Chameleon

Chameleons (family Chamaeleonidae) are a special group of lizards from the Old World. They belong to a clade, a group of animals that share a common ancestor. As of June 2015, there were 200 known species. Chameleons are best known for their many colours. They can change colour as a kind of camouflage, either to hide or to send signals to other chameleons.

Each species in this family changes colour in its own way. Some only get lighter or darker, moving between shades of brown. Others can show many colours, including reds, yellows, greens, and blues.

Chameleons have many unusual features. Their feet are zygodactylous, which means two toes point forward and two point backward. They have a prehensile tail that can grip branches. Their bodies are flat, as if they have been squeezed from the sides, and many have bony head shapes called casques. They also have long tongues that shoot out to catch food. Their eyes can move separately, so the brain can see two different pictures at once. When hunting, the eyes focus forward together, which gives them stereoscopic vision, or depth perception.

Chameleons are diurnal, which means they are active during the day. They hunt by sight and mostly eat insects, although large species can also catch small animals with backbones. Most chameleons live in trees, which is called being arboreal, but many species live on the ground. Tree-dwelling chameleons use their tail as an extra anchor, so it is often called a "fifth limb." Chameleons live in many places, from rainforest to desert, and from lowlands to highlands. Most live in Africa, and about half of all species are found only on Madagascar. One species lives in southern Europe, and a few live across southern Asia as far east as India and Sri Lanka. Chameleons have also been introduced to Hawaii and Florida.

For other uses, see Chameleon (disambiguation).

Etymology

The English word chameleon comes from the Latin word chamaeleōn. That Latin word came from the Greek word χαμαιλέων (khamailéōn). It is a compound word, made from two parts: χαμαί (khamaí), meaning "on the ground," and λέων (léōn), meaning "lion."

Classification

In 1986, the chameleon family was split into two smaller groups called subfamilies, Brookesiinae and Chamaeleoninae. Brookesiinae included Brookesia and Rhampholeon, along with Palleon and Rieppeleon, which had been split off from them. Chamaeleoninae included Bradypodion, Calumma, Chamaeleo, Furcifer and Trioceros, along with Archaius, Nadzikambia and Kinyongia, which had also been split off from them. Since then, scientists have argued about whether these subfamilies make sense. Most studies suggest that the pygmy chameleons in Brookesiinae are not a monophyletic group.

Some experts used this system for a while and then stopped, so the chameleon family was no longer split into subfamilies. In 2015, however, Glaw changed the system again. Only Brookesia and Palleon were put in Brookesiinae, and all the other groups were put in Chamaeleoninae.

Further information: Animal colouration, Chromatophore, and Signaling theory

Some chameleons can change their skin colouration. Their skin has layers of special cells called dermal chromatophores. From the surface down, there are "xanthophores" and "erythrophores" (yellow and red). In the middle is a layer of iridophores with clear, lattice-shaped guanine nanocrystals that make structural colour by bending light. At the bottom are "melanophores" with dark melanin. The colour we see comes from light passing through all these layers at once. Two layers of iridophores control colour change. The top layer has small guanine crystals in a triangular photonic crystal lattice, and the deeper layer has larger crystals that reflect light well, especially in the near-infrared range. A chameleon can change the spacing between these crystals, which changes which colours of light are reflected. When the crystals are close together, the lattice reflects blue light, which mixes with the yellow xanthophores to make green. When the spacing changes, the reflected wavelengths can quickly shift to yellow, orange, red, or back to blue. Together with the normal pigment in the other layers, this makes the final colour. Scientists do not yet fully know how chameleons adjust these crystals. Iridophores share the neural crest with other chromatophores, which are controlled by nerve or hormone signals, so a similar signal is thought to trigger the changes, though this is not certain.

Colour change helps with camouflage, but it is most often used to send signals to other chameleons and to respond to temperature and other conditions. Chameleons are ectothermic, so they change colour to control their body temperature: darker colours absorb more heat, while lighter colours reflect it. Brighter colours often show aggression, and darker colours often show submission or "giving up." Most chameleon genera (except Chamaeleo, Rhampholeon and Rieppeleon) have blue fluorescence on their skull tubercles, in a pattern unique to each species, and Brookesia also has some on its body tubercles. This glow comes from bones covered by very thin skin and may help with signalling, especially in shady places. Some species, such as Smith's dwarf chameleon and others in the genus Bradypodion, change colour for camouflage based on the type of predator, such as a bird or snake. In the Hawaiian population of Jackson's chameleon, which was introduced there, signalling colour changes have increased, while anti-predator camouflage changes have decreased compared with the native population in Kenya, where there are more predators. Chameleon colours also vary by habitat. Forest chameleons have brighter, more defined colours, while those in deserts or savannas have more basic brown and charred shades.

