Chameleons orchamaeleons (familyChamaeleonidae) are a distinctive and highly specializedclade of Old Worldlizards with 200 species described as of June 2015.[1] The members of thisfamily are best known for their distinct range of colours, being capable of colour-shiftingcamouflage. The large number ofspecies 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 theirzygodactylous feet, theirprehensile tail, their laterally compressed bodies, their head casques, their projectile tongues used for catching prey, their swaying gait,[2] 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, affordingstereoscopic vision.
Chameleons arediurnal and adapted for visual hunting of invertebrates, mostly insects, although the large species also can catch small vertebrates. Chameleons typically arearboreal, 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 fromrainforest todesert conditions and from lowlands to highlands, with the vast majority occurring in Africa (about half of the species are restricted toMadagascar), but with a single species in southern Europe, and a few across southern Asia as far east asIndia andSri Lanka. They have beenintroduced toHawaii andFlorida.[1][3]
While some authorities have previously preferred to use this subfamilial classification on the basis of the absence of evidence principle,[11] these authorities later abandoned this subfamilial division, no longer recognizing any subfamilies with the family Chamaeleonidae.[16]
In 2015, however, Glaw reworked the subfamilial division by placing only the generaBrookesia andPalleon within theBrookesiinae subfamily, with all other genera being placed inChamaeleoninae.[1]
Some chameleon species are able to change theirskin coloration. Different chameleon species are able to vary their colouration andpattern through combinations of pink, blue, red, orange, green, black, brown, light blue, yellow, turquoise, and purple.[17] Chameleon skin has a superficial layer which contains pigments, and under the layer are cells with very small (nanoscale)guanine crystals. Chameleons change colour by "actively tuning the photonic response of a lattice of small guanine nanocrystals in the s-iridophores".[18] This tuning, by an unknown molecular mechanism, changes the wavelength of light reflected off the crystals which changes the colour of the skin. The colour change was duplicatedex vivo by modifying theosmolarity of pieces of white skin.[18]
Colour change and iridophore types inpanther chameleons: (a) Reversible colour change is shown for two males (m1 and m2): during excitation (white arrows), background skin shifts from the baseline state (green) to yellow/orange, and both vertical bars and horizontal mid-body stripe shift from blue to whitish (m1). Some animals (m2) have their blue vertical bars covered by red pigment cells. (b) Red dots: time evolution in the CIE chromaticity chart of a third male with green skin in a high-resolution video; dashed white line: optical response in numerical simulations using a face-centered cubic (FCC) lattice of guanine crystals with lattice parameter indicated with black arrows. (c) Haematoxylin and eosin staining of a cross-section of white skin showing the epidermis (ep) and the two thick layers of iridophores. (d) TEM images of guanine nanocrystals in S-iridophores in the excited state and three-dimensional model of an FCC lattice (shown in two orientations). (e) TEM image of guanine nanocrystals in D-iridophores. Scale bars, 20 mm ( c); 200 nm (d,e).[18]
Colour change in chameleons has functions incamouflage, but most commonly in social signaling and in 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.[19][20] Because chameleons areectothermic, 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.[21][22] Chameleons tend to show brighter colours when displaying aggression to other chameleons,[23] and darker colours when they submit or "give up".[24] Most chameleon genera (exceptions areChamaeleo,Rhampholeon andRieppeleon) have bluefluorescence in a species specific pattern in their skulltubercles and inBrookesia there is also some in tubercles on the body. The fluorescence is derived from bones that only are covered in very thin skin and it possibly serves a signaling role, especially in shaded habitats.[25]
Some species, such asSmith's dwarf chameleon and several others in the genusBradypodion, 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.[26][27] In the introduced Hawaiian population ofJackson'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.[28]
Chameleons have two superimposed layers within their skin that control their colour and thermoregulation. The top layer contains a lattice of guanine nanocrystals, and by exciting this lattice the spacing between the nanocrystals can be manipulated, which in turn affects which wavelengths of light are reflected and which are absorbed. Exciting the lattice increases the distance between the nanocrystals, and the skin reflects longer wavelengths of light. Thus, in a relaxed state the crystals reflect blue and green, but in an excited state the longer wavelengths such as yellow, orange, green, and red are reflected.[29]
