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. âFlash' 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 - âways 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.
The Victorian naturalist Henry Bates spent the years between 1849 and 1860 wandering the Amazonian rainforests. After collecting ninety-four species of butterfly, he published an article. That article has generated heated discussion ever since.
Bates grouped together his butterflies based on their colouration, as did every collector of the day. Some nice relationships emerged - the butterflies with similar colour patterns could be placed neatly into apparently related groups. But then Bates discovered some conflicting evidence - the shapes of the butterflies' bodies told a different story. Wings apart, the shapes of the body and limbs varied considerably within a supposedly related group. In fact new groups could be formed based on body and limb shapes alone, groups very different from those based on colour. So why did unrelated butterflies share the same colouration? Was this simply a âwonder of nature', according to pre-Victorian philosophy?
Darwin and Wallace had demonstrated that wonders of nature do not exist, and Bates shared their views. He delved deeper into his dilemma of contradiction and noted that the most brightly coloured butterflies also flew the slowest, making them the easiest of prey for birds. Bates concluded, however, from the lack of evidence from discarded wings, that birds avoided them. From this he assumed these defenceless butterflies were unpalatable. Then followed an assumption which had serious repercussions - that birds understood the butterflies were distasteful based on their colouration.
Now Bates could explain his relationship dilemma. It was the shape of the body and limbs of butterflies that marked their true evolutionary relationships. While many within a genuinely related group did possess similar colours, some had departed from the norm with a purpose - that of enjoying a greater chance of survival. First, a butterfly group that has not evolved distasteful chemicals may evolve camouflage colouration, like the peppered moth. But if the camouflage code can be cracked under certain circumstances, then another evolutionary option is to pretend to be unappetising - to copy the colours of
those armed with distasteful chemicals. This behavioural and evolutionary strategy is known as mimicry.
The precise mechanism of mimicry and colouration that warns of indiscernible defences is a subject in its own right. Especially interesting is how predatory species and individuals learn to interpret visual warning codes without wiping out the potential prey species in the process. John Maynard Smith is particularly well known for untangling this academic web in the twentieth century. But for the purposes of this book it is enough to know that mimicry
does
work. After all, it exists in abundance, and that's the real proof.
In his statement on colour, Darwin used the words, â
whenever
colour has been modified for some special purpose . . .'. The use of the word âwhenever' is interesting here. Does this mean that colour can sometimes be incidental in that the colour effect has no purpose?
Black sharks may be red herrings as far as colour is concerned, where their colour provides a warning only to biologists studying adaptations to light. Kanoeohe Bay in Hawaii is a nursery for the scalloped hammerhead shark. The shark pups prefer the safety of the sea floor, even though this reaches a depth of between only 1 and 15 metres within the bay. At the deepest part of the bay, the pups are almost white in colour, but at the shallowest parts they are black. The sea floor, on the other hand, is consistently white. So do the pups require camouflage from a predator or prey only in the deeper water? Or are they making a statement with their colour in the shallows? In this case, the answer to both questions is âno'. Sometimes colour has no visual function and is said to be incidental. An example of this is the blue we see of our veins.
Pigments can serve a function other than providing a visual effect. The black or brown pigment melanin can increase the strength properties of a structure, such as a beetle's exoskeleton, or it can provide protection from the sun's ultraviolet rays. For many animals, ultraviolet light can cause tissue damage. Just as we tan in the sun, in very shallow water the hammerhead sharks do the same. At a depth of 1 metre, the ultraviolet content of the water is six hundred times greater than at 15 metres. So in the shallowest waters the hammerhead pups were gathering a layer of melanin in their skin. Melanin not only
absorbs the harmful ultraviolet light but also other wavelengths or colours. Consequently, no light is reflected and the shallow-water sharks appear as a colour void. Black, that is.
This function for pigments has no place in the literature of colour, and rightly does not appear in the
Origin
. Darwin did, nonetheless, omit a function in his bold statement on colour - the âwolf in sheep's clothing' function. We have seen examples of camouflage for indirect protection, against one's enemies. But camouflage colouration can also be employed to conceal oneself from one's prey.
I had my first active encounter with pigments while snorkelling in Greece. Although there were no coral reefs, the water was remarkably clear, blue and inviting. In the shallow water were large, brown rocks distributed randomly on the white sea floor. I noticed the rear end of a bright yellow fish emerging from the gap between two rocks and dived down to take a closer look. At first I saw nothing unusual, although I did wonder why the fish did not swim away in my presence. It appeared, through my naive eyes, to be almost jammed between the rocks, so I reached out to help it. Just as I touched its tail, something moved. Not the fish, but the rock. Part of the rock âchanged' slightly, and, on closer inspection, that part turned out to be an eye. The rock was a rock-plus-moray eel. A large brown moray eel, camouflaged perfectly against the rock it had wrapped itself around, was grasping the yellow fish head first in its gaping jaws. I was young and, since the yellow fish was about the same size as my head, I felt it was time to leave the water. Later I learnt that generally in shallow seas fish must beware all rocks . . . and stones.