Repulsion
Territorial advertising
During the reproductive season, many animals defend a particular area or territory that includes their nest or spawning site. Many other animals defend territories throughout the year. In either case, coloration is frequently important. In species in which the task of territorial defense is accomplished largely by one sex, strong sexual dimorphism usually exists, the more brightly coloured sex being the one that holds the territory. Both male- and female-territorial species are found within the diverse fish family Cichlidae. Species in which the male holds a territory are marked by large and colourful males, the females being smaller and camouflaged; in those species in which the female defends the territory the reverse is found. In still other species the fish pair and share the territory, and there is little sexual dimorphism.
Coloration frequently releases agonistic (flight or attack) behaviour in territorial animals and intimidates intruders. The flashing coloration displays of a dominant octopus are an excellent example of a visual battle in which the victor may be determined with little or no bodily contact.
Although similar advertising colorations may contribute to the spacing out of territorial animals, dissimilarity in coloration between members of a species may allow closer spacing. Many brightly coloured reef fishes, for example, defend territories or personal spaces. In many of these species the young and subadults, with radically different coloration from the adult, live within the territory of an adult but remain free from attack; after they assume adult coloration, however, they are driven away. The territories frequently function to ensure a food supply; because the juveniles utilize different food, they pose no threat to the adult’s supply. As the juveniles age, their feeding habits overlap those of the adult, and spacing is necessary.
Warning, or aposematic, coloration
Certain advertising colorations warn a third party of dangerous or inedible qualities of the organism (aposematic colorations), such as spines, poisons, or other defensive weapons, allowing the possessor to avoid a potentially damaging interaction in which the weapon is used. Red, black, and yellow are common in this context and may represent aposematic colours recognized by many animals. (See .)
As discussed above, Batesian mimicry is the imitation of aposematic coloration by benign organisms, which thereby enjoy at least a portion of the protection of the model species. While Batesian mimicry involves deceptive coloration, resemblance in warning coloration need not provide false information. Müllerian mimicry refers to instances in which several noxious species display the same warning coloration, thus enabling potential predators to learn and generalize the signal easily. The black-and-yellow coloration of bees and wasps is a typical example.
Optical functions: combination of concealing and advertising coloration
Most animals need both concealment and advertisement. An animal may need to conceal itself from predators and to advertise its presence to symbionts or to members of its own species for reproductive purposes.
Many birds that conceal courtship coloration when their feathers are held close to the body present a brilliant display upon erecting their feathers. Similar mechanisms are common in many animals, such as Anolis lizards, which have brightly coloured throat fans that are visible only when erected during courtship or threat behaviour.
Many bower birds (Ptilonorhynchidae) have bright courtship colorations, although some males of Amblyornis species do not. Instead, they decorate an elaborate bower with leaves, flower petals, and other brightly coloured objects, which attract females but provide no clue to predators as to the exact location of the male.
Some predators deceive with advertising coloration. The frogfishes, or shallow-water anglerfishes, are extremely difficult to detect against their background. They have intricate and obvious lures that are waved near the mouth on a long stalk; prey fishes attracted to the lure are eaten.
Coloration change is another obvious mechanism that can restrict advertisement to times when it is needed for purposes of communication. Many animals change from cryptic to noncryptic colorations as they change from their normal resting coloration to a display coloration during social interactions. These changes are particularly common in fishes and cephalopods, which have efficient neural mechanisms of coloration change.
Optical functions: the roles of the selective agent and of illumination
The selective agent
Of obvious importance in the evolution of coloration is the third party, which is the actual selective agent involved in the relationship between the organism and its background. Identification of the third party and the sensory and nervous system components used by it are important in order to understand thoroughly the adaptive nature of deceptive or advertising coloration.
In analyzing concealing coloration, the actual identification of the third party may have a profound influence on the interpretation of the coloration and behaviour. For example, the early stages of the green Scotch pine caterpillar (Bupalus piniarius and others) are found at the tips of pine needles, well camouflaged in this position. As they grow larger, they move into the bases of the needles and onto the branch. One explanation for the movement is that the older caterpillars are much larger than the background needle, thus rendering the camouflage less effective. Another factor appears to be a shift in the third party as the caterpillar ages; young caterpillars are preyed upon by spiders found on the twigs; larger caterpillars, by birds such as titmice (Parus).
After the initial identification of the third party, its visual capabilities must be investigated. The spectral sensitivity of its eyes must be determined, as must the way in which it perceives combinations of biochromes and their arrangements. The visual stimulus is subject to encoding and integrating steps as it passes from the eye to the cerebral cortex of the brain. Contrast and movement are amplified by some cells, while other properties, such as shape and intensity, are ignored. In humans, for example, contrast is greatly enhanced at the junction between a red and a blue stripe, producing the optical illusion that the two stripes never meet and are on different planes. Such phenomena may be of importance in disruptive coloration.
Advertisement is likewise subject to the visual capabilities of the third party, or signal receiver. Many species of plants have yellow flowers barely distinguishable to the human eye; when an ultraviolet camera is used to photograph such flowers, however, various bright patterns and nectar guides are revealed that appear to be highly species specific (see ). The importance of strong contrast and contour in the attraction of insects to flowers is related to the perceptual qualities of the insect’s compound eye, which shows maximal response to flickering stimuli and may depend upon similar qualities for much form discrimination.
