Arxiu d'etiquetes: namaqua chameleon

Colour change in chamaleons: an emotional rainbow

Many people consider chameleons to be the masters of camouflage. Their ability to change colour leads us to believe that these animals have evolved to blend in with their surroundings and to trick their predators. But, what would you think if I told you that camouflage isn’t the main reason for colour shifts in chameleons? In this new entry, apart from explaining how chameleons change their coloration, we’ll show you how these cryptic animals use colour change for a wide array of reasons.

MYTHS ABOUT CHAMELEONS

Chameleons (Chamaeleonidae family) are extremely cryptic lizards, as their coloration is usually very similar to that of their habitat’s. Also, many chameleon species present the ability to actively shift their colours, making their camouflage even more complex.

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Usambara soft-horned chameleon female (Kinyongia tenuis) displaying striking colouring. Photo by Keultjes.

There is much misunderstanding regarding chameleons’ colour changing abilities. Here you have some refuted myths about chameleons:

  • The different chameleon species can only change into a limited range of colours.
  • Chameleons do not change their coloration rapidly, as they do it subtly. If they did, they would be much easier to spot by their predators.
  • Chameleons don’t change their colours depending on what they are touching but, as we’ll see below, their reasons are much more complex.

Video from Viralweek which gives a wrong idea about how a veiled chameleon changes its colours (Chamaeleo calyptratus).

But, how do chameleons change their colours? Many other animals, like cephalopods and some fish and lizards, also have the capacity to shift colours. In most cases it is achieved using chromatophores, a type of pigmentary cell found on ectothermic animals. In colour-changing animals, chromatophores are distributed in multiple layers and have the ability to contract, expand, aggregate or disperse, causing different colour variations.

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Detail of a cuttlefish chromatophores, by Minette Layne. Depending on whether they contract or expand, different colours can be appreciated.

For a long time it was thought that chameleons changed their colours using only their chromatophores. But a recent study showed that chameleons bring colour change to the extreme. This study was being conducted by a team of biologists and physicists when they noticed something special: chameleons do not present any green pigment in their skin!

PIGMENTS AND CRYSTALS

In order to explain how chameleons change colours, first we must distinguish between two different kinds of coloration in animals: pigmentary and structural colour. Pigmentary colour is the commonest, as it’s the one that an organism presents due to pigments present in their tissues (such as melanin in human skin). Instead, as we explained in a former article, structural colour is generated by the refraction of light with some skin microstructures.

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Image of an upside down beetle in which various structural colours can be seen. Photo by David López.

And what happens with chameleons? Well, it’s a combination of both mechanisms. Chameleons present black, red and yellow chromatophores, which they can contract and expand voluntarily. Also, in a study conducted with panther chameleons (Furcifer pardalis), it’s been proved that they also present two layers of guanine nanocrystal-bearing cells, called iridiophores, which reflect light. Then a chameleon’s green coloration is acquired by the blue light reflected by the iridiophores that goes through the outer yellow chromatophores.

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Scheme of a chameleon’s skin section in which the iridiophores (blue) with nanocrystal layers and the different kinds of chromatophores can be seen; xanthophores (yellow), erythrophores (red) and melanophores (black). Image by David López.

Chameleons also present a series of neural circuits that allow them to control de composition and the distance between the iridiophores’ nanocrystals in different parts of their skin. This allows them to control the wavelength of the light reflected by the iridiophores and so, the colour. Combined with the chromatophores, the different chameleon species can cover most of the visible spectrum of colours.

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Differences in the colouring of a panther chameleon when it’s relaxed and excited, and its relation with the composition and distribution of the iridiophore nanocrystals. Image extracte from Teyssier & Saenko.

CHANGING COLOURS FOR WHAT?

Even if there are other squamosal species that can shift colours, this usually is because of a physiological response to thermoregulation, excitement or changes related to reproduction. Chameleons, also have an important part of their nervous system dedicated to changing colour rapidly, consciously and reversibly. They can even change different skin regions to different colours and, while one region becomes more orange or red, another one becomes more bluish or whitish, creating pretty striking colour effects and contrasts.

But then, why do chameleons change their colours? Well, the truth is that the kaleidoscopic abilities of these lizards have different functions, varying among the different species.

CAMOUFLAGE

The most obvious motive (even if not the most important) is camouflage. Even if the standard coloration of most chameleon species is cryptic enough, in case of necessity chameleons are able to blend in even more with their surroundings. This helps them not to be detected by their prey, but mainly to go unnoticed by their predators.

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Mediterranean chameleon (Chamaeleo chamaeleon) perfectly blending in with its surrounding. Photo by Javier Ábalos Álvarez.

