In The Blink Of An Eye (16 page)

Back on the Barrier Reef, as described at the beginning of this book, the cuttlefish's brown ink contains a pigment released into the water. While following the cuttlefish, the corals I passed over had as great a diversity of colour as forms. The saturated reds, yellows and oranges looked the same from all directions, indicating that pigments were behind them all. The sponges covered the complete spectrum, and like the reds of anemones, lobsters and starfish, they bore the saturated coloured effects of pigments. The purples and browns of sea urchins indicated unsaturated colours. But as I followed the cuttlefish around them, again their colours did not change, indicating pigments all the same. Then, as I mentioned previously, something happened to the colour of my guide. The otherwise brown and white cuttlefish turned red . . . then green.

Take a close look at a colour TV screen. When it is switched on, clusters of blue, green and red ‘sub-dots' are distinguishable. Each of these sub-dots continually, and independently, becomes brighter and dimmer as the overall picture on the screen changes successively. This, again, is Young's colour mixing in action. Black and white photographs in newspapers are constructed from regularly spaced black dots on a white background, just like the shades of grey achieved from the black and white scales of Atlas moth wings. The size of each dot determines the shade of grey in its particular region. The picture on a TV screen is constructed from dots, too. But here a comparable dot is made up of three sub-dots - one green, one blue and one red. And by changing the brightness (rather than size) of each sub-dot, the overall dot can appear any colour of the spectrum. So as a yellow tennis ball flashes across the grass court on the TV screen, different combinations of sub-dots glow. When the ball is over a dot, the green and red sub-dots light up, while the blue is off, to produce yellow. As the ball passes, the red sub-dot also turns off to leave green. And a yellow wave of colour travelling across a green cuttlefish works in the same way. But how can this be? Pigments produce permanent colours; they cannot suddenly change. The leopard, for instance,
cannot
change its pigmented spots. It was the Victorians, again, who made sense of this paradox, although not at the first attempt.

In 1802, Tom Wedgwood, son of the potter Josiah Wedgwood and
uncle of Charles Darwin, took one of the first forms of a photograph. He painted leather or paper with a solution of silver nitrate, which is sensitive to light. He placed leaves on top and exposed the apparatus to sunlight for about half an hour. The light turned the exposed silver nitrate to silver metal, which reappeared, and the shape of the leaves emerged as pale silhouettes. A negative had been made, albeit in black and white. So of course colour photography became the next great Victorian goal, one eventually achieved by James Clerk Maxwell.

Prior to Maxwell's accomplishment, the nineteenth-century scientist Otto Wiener believed the colour breakthrough lay with compounds of silver chloride that react with different wavelengths of light. The new compounds formed at the end of the reaction would have the colour corresponding to the catalysing wavelength. Wiener also thought that organic substances, such as those found in animals, could possess a similar property. Then came a theory of adaptive camouflage. A caterpillar, Wiener argued, might vary its colour to match a changing environment because its skin ‘photographs that environment by means of the sensitive compounds of its own tissues'. A nice idea, but pure fiction.

The eminent Victorian naturalist Henri Milne Edwards made amends in 1848. Like Aristotle, and philosophers, scientists and poets since, Milne Edwards was intrigued by the chameleon. Chameleons change their colour dramatically. The big question is, ‘How?' Milne Edwards realised the answer lay not with any chemical change in the skin, but with the mechanical distribution of pigments. This was a breakthrough.

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.

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.

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 continued 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.