In social signals, the visual system of a species is frequently maximally responsive to its own range of colorations. Butterflies of the genus Dardanus, for example, are maximally responsive to their own blue courtship coloration. The visual system and coloration are coadapted to provide an efficient signal mechanism.
George S. Losey Edward Howland BurttIllumination
Most optical signals depend on sunlight reflected from the animal or plant. Therefore, the receiver’s perception of the signal depends on the characteristics of the ambient illumination, which, in turn, depends on such variables as time of year, time of day, amount of cloud cover, amount of vegetation between the light source and the optical signal, and spectral reflectance of the habitat. Clear-sky sunlight with the Sun overhead is essentially white, but with the Sun low in the sky the light has a yellow or orange spectral emphasis. Light in broadleaf forests has a yellow-green emphasis, whereas light in conifer forests has a slight bluish emphasis. These small but consistent differences may affect the evolution of optical coloration.
Visual functions
Biological coloration can play a variety of roles in an animal’s visual system. For example, facial coloration can help determine the amount of light that is reflected into the eyes. Among animals living in brightly lit habitats, too much reflected light could have undesirable effects on vision. It could, for example, produce blinding glare or dazzle; it might result in high luminance in parts of the visual field, thereby diminishing contrast in other parts of the field; or it could cause adaptation to a higher illuminance level than is appropriate for the remainder of the visual field. Birds that forage in sunlight for aerial insects—a visually demanding task—have bills that are black. Apparently the black coloration reduces reflectance that interferes with their vision.
Vision itself depends on a biochrome that consists of a protein, opsin, attached to a chromophore. The chromophore may be either retinal (vitamin A1), in which case the molecule is called rhodopsin; or 3-dehydroretinal (vitamin A2), in which case the molecule is called porphyropsin. When light enters the eye and strikes the visual biochrome, the molecule undergoes a chemical change that stimulates the receptor nerve and thereby produces a visual stimulus.
In addition to the visual pigments, the eyes of many invertebrates contain biochromes that affect the spectrum of light that reaches the photoreceptors. Similarly, oil droplets in the retina and epithelium of vertebrate eyes contain carotenoids that may affect colour perception. More importantly, the epithelium contains melanin, which absorbs stray light that penetrates the retina without being absorbed by the visual pigments. In insect eyes a similar function is performed by ommochromes in secondary pigment cells surrounding the photoreceptors.
Among many nocturnal vertebrates the white compound guanine is found in the epithelium or retina of the eye. This provides a mirrorlike surface, the tapetum lucidum, which reflects light outward and thereby allows a second chance for its absorption by visual pigments at very low light intensities. Tapeta lucida produce the familiar eyeshine of nocturnal animals.
Physiological functions
The discussion of biochromes earlier in this article touched upon the many important physiological roles of biological pigments, including that of the chlorophylls in photosynthesis and of the hemoglobins in oxygen transport. This section provides examples of other physiological effects of biological coloration.
Hair and feathers that contain melanin are more durable than those that lack this biochrome. Increased durability probably accounts for the dark, melanic wing tips of most birds. It may also be a contributing factor to the high proportion of black among birds that live in deserts, which are exceptionally abrasive habitats.
The absorption of solar energy by dark skin, scales, feathers, or hair is often associated with increased heat gain and reduced metabolic rates. Because birds lose a large amount of body heat through their uninsulated legs, dark leg coloration may help to warm the legs by absorbing solar energy, thereby reducing heat loss. Such reduced heat loss may explain why dark-legged North American woodwarblers (Parulidae) arrive in their northern breeding areas earlier than light-legged woodwarblers. Dark feathers, however, may actually reduce the amount of solar energy that penetrates to and is absorbed by a bird’s skin. With fully erect plumage in moderate winds, a dark bird in full sunlight absorbs less heat into its body than a light bird does. This may also be a factor contributing to the high proportion of black among desert-dwelling birds.
Photoactivation of 7-dehydrocholesterol into vitamin D occurs throughout the epidermis of humans in the presence of ultraviolet light. The melanization of human skin may be an adaptation to optimize synthesis of vitamin D by permitting more or less ultraviolet radiation to penetrate the epidermis.
A widespread response to increased light levels is the addition of melanin, or darkening of the body—for example, tanning in humans. Such melanic shielding protects the tissues of the organism from potentially dangerous levels of ultraviolet radiation. Since the ultraviolet shield need protect only easily damaged cells in the nervous and reproductive systems, it does not necessarily have to lie in the skin but can instead be located internally, immediately around sensitive organs. When the ultraviolet shield is internal, external coloration may conform to other selection pressures.
Water is conserved by reducing evaporative loss and by reducing excretory water loss. Insects reduce evaporative water loss by adding melanin to the cuticle, melanin being more waterproof than other biochromes. The black-coloured beetle Onymacris laeviceps loses significantly less water than does the white-coloured beetle O. brincki when both species are kept without food under identical conditions. Quinones also darken insect exoskeletons, and in Drosophila quinones contribute to the low permeability of the exoskeleton. Some insects avoid excretory water loss by depositing nitrogenous wastes in the exoskeleton, which is shed periodically. In these species external coloration is a consequence of nitrogen excretion.
Some arthropods produce offensive odours as a means of defense against predators. These odours derive from p-benzoquinones in the exoskeleton and are correlated with the chromatic properties of the molecules. Consequently, coloration in these species may be a consequence of selection for chemical defense.
