There is a huge amount of variety in the colours and patterns exhibited by plants and animals. However, most of this variation is fixed at the individual level; only when comparing individuals do we see differences. The ability to change your colour during your lifetime is a trait possessed by only a few animals, which have converged on remarkably similar mechanisms. Colour changes that occur during an animal’s lifetime can occur slowly, with seasonal changes or age (morphological colour change). More dramatically, and more interestingly, some species also have the ability to change their colour or pattern very rapidly, in response to environmental or social conditions (physiological colour change).
For the vast majority of species, physiological colour changes are achieved by altering the light reflected by the skin or exoskeleton. This is commonly facilitated by pigment-containing cells called chromatophores. There are several types of chromatophore, which contain different pigments and therefore produce different colours. Melanophores are the best known of these, containing the dark pigment melanin. Often, colour-change is achieved by the movement of pigments in and out of the chromatophores, however more sophisticated changes are achieved in some species through muscular control of the chromatophores.
One rather beautiful example of colour change in animals comes in the form of a beetle. The golden tortoise beetle (Charidotella sexpunctata) is a North American species which is capable of spectacular colour changes during both love and war. This is a talent that the Golden Tortoise beetle shares with its close relative the Panamaian Tortoise beetle. Love in this species can last anywhere between 15 minutes and 9 hours. Just two minutes in, the beetles have changed colour, from a metallic golden colour to bright red colour, and gained black spots. The same response is triggered when the beetle feels threatened by a predator or a nosy human.
Tortoise beetles are closely related to asparagas beetles and longhorn beetles. The former show no colour-change abilities, while the later change from golden to red with changes in moisture levels on their scales. However, this colour change is dependent on the environment but is not under the direct control of the beetle. Tortoise beetles are particularly interesting because they are one of just a handful of species that have conscious control over their colouration.
In contrast to the chromatophore-based colour-changing system exhibited by some reptiles and squid, Tortoise beetles achieve this startling colour change using microscopic valves which control the moisture level of their shell, thereby altering the light it reflects. Their shells are translucent and composed of three tiers; each tier being made up of densely packed reflective layers covered in tiny pores. When dry, these layers reflect light, creating a gold colouration. The colour-change occurs when liquid is released from the pores, filling the grooves in the reflector and disrupting its reflective properties. The reflective layer becomes translucent, revealing a red pigmented layer below.
Colour change in the tortoise beetles is believed to serve a purpose both in male-female communication, being used as a signal of maturity. Younger beetles cannot produce the golden colouration and do not get to mate. It’s use in response to stress may indicate that it also plays a role in camouflage or mimicry.
One famous example of this extraordinary control of colour change is the Chameleon. There are around 160 species of chameleon found across Africa, Madagasgar, the Mediterranean and Southern Asia. Chameleons can change their colours in response to temperature, environmental and social cues, and are used for communication as well as camouflage. Their colours are based upon varying contributions of four pigments: Xanthophores (yellow), Erythrophires (red), Leucophores (white) and Melanophores (black), but chameleons are capable of producing a wide variety of different colourations from pink, red and purple to black, brown, blue and turquoise. Chameleons achieve colour change by moving pigment-containing organells inside the chromatophores.
The primary function of colour change in chameleons is believed to be social signalling, with chameleons using colours to signal their intentions to other members of the same species; to attract mates and to ward off enemies. Males may use bright colouration to signal social dominance, and colour-changes are widely used in chameleons to avoid physical confrontation. Males will signal submission with pale, dull colourations. Females, too, can signal their status using colour-changes, rejecting males or indicating that they are pregnant.
Their sophisticated control of colour and pattern enables them to produce dramatic changes in seconds or even milliseconds. In many species this remarkable capability has also been co-opted for camouflage. 
Smith’s Dwarf Chameleon, found in South Africa, uses facultative camouflage – it alters the sophistication of its disguise depending on the visual system of its predators. Snakes have less advanced colour perception than some of the chameleon’s other predators, such as birds, and so they don’t have to try as hard when hiding from snakes. Of the 21 species of Dwarf Chameleon, at least 11 are believed to exhibit this facultative camouflage.
Other species use their colour-changing powers for thermoregulation; the desert-dwelling Namaqua chameleon (Chamaeleo namaquensis) changes from a dark brown or black colour in the morning (absorbing heat) to a lighter grey colour during the heat of the day (reflecting heat). As chameleons are cold-blooded (exothermic), their body temperature is determined by their surroundings, and using body colour allows finer control of body temperature and adaptation to this extreme environment. The Namaqua chameleon will even sometimes use both colourations at once, with one side of its body pale and the other dark, in relation to its orientation to the sun.
The true masters of colour change both for communication and for camouflage are the cephalopods; squid, octopuses and cuttlefish. For many cephalopod species, particularly squid and cuttlefish, mating rituals and social interaction depend upon colour and pattern communication. Squid have also been observed to use colour communication between males as a means to avoid escalated, physical conflict. Some species are even able to multi-task; displaying one pattern on one side to attract a female, and another on the other side to deter competing males.
Cephalopods are able to produce an impressive array of dramatic colours and patterns, from the zebra stripes of the male cuttlefish to the vibrant warning display of the Flamboyant Cuttlefish (Metasepia pfefferi). So rapid and fine is their control of colouration, that some species of cuttlefish have been observed to produce bands of stripes that move down the body as they move.