The skin of a chameleon also contains some yellow pigments, which combined with the blue reflected by a relaxed crystal lattice results in the characteristic green colour which is common of many chameleons in their relaxed state. Chameleon colour palettes have evolved through evolution and the 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.[30]
The oldest described chameleon isAnqingosaurus brevicephalus from the Middle Paleocene (about 58.7–61.7mya) of China.[31]Other chameleon fossils includeChamaeleo caroliquarti from the Lower Miocene (about 13–23 mya) of the Czech Republic and Germany, andChamaeleo intermedius from the Upper Miocene (about 5–13 mya) of Kenya.[31]
The chameleons are probably far older than that, perhaps sharing a common ancestor withiguanids andagamids 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.[32] In fact, it has recently been shown that chameleons most likely originated in mainland Africa.[15] 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.[33]
Dazaet al. (2016) described a small (10.6 mm in snout-vent length), probablyneonatal lizard preserved in the Cretaceous (Albian-Cenomanian boundary) amber fromMyanmar. 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 astem-chamaeleonid.[34] However, Matsumoto &Evans (2018) reinterpreted this specimen as analbanerpetontid amphibian.[35] This specimen was given the nameYaksha perettii in 2020, and was noted to have several convergently chameleon-like features, including adaptations for ballistic feeding.[36]
Chameleon in Ghana
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.[37] 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.[37]
Description
Nearly all species of chameleon have prehensile tails, but they most often grip with the tail when they cannot use all four feet at once, such as when passing from one twig to another.
Chameleons vary greatly in size and body structure, with maximum total lengths varying from 22 mm (0.87 in) in maleBrookesia nana (one of the world'ssmallest reptiles) to 68.5 cm (27.0 in) in the maleFurcifer oustaleti.[38][39] Many have head or facial ornamentation, such as nasal protrusions, or horn-like projections in the case ofTrioceros jacksonii, or large crests on top of their heads, likeChamaeleo calyptratus. Many species aresexually 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:
The feet of chameleons are highly adapted toarboreal locomotion, and species such asChamaeleo namaquensis that have secondarily adopted a terrestrial habit have retained the same foot morphology with little modification. On each foot, the five distinguished toes are grouped into two fascicles. The toes in each fascicle are bound into a flattened group of either two or three, giving each foot atongs-like appearance. On the front feet, the outer,lateral, group contains two toes, whereas the inner,medial, group contains three. On the rear feet, this arrangement is reversed, the medial group containing two toes, and the lateral group three. These specialized feet allow chameleons to grip tightly onto narrow or rough branches. Furthermore, each toe is equipped with a sharp claw to afford a grip on surfaces such as bark when climbing. It is common to refer to the feet of chameleons asdidactyl orzygodactyl, though neither term is fully satisfactory, both being used in describing different feet, such as the zygodactyl feet of parrots or didactyl feet of sloths or ostriches, none of which is significantly like chameleon feet. Although "zygodactyl" is reasonably descriptive of chameleon foot anatomy, their foot structure does not resemble that of parrots, to which the term was first applied. As for didactyly, chameleons visibly have five toes on each foot, not two.
Some chameleons have acrest of small spikes extending along the spine from theproximal part of the tail to the neck; both the extent and size of the spikes vary between species and individuals. These spikes help break up the definitive outline of the chameleon, which aids it when trying to blend into a background.
Chameleons have the most distinctive eyes of any reptile. The upper and lower eyelids are joined, with only a pinhole large enough for the pupil to see through. Each eye can pivot and focus independently, allowing the chameleon to observe two different objects simultaneously. This gives them a full 360-degree arc of vision around their bodies. Prey is located using monoculardepth perception, notstereopsis.[40] Chameleons have the highest magnification (per size) of any vertebrate,[41] with the highest density ofcones in theretina.[42]
Likesnakes, chameleons do not have an outer or a middleear, so there is neither an ear-opening nor an eardrum. However, chameleons are not deaf: they can detect sound frequencies in the range of 200–600 Hz.[43]
Chameleons can see in both visible andultraviolet light.[44] Chameleons exposed to ultraviolet light show increased social behavior and activity levels, are more inclined to bask, feed, and reproduce as it has a positive effect on thepineal gland.
