamaeleo calyptratus. Many species are sexually dimorphic, and males are typically much more ornamented than the female chameleons.
Typical sizes of species of chameleon commonly kept in captivity or as pets are:
Scientific name Common name Length (male) Length (female) Colour Lifespan (years)Chamaeleo calyptratus Veiled chameleon 35–60 cm 25–33 cm Green and light colours about 5Trioceros jacksonii Jackson's chameleon 23–33 cm 25–33 cm Green and light colours 5–10Furcifer pardalis Panther chameleon 38–53 cm 23–33 cm Darker colours about 5 (2–3 for birthing females)Rieppeleon brevicaudatus Bearded pygmy chameleon 5–8 cm 5–8 cm Brown, beige, green about 3–5Rhampholeon spectrum Spectral pygmy chameleon 8–10 cm 5–10 cm Tan and gray 3–5Rhampholeon temporalis Usambara pitted pygmy chameleon 6–10 cm 5–9 cm Gray and brown 5–11