Evolution

The oldest described chameleon is Anqingosaurus brevicephalus. It lived in China during the Middle Paleocene, about 58.7 to 61.7 mya. Other fossils, such as Chamaeleo caroliquarti, have been found in Europe. Chameleons are probably much older. They may have shared a common ancestor with iguanids and agamids more than 100 mya. Fossils found in Africa, Europe, and Asia show that chameleons were once more widespread. Today, nearly half of chameleon species live in Madagascar, but chameleons most likely began in mainland Africa. Several studies support that the family is a monophyly group.

In 2016, Daza et al. described a tiny lizard, probably newborn, preserved in amber from the Cretaceous (Albian-Cenomanian boundary) in Myanmar. The authors thought it was a stem chamaeleonid. Later, Matsumoto & Evans reinterpreted it as an albanerpetontid amphibian. In 2020, the specimen was named Yaksha perettii.

Scientists have studied how colour change in chameleons developed. Dwarf chameleons in brighter areas tended to show brighter signals. Those in darker areas tended to show signals that contrasted more with their backgrounds. This suggests that habitat has shaped how chameleons use colour signals. Some chameleon species give birth to live young (viviparity). This trait evolved twice, and both developments are believed to have happened by the Early Oligocene, about 30 to 29 million years ago.

Description

Chameleons come in many sizes and shapes. The smallest, a male Brookesia nana, is only 22 mm (0.87 in) long. It is one of the world's smallest reptiles. The largest, a male Furcifer oustaleti, can grow to 68.5 cm (27.0 in). Many have head decorations, such as bumps on the nose, horn-like points on Trioceros jacksonii, or a large crest on the head of Chamaeleo calyptratus. Males and females often look different, and males are usually much more decorated. Here are typical sizes of chameleons kept as pets:

Chameleon feet are good for climbing in trees. Each foot has five toes split into two groups, so it looks a bit like tongs. This helps the animal grip thin or rough branches. Sharp claws help them hold onto bark. Some chameleons also have a crest of small spikes along their backs, running from the proximal part of the tail to the neck. This helps break up their outline. Their eyelids are joined, leaving only a small opening. Each eye moves and focuses on its own, so they can look at two things at once. They can see ultraviolet light and use cones in the retina to see well. Like snakes, they have no outer or middle ear, but they can still hear some sounds.

Chameleons mostly eat insectivores, which are insects. They catch prey by quickly shooting their long tongues out of their mouths. Their tongues are powered by hyoid bones, tongue muscles, and collagenous elements. The tongue can reach prey in a very short time, less than a tenth of a second. Some chameleons have bones that glow under ultraviolet light. This is called biogenic fluorescence. Scientists saw this glow in CT scans of many chameleons from Madagascar.

Distribution and habitat

Chameleons mostly live in the mainland of sub-Saharan Africa and on the island of Madagascar. A few kinds live in northern Africa, southern Europe (Portugal, Spain, Italy, Greece, Cyprus and Malta), the Middle East, southeast Pakistan, India, Sri Lanka, and some smaller islands in the western Indian Ocean. Introduced chameleons, which did not originally come from these places, live in Hawaii and Florida.

Chameleons live only in warm places, in subtropical and tropical areas. They live in forests, woodlands, shrublands, savannas, and sometimes deserts. Each kind usually lives in just one or a few habitat types. Most chameleons in the subfamily Chamaeleoninae live in trees or bushes. This is called being arboreal. A few, such as the Namaqua chameleon, spend most or part of their time on the ground. This is called being terrestrial. The genus Brookesia includes most of the species in the subfamily Brookesiinae. These chameleons live low in plants or on the ground among leaf litter. Many chameleon kinds live in only small areas, and some are in danger of disappearing. Their numbers are falling, mostly because their homes are being lost.

Reproduction

Most chameleons are oviparous, which means they lay eggs. All Bradypodion species and many Trioceros species are ovoviviparous. This means their babies grow inside the mother's body until they are ready to hatch.

Egg-laying chameleons lay their eggs about three to six weeks after copulation. The female digs a hole, 10 to 30 cm deep, and puts her eggs inside. The number of eggs can be very different. Small Brookesia species may lay only two to four eggs. Large veiled chameleons can lay 20 to 200 eggs. Most eggs hatch after four to 12 months. The eggs of Parson's chameleon usually take 400 to 660 days.