Feeding
All chameleons are primarilyinsectivores that feed byballistically projecting their longtongues from their mouths to capture prey located some distance away.[45] While the chameleons' tongues are typically thought to be one and a half to two times the length of their bodies (their length excluding the tail), smaller chameleons (both smaller species and smaller individuals of the same species) have recently been found to have proportionately larger tongue apparatuses than their larger counterparts.[46] Thus, smaller chameleons are able to project their tongues greater distances than the larger chameleons that are the subject of most studies and tongue length estimates, and can project their tongues more than twice their body length.[47]
The tongue apparatus consists of highly modifiedhyoid bones,tongue muscles, andcollagenous elements.[48][49][46][50] The hyoid bone has an elongated, parallel-sided projection, called the entoglossal process, over which a tubular muscle, the accelerator muscle, sits.[46][50][48][49] The accelerator muscle contracts around the entoglossal process and is responsible for creating the work to power tongue projection, both directly and through the loading of collagenous elements located between the entoglossal process and the accelerator muscle.[45][46][48][49] The tongue retractor muscle, the hyoglossus, connects the hyoid and accelerator muscle, and is responsible for drawing the tongue back into the mouth following tongue projection.[45][46][50][48]
Tongue projection occurs at extremely high performance, reaching the prey in as little as 0.07 seconds,[48][49][51] having been launched at accelerations exceeding 41g.[51] Thepower with which the tongue is launched, known to exceed 3000 W kg−1, exceeds that which muscle is able to produce, indicating the presence of an elastic power amplifier to power tongue projection.[49] The recoil of elastic elements in the tongue apparatus is thus responsible for large percentages of the overall tongue projection performance.
One consequence of the incorporation of an elastic recoil mechanism to the tongue projection mechanism is relative thermal insensitivity of tongue projection relative to tongue retraction, which is powered by muscle contraction alone, and is heavily thermally sensitive.[51][52] While otherectothermic animals become sluggish as their body temperatures decline, due to a reduction in the contractile velocity of their muscles, chameleons are able to project their tongues at high performance even at low body temperatures.[51][52] The thermal sensitivity of tongue retraction in chameleons, however, is not a problem, as chameleons have a very effective mechanism of holding onto their prey once the tongue has come into contact with it, including surface phenomena, such as wet adhesion and interlocking, and suction.[53] The thermal insensitivity of tongue projection thus enables chameleons to feed effectively on cold mornings prior to being able to behaviorally elevate their body temperatures throughthermoregulation, when othersympatric lizards species are still inactive, likely temporarily expanding their thermalniche as a result.[51]
Use of tongue in feeding
Tongue structure, with cup-like end
Tongue begins strike
Capturing prey
Bringing prey to the mouth
Bones
Certain species of chameleons have bones that glow when underultraviolet light, also known asbiogenicfluorescence.[25] Some 31 different species ofCalumma chameleons, all native toMadagascar, displayed this fluorescence inCT scans.[54] The bones emitted a bright blue glow and could even shine through the chameleon's four layers of skin.[54] The face was found to have a different glow, appearing as dots otherwise known astubercles on facial bones.[25] The glow results fromproteins,pigments,chitin, and other materials that make up a chameleon's skeleton,[25] possibly giving chameleons a secondary signaling system that does not interfere with their colour-changing ability, and may have evolved fromsexual selection.[25]
Distribution and habitat
Brookesia minima, Lokobe Strict Reserve. The 30 species of chameleons in the genusBrookesia are tiny, usually brown-colored and mainly terrestrial.
