The skin of the chameleon or cuttlefish is packed with chromatophores - colour cells. These are simply cells packed (usually) with pigment. Each colour cell contains just one type of pigment that causes one colour. But the cell is elastic - it can change its shape. Under nervous control, it can become flat and thin, lying parallel with the surface of the animal, or short and squat. And the pigment is spread evenly throughout the cell in each case. Looking at the animal, the short, squat cells reveal only a small area of pigment, and the visual effect is negligible. But the thin, flat cells reveal much more of their pigment, and can be seen by the naked eye. Compare these two possible forms of the colour cell, considered off and on, with a coin. A coin is easily observed when lying flat, but it is more difficult to see edge on.
Chameleon and cuttlefish skin is actually packed with colour cells of various hues. In comparison with a TV screen, individual cells can be considered sub-dots, collectively forming dots that can independently cause any colour. By being turned on and off, or by becoming an intermediate phase, the different sub-dots contribute to a dot that is capable of assuming any colour of varying brightness. At high magnification, imagine the skin as an assortment of juxtaposed and coloured coins. When some coins are turned on their sides, different overall colours are achieved. And this works - it really is extremely effective. One would hope so, too, considering the evolutionary trouble involved and the physical costs of such a mechanism. Significant electrical wiring, brain space, production of pigment and specialised cells, muscles, and sensors are required. With these costs in mind we can begin to consider the importance of light as an evolutionary factor and behavioural concern. The importance of this cannot be overstated.
Evolutionary interlude
If an animal does not adapt to the light in its environment, it will not survive. Today light could be considered the most powerful stimulus in most environments on Earth. In this chapter I will continue to demonstrate this point, using examples of how the world we see is one adapted to light. I do not intend to diminish the significance of other stimuli, such as touch, sound and chemicals, for these are hugely important, too. But light is an exception among stimuli because it is always there. If you don’t make a scent, you will not be smelt. If you don’t make a sound, you won’t be heard, although for some animals silence and lack of scent are difficult to achieve. Touch is a little different because it operates, obviously, only over very short distances. The adaptation to light is a vital necessity. Light is where the sun’s radiation peaks. It exists in many environments on Earth. If it did not, life today would be very different.
There are a couple of exceptions to this rule of exclusivity. Two other stimuli exist in the environment that also cannot be avoided. Many bats hunt using radar. They produce pulses of ultrasound that return to the bat after rebounding from an object, just like the military radar system that detects aircraft. If, at night, the bat’s radar detects an object that is small and in mid-air, it is probably a moth. That’s food to a bat. But just as animals living under the sun are adapted to light, so moths are adapted to radar. They are covered in a sort of radar-absorbing fur, which reduces the signal reflected back towards the bat. When the radar source is very close, they can stall and dodge the oncoming bat. A similar cat-and-mouse game takes place underwater, where dolphins hunt fish using a comparable stimulus - they produce sonar.
Also in the water, some fish produce a different stimulus. Electric fish such as the numb ray and electric eel were once targets for those who doubted evolution. How could such a strong, complex and specialised characteristic suddenly appear in the history of animals, as if out of nowhere? Any evolutionary shudders were stilled on the discovery of the ‘missing link’ - weakly electric fishes. These fishes do not produce the high voltages capable of killing prey by their mere touch. Instead, weakly electric fishes emit faint electric fields that work in a similar way to sonar. They can select prey based on the electrical signal that is returned. And from this the strongly electric fish could evolve.
Radar, sonar and electric fields, however, are rare on the surface of the Earth in comparison to sunlight. To begin with, an animal must produce its own stimulus, although this is sometimes worthwhile because, like light, it becomes a stimulus that other animals cannot avoid without taking action. Stimulus production is an expensive exercise all the same. So the fact that it exists in nature indicates that it does work, and works well. But still the environments that carry these stimuli are very limited. Also, sonar and electric fields only affect animals of a very specific size - the size of food for the stimuli producers. Yet with light, there is always an animal, or more realistically many animals, which will have an interest in the optical signature of every animal living under sunlight.
So animals have to accept, or in evolutionary terms adapt to, the sunlight that strikes them. There are two routes an animal can take - the path to camouflage or the path to conspicuousness. At the foot of this evolutionary junction, the balance may be even. The path to take could be purely under the influence of chaos. It could also be influenced by the materials available for evolution - the building blocks, or atoms in the case of pigments. But, as will be demonstrated in Chapter 5, once the balance has tipped one way, evolution can continue full speed ahead along its chosen path, until there’s no turning back. And it is this balance of camouflage (‘indirect protection’) and conspicuousness (‘direct protection or attraction between sexes’) to which Darwin referred in the epigraph at the beginning of this chapter.
The purpose of pigments
When the Australian colonists entered the mountainous terrain of Papua New Guinea in the 1930s, they were amazed to find some of the population still in the Stone Age. Tribes there lived under a cyclical regime of peace and warfare.