Live-bearing species, such as Jackson's chameleon, are pregnant for five to seven months. Each baby is born inside a sticky, clear covering. The mother presses each baby onto a branch, where it sticks. The covering then breaks open, and the baby chameleon frees itself and climbs away. It must hunt for food and hide from animals that might eat it. A female can have up to 30 live babies from one pregnancy.

Diet

Chameleons usually eat insects, but larger species, such as the common chameleon, may also eat other lizards and young birds. Different species eat different foods. The veiled chameleon from Arabia mostly eats insects. When other water sources are not available, it eats leaves. It can be kept on a diet of crickets, and it can eat 15 to 50 large crickets a day. Jackson's chameleon (Trioceros jacksonii) lives in Kenya and northern Tanzania. It eats many small animals, such as ants, butterflies, snails, and other chameleons. It also eats plants like leaves, shoots, and berries.

The common chameleon (Chamaeleo chamaeleon) lives in Europe, North Africa, and the Near East. It mainly eats wasps and mantises. These are arthropods, and they make up over three-quarters of its diet. Some experts advise that crickets should be no more than half of its diet. The rest can include waxworms, earthworms, grasshoppers, flies, and plants such as green leaves, oats, and fruit. Some chameleons, like the panther chameleon of Madagascar, get vitamin D3 from sunlight. Their insect diet is a poor source of this vitamin, so they sit in the sun. UV rays help their bodies make more of it. The sunlight itself is sunlight.

Anti-predator adaptations

Many animals hunt chameleons. Birds and snakes are the most important predators of adult chameleons. Insects, especially ants, can be a danger to chameleon eggs and young chameleons. Chameleons cannot run away quickly, so they mostly rely on hiding. They can change their colours and patterns, to different degrees, to look like their surroundings or to hide the shape of their body from enemies.

Chameleons only defend themselves when they are spotted. They take on a threatening body posture and make their body look bigger by flattening it from side to side. They warn attackers by opening their mouths and, if needed, use their feet and jaws to fight back. Sometimes they also make sounds as part of their threat display.

Parasites

Chameleons can get sick from parasites called nematode worms, including threadworms (Filarioidea). Threadworms can spread through biting insects such as ticks and mosquitoes. Other roundworms spread through food that contains their eggs. The young worms move through the intestine wall and into the bloodstream.

Chameleons can also have tiny one-celled parasites. Plasmodium causes malaria, Trypanosoma causes sleeping sickness, and Leishmania causes leishmaniasis. Chameleons can also get infected by coccidia, including types from the groups Choleoeimeria, Eimeria, and Isospora.

As pets

See also: Chameleon ranching

Chameleons are popular pets. Most are brought in from African countries like Madagascar, Tanzania, and Togo. The most common kinds sold as pets are the Senegal chameleon (Chamaeleo senegalensis), the Yemen or veiled chameleon (Chamaeleo calyptratus), the panther chameleon (Furcifer pardalis), and Jackson's chameleon (Trioceros jacksonii). Some other kinds are kept as pets now and then, such as the carpet chameleon (Furcifer lateralis), Meller's chameleon (Trioceros melleri), Parson's chameleon (Calumma parsonii), and several small pygmy and leaf-tailed chameleons. These are among the most sensitive reptiles to keep, so they need special care.

The U.S. has been the main importer of chameleons since the early 1980s, bringing in a large share of African reptile exports. However, imports have fallen a lot. Stricter rules now protect wild species, and many chameleons have become invasive in places like Florida. Chameleons are still popular, perhaps because many are now bred in captivity in the U.S. This breeding has grown so much that the U.S. can meet its own demand and has even become a major exporter. Chameleons are still in demand in the U.S., and hobbyists there look for them to keep or to breed and sell. Florida already has six invasive chameleon species from the pet trade. Some chameleons sell for up to a thousand dollars.

Historical understandings

Aristotle (4th century BC) wrote about chameleons in his History of Animals. Pliny the Elder (1st century AD) also wrote about them in his Natural History. He said that chameleons can change colour to hide themselves.

The chameleon appeared in Conrad Gessner's Historia animalium (1563). Gessner copied much of it from De aquatilibus (1553) by Pierre Belon. In Shakespeare's Hamlet, the Prince says, "I eat the air, promise-crammed." This line comes from an old belief in Shakespeare's time that chameleons lived on nothing but air.