Chameleons are found only intropical andsubtropical regions and inhabit all kinds of lowland and mountain forests, woodlands, shrublands,savannas, and sometimesdeserts, but each species tends to be a restricted to only one of a few different habitat types. The typical chameleons from the subfamily Chamaeleoninae arearboreal, usually living in trees or bushes, although a few (notably theNamaqua chameleon) are partially or largelyterrestrial. The genusBrookesia, which comprises the majority of the species in the subfamily Brookesiinae, live low in vegetation or on the ground amongleaf litter. Many chameleon species have small distributions and are considered threatened. Declining chameleon numbers are mostly due to habitat loss.[55]
Image published in the Manual Training Highschool Annuel, 1900
Reproduction
Most chameleons areoviparous, but allBradypodion species and manyTrioceros species areovoviviparous (although some biologists prefer to avoid the term ovoviviparous because of inconsistencies with its use in some animal groups, instead just usingviviparous).[56]
The oviparous species lay eggs three to six weeks aftercopulation. The female will dig a hole—from 10–30 cm (4–12 in), deep depending on the species—and deposit her eggs. Clutch sizes vary greatly with species. SmallBrookesia species may only lay two to four eggs, while largeveiled chameleons (Chamaeleo calyptratus) have been known to lay clutches of 20–200 (veiled chameleons) and 10–40 (panther chameleons) eggs. Clutch sizes can also vary greatly among the same species. Eggs generally hatch after four to 12 months, again depending on the species. The eggs ofParson's chameleon (Calumma parsoni) typically take 400 to 660 days to hatch.[57]
Chameleons lay flexible-shelled eggs which are affected by environmental characteristics during incubation. The egg mass is the most important in differentiating survivors of Chameleon during incubation. An increase in egg mass will depend on temperature and water potential.[58] To understand the dynamics of water potential in Chameleon eggs, the consideration of exerted pressure on eggshells will be essential because the pressure of eggshells play an important role in the water relation of eggs during entire incubation period[59]
The ovoviviparous species, such as theJackson's chameleon (Trioceros jacksonii) have a five- to seven-month gestation period. Each young chameleon is born within the sticky transparent membrane of its yolk sac. The mother presses each egg onto a branch, where it sticks. The membrane bursts and the newly hatched chameleon frees itself and climbs away to hunt for itself and hide from predators. The female can have up to 30 live young from one gestation.[60]
Diet
Chameleons generally eatinsects, but larger species, such as the common chameleon, may also take other lizards and youngbirds.[61]: 5 The range of diets can be seen from the following examples:
The veiled chameleon,Chamaeleo calyptratus fromArabia, is insectivorous, but eats leaves when other sources of water are not available. It can be maintained on a diet ofcrickets.[62] They can eat as many as 15–50 large crickets a day.
Jackson's chameleon (Trioceros jacksonii) from Kenya and northern Tanzania eat a wide variety of small animals including ants, butterflies, caterpillars, snails, worms, lizards, geckos, amphibians, and other chameleons, as well as plant material, such as leaves, tender shoots, and berries. It can be maintained on a mixed diet including kale, dandelion leaves, lettuce, bananas, tomatoes, apples, crickets, and waxworms.[60]
The common chameleon of Europe, North Africa, and the Near East,Chamaeleo chamaeleon, mainly eatswasps andmantises; sucharthropods form over three-quarters of its diet.[61]: 5 Some experts advise that the common chameleon should not be fed exclusively on crickets; these should make up no more than half the diet, with the rest a mixture ofwaxworms,earthworms,grasshoppers,flies, and plant materials such as green leaves, oats, and fruit.[61]: 5–6
Some chameleons like thepanther chameleon ofMadagascar regulate theirvitamin D3 levels, of which their insect diet is a poor source, by exposing themselves tosunlight since itsUV component increases internal production.[63]
Anti-predator adaptations
Chameleons are preyed upon by a variety of other animals.Birds andsnakes are the most importantpredators of adult chameleons. Invertebrates, especially ants, put a high predation pressure on chameleon eggs and juveniles.[64] Chameleons are unlikely to be able to flee from predators and rely oncrypsis as their primary defense.[65] Chameleons can change both their colours and their patterns (to varying extents) to resemble their surroundings or disrupt the body outline and remain hidden from a potential enemy's sight. Only when detected do chameleons actively defend themselves. They adopt a defensive body posture, present an attacker with a laterally flattened body to appear larger, warn with an open mouth, and, if needed, utilize feet and jaws to fight back.[66] Vocalization is sometimes incorporated into threat displays.[64]
Chameleons areparasitized bynematode worms, including threadworms (Filarioidea). Threadworms can be transmitted by biting insects such asticks andmosquitoes. Other roundworms are transmitted through food contaminated with roundworm eggs; the larvae burrow through the wall of theintestine into the bloodstream.[67]
Chameleons are subject to several protozoan parasites, such asPlasmodium, which causesmalaria,Trypanosoma, which causessleeping sickness, andLeishmania, which causesleishmaniasis.[68]
Chameleons are popular reptile pets, mostly imported from African countries like Madagascar, Tanzania, and Togo.[70] The most common in the trade are the Senegal chameleon (Chamaeleo senegalensis), the Yemen or veiled chameleon (Chamaeleo calyptratus), the panther chameleon (Furcifer pardalis), and Jackson's chameleon (Trioceros jacksonii).[70] Other chameleons seen in captivity (albeit on an irregular basis) include such species as the carpet chameleon (Furcifer lateralis),Meller’s chameleon (Trioceros melleri), Parson’s chameleon (Calumma parsonii), and several species of pygmy and leaf-tailed chameleons, mostly of the generaBrookesia,Rhampholeon, orRieppeleon. These are among the most sensitive reptiles one can own, requiring specialized attention and care.