Until the late 1980s, battle in New Guinea involved spears, arrows and shields. Shields were carved from tree trunks and were often as tall as their owners. These shields were painted with locally available pigments, in geometric designs. Anthropologists made early attempts to interpret these designs, but they were on the wrong track. The designs carried no meaning; they were there simply to intimidate the enemy. Indeed, the warriors also painted themselves, making them ‘glint terrifyingly’. The overall bearing and brilliance of a warrior with his shield warned of his support by ancestral ghosts . . . and this was backed up by a large spear. The pigments were warning colours advertising the threat posed by the warrior. In this context, his weapons were also ornaments. Warrior colours may have incited surrender or retreat before battle had chance to commence.
Following the decommissioning of armour, European armies employed warning colours up until the nineteenth century. The bright red and white uniforms, with tall headwear, provided a warning message or two. Like much of the armour before, a large hat provides a false impression of body size. The larger the individual, the greater the threat perceived. And the immaculate dress itself was a clear symbol of a well-disciplined army. Then, of course, there were the regimented manoeuvres. This was an army that was prepared and knew what it was doing, at least in the eyes of its enemy.
During the nineteenth century the philosophy of battle colours changed. With the introduction of accurate, long-range guns came a new form of advantage for the soldier.
Until this time, although conspicuousness had been the soldier’s battle principle, there was always an alternative lurking in the back of the brigadier’s mind - camouflage. Merging into his surroundings, a soldier could either avoid or surprise the enemy. But then armaments really would be armaments, and the enemy would be fearless. Ornaments would become obsolete. So there was always a balance within military intelligence, just like the balance within nature, between conspicuousness and camouflage. And the military balance eventually tipped the other way.
New weaponry called for new tactics. Armies fought at greater distances apart - so far in fact that the smart uniforms, never mind their shiny buttons, were simply not visible. Although the regimented formations co
ntinued to instil some degree of fear, in general it was fading, like the pigments themselves over distance. Now the bright red uniforms served only as targets, and the path to camouflage became the route to take.
The balance between camouflage and conspicuousness lies behind every case of purposeful colouration in nature. Whether the colour seen is conspicuous or inconspicuous indicates the way the balance has tilted. This is the direction of evolution - the direction with the greatest difference between positive and negative selective pressures.
Dropping the military metaphor, the employment of pigments to provide an ‘attraction between sexes’ is a simple and straightforward concept in nature. Many obvious examples could be listed. Think of the birds of paradise, with their dull females and flamboyantly costumed males. Then there are the male hornbills that actively wear alluring (to a female hornbill) yellow make-up, secreted from preen glands and applied to their wings by the bill. But the other functions of colour as listed by Darwin are equally bountiful in nature.
Pigments are employed to provide ‘direct protection’ through advertising. The unicorn fish inhabits Hawaiian waters. Its name derives from a single, horn-like protrusion from its head. But another obvious characteristic of this fish is a strong spine on either side of its tail. The spines have a protective function - they can potentially slice open an aggressive fish with a single swish of the tail. And they are made obvious by their bright yellow pigments - a warning not to disturb this species. The warning is heeded well and the fish is left alone. The armaments are, again, ornaments.
Pigments may provide ‘indirect protection’ through camouflage. The peppered moth provides the case that first springs to mind. This well-known species is, as seen in its seventeenth-century guise, a pale grey colour so that it can camouflage itself against the silvery bark of trees as protection from predatory birds. During the Industrial Revolution, trees growing near factories became blackened by smoke pouring from factory chimneys. The pale grey moths were suddenly conspicuous against the black trees . . . or they would have been if it hadn’t been for evolution. As selective pressures changed, new genetic mutations became advantageous - the ones that coded for black pigments. Thus the peppered moth became black in industrialised areas - its camouflage was restored. The moth had adapted to its new light environment, and it survived there.
Unfortunately for some other moths, their camouflage code is all too often cracked. But the moths are prepared for this. In the event that their cover is blown, they opt for conspicuousness as a last resort. The camouflage of these moths is confined to their upper wings - the only wings visible during rest. But when danger comes too close for comfort, their lower wings are quickly displayed, along with their warning colouration. Predators are confused by these unexpected blazes of bright colour and, in theory, the moths buy some time to escape. ‘Flash’ colouration is employed commonly by camouflaged animals, and so it must work . . . so long as the predators’ approach is detected.
A variation on regulation camouflage is disruptive colouration. The tiger’s stripes and giraffe’s patchwork patterning break up the outline of the animals themselves against their natural backgrounds. Then at times they may provide good old regulation camouflage. Sometimes repetitive patterns are less noticeable than a continuous, albeit camouflage, colour against a busy, varied background. Closely packed trees provide vertical lines with leaves of different colours, shapes and, according to Pissarro, finely pixillated patterns. This situation calls for equally busy camouflage patterning, and the precise colours may be less important.
Outside Sydney University, there is a large pond full of water lilies, complete with lily pads. Admiring the plant life there one day, it was some time before I realised I was also watching a large black and white bird. But how could this be? The bird was black and white against a background of green leaves - surely the bird would be conspicuous?