Cephalopod colour-change is achieved not by the transport of pigments in and out of the chromatophores, but by altering the relative contribution of different chromatophores to the overall pigmentation. Pigment is stored inside elastic saccules within the chromatophores, which are attached to muscles. Contracting and relaxing these muscles causes the saccule to change size, and thus increase or decrease the amount of that pigment contributing to colouration. Chromatophores can be black, brown, red, orange or yellow.
As well as the basic chromatophore colour-changing system they also possess two additional, specialised chromatophores. Iridophores are found directly underneath the chromatophores, and are responsible for metallic greens, blues and golds. Iridophores relect light and can also cause silver colouration around the eyes and ink sacs of some squid. Iridophores, however, are not under such minute control as chromatophores, and their response is somewhat delayed. Last, but not least, are the leucophores, which produce the white spots seen in some species of cuttlefish, squid and octopus. They scatter and reflect the prominent colour of the environment back, and can therefore be used to aid camouflage.
In addition to changing colour using chromatophores, a few species actually emit light themselves, in a process known as bioluminescence. Luminiscence in squid as well as other marine organisms such as jellyfish and fireflies is achieved by exposuing luciferin to oxygen and an imaginatively named enzyme called luciferase. The result of this interaction is a fluorescent (blue or green) molecule which is stored in a photophore.
The other well-known glowing animal is the moon jellyfish (Aurelia aurita), a translucent jellyfish about 30 – 40cm in diameter. It feeds on plankton and molluscs, grasping them with its tentacles. Most interestingly, it possesses bioluminescence. The glow is achieved in exactly the same way as in cephalopods; luciferase reacts with luciferin to produce an electronically excited oxyluciferin, which decays rapidy to release a photon of light. Another jellyfish, Aequorea victoria, is also a bioluminescent jellyfish, from which scientists isolated one form of luciferin known as coelenterazine. This species uses a slightly different method to produce its greenish glow, a photoprotein known as the green-fluorescent protein (GFP). GFP emits a bright green fluorescence (surprise, surprise!) when exposed to blue or UV light. A very similar protein, called aequorin, is used in other species of the Aequorea genus. It generates light in reaction to calcium, and is believed to interact with coelenterazine.
The discovery of GFP represented a major scientific breakthrough, as GFP has been extensively used by molecular biology as a label to identify successful genetic modifications. The GFP gene can be introduced to an animal using a viral vector, or by cell transformation. It is usually transferred along with a gene of interest, enabling a speedy identification of successes and failures. It has now been using for research on bacteria, yeaste, fungi, amphibians, fish, plants, flies and mammalian cells, and has been used to track the internal spread of HIV infections and cancer, as well as tracking the success of individual sperm.
In the grand scheme of things, this fascinating ability to change colour at will is really quite unusual. But it seems that the more we look, the more examples we find. Other species showing short-term changes in colouration include: Fiddler crabs (Uca pugilator, Uca capircornis), Chameleon grasshoppers (Hosciuscola tristis), stick insects (Carausius morosus), Australian damselflies (Austrolestes) and the Golden Crab Spider (Misumenavatia). Many of these species change colour in response to temperature changes, such as the Fiddler Crab, who can change colour within 15 minutes of temperature changes. Colour changes vary between males and females has been suggested that they play a role in mate recognition in this species. Chameleon grasshoppers, too, show colour changes in response to temperature, turning from black to bright turquoise as temperature increases. However, their response lags behind both the environmental and internal temperature, indicating that the control mechanism is more complex.
Other colour changes follow a daily (circadian) rhythm, as found in at least 15 species of Fiddler Crab. Environmental cues used to control colour changes can also be visual, as in the Goldenrod crab spider, which changes from yellow to black and white in response to the appearance of flowers. This change, however takes many months to complete, and is due to the release of pigments from glands under the exoskeleton. A speedier response is seen in the White’s tree frog (Litoria caerulea). These frogs can change through a variety of blues, greens, grays and browns, based on the brightness of their environment. This is almost certainly a mechanism of camouflage. The bullethead parrot fish (Chlorurus sordidus) is another species capable of colour-change for the purpose of camouflage.
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Across the animal kingdom, species have developed the ability to change their body colour in response to environmental and social cues. The speed of this change can vary from months or even years down to milliseconds, and is used for social communication, camouflage and thermoregulation. However, in the majority of species, the same basic mechanism is used; movement of pigmented organelles within the coloured chromatophores. In cephalopods, chromatophores are contracted or expanded using muscles. Highly sophisticated and adaptable colour-change is achieved by direct neural control of these contractions. Other, more basic, colour-changes are achieved through chemical reactions in the exoskeleton, but these are generally slower and neural control is limited or absent. The tortoise beetles have found another, innovative mechanism for achieving rapid colour-change, by modifying the reflective properties of their shell.
All these changes are based upon the differential reflection of light from the outer surface of the animal. But some species have taken it one step further, actually emitting their own light. Bioluminescence is observed in several species of jellyfish, as well as some corals and insects such as fireflies. The substances involved in bioluminescence have been hugely important in biomedical science.
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By: Claire Asher
Claire is a Freelance Science Writer and Communicator, and Innovation Officer for the London NERC Doctoral Training Partnership. She created Curious Meerkat in 2009, and has also written for a variety of publications including New Scientist, BBC Earth, Science, Nature, Mongabay, and The Scientist, as well as contributing to several books.. She completed her PhD in 2013 on the social behaviour of dinosaur ants.
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