The U.S. has been the main importer of chameleons since the early 1980s accounting for 69% of African reptile exports.[70] However, there have been large declines due to tougher regulations to protect species from being taken from the wild and due to many becoming invasive in places like Florida.[70] They have remained popular though which may be due to the captive-breeding in the U.S. which has increased to the point that the U.S. can fulfill its demand, and has now even become a major exporter as well.[70] In the U.S. they are so popular, that despite Florida having six invasive chameleon species due to the pet trade, reptile hobbyists in these areas search for chameleons to keep as pets or to breed and sell them, with some selling for up to a thousand dollars.[3]
Historical understandings
Chameleon in a German translation of Gessner's book (1563).
In Shakespeare'sHamlet, the eponymous Prince says "Excellent, i' faith, of the chameleon's dish. I eat the air, promise-crammed." This refers to the Elizabethan belief that chameleons lived on nothing but the air.
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^"Chamaeleon News".Chameleonnews.com. August 2004. Archived from the original on 22 January 2008. Retrieved1 November 2017.{{cite web}}: CS1 maint: bot: original URL status unknown (link)
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^abcdeAnderson, C. V.; Sheridan, T.; Deban, S. M. (2012). "Scaling of the ballistic tongue apparatus in chameleons".Journal of Morphology.273 (11):1214–1226.doi:10.1002/jmor.20053.PMID22730103.S2CID21033176.
^abcdeHerrel, A.; Meyers, J. J.; Nishikawa, K. C.; De Vree, F. (2001). "Morphology and histochemistry of the hyolingual apparatus in chameleons".Journal of Morphology.249 (2):154–170.doi:10.1002/jmor.1047.PMID11466743.S2CID3246256.
^abcAnderson, C. V.; Higham, T. E. (2014), "Chameleon anatomy", in Tolley, K. A.; Herrel, A. (eds.),The Biology of Chameleons, Berkeley, CA: University of California Press, pp. 7–55,ISBN9780520276055
^Diaz-Paniagua C, Cuadrado M (2003), "Influence of incubation conditions on hatching success, embryo development and hatchling phenotype of common chameleon (Chamaeleo chamaeleon) eggs",Amphibia-Reptilia,24 (4):429–440,doi:10.1163/156853803322763891
^Andrews (2008), "Effects of incubation temperature on growth and performance of the veiled chameleon (Chamaeleo calyptratus",Journal of Experimental Zoology. Part A, Ecological Genetics and Physiology,309 (8), Journal of Experimental Zooly:435–446,Bibcode:2008JEZA..309..435A,doi:10.1002/jez.470,PMID18512704
^Karsten, K. B.; Ferguson G. W.; Chen T. C.; Holick M. F. (2009). "Panther chameleons, Furcifer pardalis, behaviorally regulate optimal exposure to UV depending on dietary vitamin D3 status".Physiol. Biochem. Zool.82 (3):218–25.doi:10.1086/597525.PMID19335229.S2CID205990383.
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^Berg, Philipp; Berg, Jessica; Berg, Rainer (2020). "Predator–prey interaction between a boomslang, Dispholidus types, and a flap-necked chameleon, Chamaeleo dilepis".African Journal of Ecology.58 (4):855–859.doi:10.1111/aje.12782.S2CID225209615.
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Davison, Linda J.Chameleons: Their Care and Breeding. Hancock House Publishers, 1997.
de Vosjoli, Philippe.Essential Care of Chameleons. Advanced Vivarium Systems, 2004.
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