Although green, the lily pads were also curled and shiny, and where they reflected sunlight into my eyes they appeared white. Standing on the lily pads the white patches of the pied bird matched those of the reflections from the leaves. So the white areas of the bird were removed from possible conspicuousness. The remaining black areas of the bird should, in theory, have been obvious against the green leaves. But they no longer formed the shape of a bird . . . or anything recognisable as such to me. And the bird itself had escaped my attention. I learnt that having more than one colour can provide camouflage even if only one of those colours matches the background. And another lesson learnt was to consider nature’s colours only in their natural environments. The green leaves would, in the laboratory, have appeared a continuous green colour, against which the pied bird would have been quite prominent. This was not the case in the natural environment, under bright sunlight.
Monet provided a warning that one should beware the fixed, stereotypical image of an environment. He painted most of his landscapes many times, but at different times of the day . . . and his paintings were all unique. The epitome of this concept of immediacy is recorded in two of his haystack paintings of 1891. Painted at midday, the haystacks appear yellow, but in his evening interpretation the haystacks are glowing red. Under yellow light an object with a complete spectral repertoire will appear yellow; under red light it will appear red. To see this principle in action, try looking at the pages of this book under different light. The paper reflects all spectral colours, but under shaded sunlight it appears a bluish white, and under a light bulb a yellowish white. These are just two of the light conditions that call for different camouflage colours. So different constraints are placed on animals active at different times of the day, when different selective pressures are in action.
The Atlas moth has been considered so far under white light only. But under different colours, the moth assumes different appearances. Under red light, such as would be observed during the evening, the Atlas moth reveals patterns of stripes, providing disruptive colouration. Under green light, the moth exhibits a similar pattern to that under white light; that of regulation camouflage. So depending on whether the time is midday or evening, the Atlas moth sends out a slightly different message, albeit one intended to avoid the attention of predators in both cases. But there is more to this story. There is another colour contained within the sun’s rays, just before violet in the spectrum. It is a colour that thwarted Leonardo, Newton and the Victorians, because humans cannot see it. That colour is ultraviolet.
Beetles and birds send secret messages written in ultraviolet through the atmosphere. We know this because their ultraviolet colouration can be recorded on camera film. Like the lenses in our eyes, glass absorbs ultraviolet wavelengths. Fix a quartz lens to a camera, however, and the ultraviolet transmits, and affects the camera film in the same way as violet or blue light. When this film is developed, we can observe the ultraviolet plumage of the budgerigar, for instance. But if we cannot normally see ultraviolet, why should we even consider it for biological purposes? Well, other animals, especially birds and insects, can see it.
Many flowers include ultraviolet in their colour palettes to attract pollinating insects. If birds can generally see in ultraviolet, and birds eat Atlas moths, it is important to know how the Atlas moth appears under ultraviolet light. Does it continue its camouflage or disruptive colouration into the ultraviolet? The answer is no. Under ultraviolet light the Atlas moth takes on a remarkable transformation. It appears as two snakes, with prominent bodies and heads, with eyes and mouths. The purpose of this will emerge in my discussion of Henry Bates’s work, later in this chapter.
Enchanting as this case may seem, there is nothing magical about ultraviolet light; it is just another colour in the rainbow. But again, it does vary in content depending on the time of day - there is little ultraviolet present at dawn and dusk. It is the colour that transmits least well through the atmosphere, and can be almost completely absent under forest canopies, where light bounces around like a pinball and is absorbed by the leaves. Now it is time to consider
light as a creator of niches - ‘ways of life’ for animals.
West Indian Anolis lizards inhabit forested areas. Different species reveal different colours, and it is easy to assume that their colours simply attract their own species within a busy environment. Their environment is busy - the forest contains a variety of microhabitats, constructed by the physical nature of the plant life - but the Anolis lizards are not all spread throughout the entire forest. They do all occupy the same forest, but they divide up the height or profile of the plant life into microenvironments based on light conditions, including ultraviolet content. And the colours of each species are adapted exactly to the light of their specific microenvironment. So in each microenvironment, one type of colouration will be most adaptive, and the owners of that colouration will be the most successful there. In their correct microenvironment they can attract mates and defend territory most efficiently, allowing them to devote more time and energy to other activities. In this case, light is the foremost stimulus. The Anolis lizards have adapted to light most significantly, and other selective pressures secondarily. Adaptation to light is necessary for survival. A similar story could be told for many other animals, including birds and fishes.
A more unusual form of adapting to light is found where animals take their colour directly from their environment, without drawing on their body chemistry. The pink colour of flamingos derives from the carotenoid pigments in their crustacean food. And in a case of camouflage, flatworms parasitic on marlin take up pigment from the marlin’s skin below them to match their backgrounds and effectively disappear. But other animals, including the cuttlefish and chameleon in some situations, use chromatophores to gain camouflage. The skin may be equipped with sensors that detect the colour and brightness of the animal’s immediate background. This is possibly the ultimate in adaptation to light. A disguise from predators can be conjured up in any environment, and then warning or mating colours can be flashed when appropriate. But when chromatophores are not a possibility, the balance between direct and indirect protection can, throughout evolution, tip one way . . . and then another.
In The Blink Of An Eye Page 12