Also, in a study conducted with Smith’s dwarf chameleons (Bradypodion taeniabronchum), is was proved that these were able to adjust the degree of their colour shifts to the visual capacities of their predators. Birds and snakes both feed on chameleons but, while the former have a great perception of shapes and colours, the latter doesn’t have such a sharp vision. It’s seen that Smith’s dwarf chameleons show more convincing colour changes when faced with a predator bird, than they do when faced with a snake.

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Photos of a Smith’s dwarf chameleon blending in when facing two decoy predators, a shrike and a mamba. Photo by Devi Stuart-Fox.

THERMOREGULATION

Chameleons are ectothermic and, like most reptiles, depend on external sources of heat. Apart from the more superficial iridiophores (called S-iridiophores), chameleons have a deeper layer of iridiophores called D-iridiophores, which (even if they present a much messier nanocrystal structure that cannot be modified) highly reflect infrared light, and it is thought that they must have some thermoregulation-related function. Many other lizards also present an iridiophore layer similar to D-iridiophores.

Apart from D-iridiophores, chameleons also shift to darker or lighter colours in order to regulate their body temperature. This becomes more apparent in species that live in habitats with more extreme climates. As we explained in an earlier entry, the Namaqua chameleon (Chamaeleo namaquensis), which inhabits deserts in south-western Africa, presents an almost black colour during the early morning hours, in order to absorb the maximum heat, while during the hottest hours it shows a whitish coloration, in order to reflect the maximum solar radiation.

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Two different coloration patterns in a Namaqua chameleon, a lighter one (photo by Hans Stieglitz) and a darker one (photo by Laika ac).

COMMUNICATION

The main function of chameleons colour change is intraspecific communication. Chameleons use different colour patterns known as liveries in some countries, which are changed in order to transmit information to other individuals of their same species like their stress degree, their reproductive or health status, etc… A chameleon’s standard coloration is usually similar to that of their habitat. So, this colour pattern usually indicates a healthy animal, while if they feel sick or have some physical problem, they usually present paler and duller colorations.

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Dominance and submission patterns on three dwarf chameleon species (Bradypodion sp.) Image from Adnan Moussalli & Devi Stuart-Fox.

In many species, females present more conspicuous and contrasted patterns when they are in heat, while they show a darker coloration after mating. When seeing these signals, males know which females are available and with which females they should better save their energy. Males also present more eye-catching patterns during the mating season, in order to indicate their intentions to females and to warn their rivals.

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Female carpet chameleon (Furcifer lateralis) with a pattern that indicates that it’s already pregnant and that it has no interest in mating. Photo by Bernard Dupont.

Finally, outside mating season, all chameleons use their boldest colours during their encounters with rivals of their same species. It’s in these situations when chameleons show the most contrasted patterns, apart from inflating and looking bigger and more aggressive, in order to scare off their rivals.

Video of a panther chameleon (Furcifer pardalis) acting aggressively when presented with a “rival”. Video from The White Mike Posner.

As we’ve just seen, the variety of colorations among the distinct chameleon species is huge. Yet, their incredible abilities haven’t saved chameleons from being on the endangered species list, as many of them are in danger of extinction, mainly because of the destruction of their habitat due to the logging industry and because of poaching for the illegal exotic animal trade. We hope that with a better awareness of these spectacular and colourful lizards, future generations can still delight with chameleon colour shifts for a long time.

REFERENCES

The following sources have been consulted during the elaboration of this entry:

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Desert reptiles

Deserts are some of the most extreme habitats on the planet. The Sahara, the Gobi and the Sonora are some examples of warm deserts where the high temperatures and the lack of water pose a great challenge to animals that live in them. Reptiles are one of the animal groups that present the most incredible adaptations for life in deserts. In this entry we’ll explain the difficulties that desert reptiles must face in order to survive, and we’ll introduce you to different species of snakes and lizards that in the deserts have found their home.

REPTILES IN THE DESERT

The characteristic which unites all deserts is the scarce precipitation as, unlike most people think, not all deserts present high temperatures (there are also cold deserts, like the Arctic and the Antarctic, both in danger because of the climate change). Reptiles thrive better in warm deserts than in cold deserts, because the low temperatures would not allow them to develop their life activity.

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Map by Vzb83 of the warm deserts, both arid and semiarid, of the world.

Warm deserts not always have extremely high temperatures. While during the day temperatures may rise up to 45°C, when the sun goes down temperatures fall below freezing point, creating daily oscillations of up to 22°C. The different desert reptiles, being poikilotherms and ectotherms, use different behavioural strategies in order to avoid overheating during the day and to keep their body heat during the night (for example, climbing to elevated areas or sleeping in burrows).

The Namaqua chameleon (Chamaleo namaquensis) regulates its body heat changing its colouration. During sunrise it is black in colour in order to absorb as much radiation of the sun and activate its metabolism. When temperatures become higher, it turns white to reflect solar radiation. Video from BBC.

As we have already stated, the main characteristic of any desert is the lack of water. Generally, in a desert, it rains less than 250 mm of water a year. The scaly and impervious skin of reptiles prevents the loss of water, and their faeces contain uric acid which, compared to urea, is much less soluble in water, allowing them to retain more liquids. Most desert reptiles extract the water they need from their food and some drink water from the dew.

Both the extreme temperatures and the shortage of precipitations make the desert a place with very few living beings. Vegetation is scarce and animals are usually small and secretive. This lack of resources causes desert reptiles to be usually smaller than their cousins from more benevolent environments. Also, these saurians usually exploit any available food resource, although they think twice before wasting their precious energy to get their next meal.

SAND SNAKES

In many sandy deserts we can find various species of snakes (and legless lizards) that have adapted to a life among the dunes. Many of these ophidians share a locomotion method called “sidewinding”, in which they raise their head and neck from the ground and move them laterally while the rest of the body stays on the ground. When they place their head on the ground again they raise their body, making these snakes move laterally in a 45° angle. This method of locomotion makes these snakes move more efficiently in an unstable terrain. It also reduces the contact of their body with an extremely hot substrate, as the body of these ophidians only touches the ground in two points at a time.

As we can see in this video from RoyalPanthera, sidewinding allows desert snakes to move minimizing the contact with the hot terrain.

Many desert ophidians bury themselves in the sand both to avoid sun exposure and to blend in and catch their prey unaware. This has made many desert-dwelling snakes very sensitive to vibrations generated by their prey as it moves through the sand. In addition some species present an overly developed rostral scale (the scale at the tip of their snout), being much thicker in order to aid during excavation in sandy soils.

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An example of this are the North American snakes of the Heterodon genus, also known as hog-nosed snakes, as they present an elevated rostral scale giving their snout a characteristic shape. Photo of Heterodon nasicus by Dawson.

The horned vipers of the Cerastes genus also present various characteristics that facilitate life in the deserts. These vipers evade high temperatures becoming active at night and they spend the day buried in the sand. Their hunting method consists in burying themselves waiting for a prey to pass by, this way saving most of their energy. It is believed that their horn-shaped supraocular scales cover their eyes when they are buried in order to protect them from the sand.

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Photo by Tambako The Jaguar of a Sahara sand viper (Cerastes vipera), a species from North Africa and the Sinai Peninsula.

SPINY CRITTERS

In different deserts of the world we find reptiles with their bodies covered in spines. This not only provides them with certain protection against predators, but is also helps them blend in in a habitat with plenty of thorny plants. Two of these animals are members of the Iguania suborder: the thorny devil and the horned lizards.

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Photo of a thorny devil (Moloch horridus) by Christopher Watson.

The thorny devil (Moloch horridus) is an agamid that lives in the Australian sandy deserts. This lizard presents spines all over its body, making it difficult for its predators to swallow. It also has a protuberance behind its head that acts as a fat storage.  When it feels threatened, it hides its real head between its legs and it exposes its neck protuberance as a decoy head. Probably, the most interesting adaptation of this animal is the system of small grooves among its scales, which collect any water that contacts its skin and conducts it directly to its mouth.

Horned lizards (Phrynosoma genus, affectionately called “horny toads”) are iguanids which are found in different arid habitats of North America. Similarly to the thorny devil, their body is covered in spines making them hard to eat for their predators. Also, when they are caught, they inflate their bodies to make the task even more difficult. Finally, some species like the Texas horned lizard (Phrynosoma cornutum) are known for their autohaemorrhagic abilities: when they feel cornered they squirt a stream of stinky blood from their eyes which scares away most predators.

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Photo from the U.S. Fish & Wildlife Service of a Texan horned lizard (Phrynosoma cornutum).

As you have seen, in the deserts we can find reptiles with some of the most inventive (and disturbing) adaptations of the world. These are only a few examples of the astonishing diversity of squamates that are found in the deserts of the world, which only seek to survive the harsh conditions of these extreme environments. Sometimes, it’s just a matter to avoid burning your feet with the hot sand.

Video from BBCWorldwide of a shovel snouted lizard (Zeros anchietae) making the “thermal dance” in order to diminish the contact with the hot sand.

REFERENCES

The following sources have been consulted during the elaboration of this entry:

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