Animal colouration
Animal colouration has been a topic of interest and research in biology for well over a century. Colours may be cryptic (functioning as an adaptation allowing the prevention of prey detection; aposematic (functioning as a warning of unprofitability) or may be the result of sexual selection. Colouration may also be function in mimicry of other organisms. The subject may be investigated in terms of both the chemical and physical basis of the colours (proximate cause) and the evolution of colouration (ultimate cause).
Camouflage is generally viewed as a result of natural selection, and involves an organism's colour blending in with its biotic (e.g. moss) or abiotic (e.g. sand) surroundings. Camouflage is often accompanied by behavioural adaptations that make the most of it, such as landing on areas of similar colour, and aligning the body correctly. It may involve costs as well as benefits, such as the cost of finding a suitable resting spot. Colour may change during the seasons, during an organism's life cycle, or even over very brief intervals, such as with the chameleon. Polymorphism may also occur, allowing individuals of the same species to have different camouflage, and making prey detection more difficult for predators. Organisms living in the same environment may come to have similar colouration through convergent evolution. Colours are an aspect of only one of the senses, and although the visual system is most important for humans, some animals cannot even see (such as those living in caves, underground, in the deep sea, or those active at night) and their colour may be of little or no adaptive value. These organisms rely primarily on other senses, such as olfaction and hearing, and even electroreception.Contents.
A camouflaged Jumping spider can easily capture prey.
The Goldenrod Crab Spider (Misumena vatia) has the capacity of changing colour by secreting a liquid yellow pigment into the outer cell layer of the body.
Main article: Camouflage
Cryptic colouration has evolved in many species that have been subjected to the pressures of predation and also in predatory species. Such colours help predators (aggressive resemblance or anticryptic colouring) and prey (protective resemblance or procryptic colouring). Protective resemblance is far commoner among animals than aggressive resemblance, in correspondence with the fact that predaceous forms are as a rule much larger and much less numerous than their prey. In the case of insectivorous vertebrates and their prey such differences exist in an exaggerated form. Cryptic colouring, whether used for defence or attack, may be either general or special. In general resemblance the animal, in consequence of its colouring, produces the same effect as its environment, but the conditions do not require any special adaptation of shape and outline. General resemblance is especially common among the animals inhabiting some uniformly coloured expanse of the Earth's surface, such as an ocean or a desert. In the former, animals of all shapes are frequently protected by their transparent blue colour; on the latter, equally diverse forms are defended by their sandy appearance. The effect of a uniform appearance may be produced by a combination of tints in startling contrast. Thus the black and white stripes of the zebra blend together at a little distance, and their proportion is such as exactly to match the pale tint which arid ground possesses when seen by moonlight (F Galton, South Africa, London, 1889).
Special resemblance is far commoner than general, and is the form which is usually met with on the diversified surface of the earth, on the shores, and in shallow water, as well as on the floating masses of algae on the surface of the ocean, such as the Sargasso Sea. In these environments the cryptic colouring of animals is usually aided by special modifications of shape, and by the instinct which leads them to assume particular attitudes. Complete stillness and the assumption of a certain attitude play an essential part in general resemblance on land; but in special resemblance the attitude is often highly specialized, and perhaps more important than any other element in the complex method by which concealment is effected. In special resemblance the combination of colouring, shape and attitude is such as to produce a more or less exact resemblance to some one of the objects in the environment, such as a leaf or twig, a patch of lichen, or flake of bark. In all cases the resemblance is to some object which is of no interest to the enemy or prey respectively. The animal is not hidden from view by becoming indistinguishable from its background, as in the cases of general resemblance, but it is mistaken for some well-known object.
In the past these effects were explained as a result of the direct influence of the environment upon the individual (GLL Georges-Louis Leclerc, Comte de Buffon), or by the inherited effects of effort and the use and disuse of parts (JEP Jean-Baptiste Lamarck), but natural selection, which can accumulate any and every variation which tends towards survival, has been the accepted explanation now for almost a century. A few of the chief types of methods by which concealment is effected may be briefly described. The colours of large numbers of vertebrate animals are darkest on the back, and become gradually lighter on the sides, passing into white on the belly. Abbott Handerson Thayer (The Auk, vol. xiii., 1896) has suggested that this gradation obliterates the appearance of solidity, which is due to shadow.
The colour-harmony, which is also essential to concealment, is produced because the back is of the same tint as the environment (e.g. earth) bathed in the cold blue-white of the sky, while the belly, being cold blue-white bathed in shadow and yellow earth reflections, produces the same effect. Thayer has made models (in the natural history museums at London, Oxford and Cambridge) which support his interpretation in a very convincing manner. This method of neutralizing shadow for the purpose of concealment by increased lightness of tint was first suggested by EB Poulton in the case of a larva (Trans. Ent. Soc. Loud., 1887, p. 294) and a pupa (Trans. Ent. Soc. Loud., 1888, pp. 596, 597), but he did not appreciate the great importance of the principle. In an analogous method an animal in front of a background of dark shadow may have part of its body obliterated by the existence of a dark tint, the remainder resembling, e.g., a part of a leaf (W Müller, Zool. Jahr. JW Spengel, Jena, 1886).
A camouflaged Orange Oak Leaf butterfly (centre)
This method of rendering invisible any part which would interfere with the resemblance is well known in mimicry. A common aid to concealment is the adoption by different individuals of two or more different appearances, each of which resembles some special object to which an enemy is indifferent. Thus the leaf-like butterflies (Kallima) present various types of colour and pattern on the under side of the wings, each of which closely resembles some well-known appearance presented by a dead leaf; and the common British yellow under-wing moth (Tryphaena pronuba) is similarly polymorphic on the upper side of its upper wings, which are exposed as it suddenly drops among dead leaves. Caterpillars and pupae are also commonly dimorphic, green and brown. Such differences as these extend the area which an enemy is compelled to search in order to make a living. In many cases the cryptic colouring changes appropriately during the course of an individual life, either seasonally, as in the ptarmigan or Alpine Hare, or according as the individual enters a new environment in the course of its growth (such as larva, pupa, imago, etc.). In insects with more than one brood in the year, seasonal dimorphism is often seen, and the differences are sometimes appropriate to the altered condition of the environment as the seasons change. The causes of change in these and Arctic animals are insufficiently worked out: in both sets there are observations or experiments which indicate changes from within the organism, merely following the seasons and not caused by them, and other observations or experiments which prove that certain species are susceptible to the changing external influences. In certain species concealment is effected by the use of adventitious objects, which are employed as a covering. Examples of this allocryptic defence are found in the tubes of the caddis fly larvae (Trichoptera), or the objects made use of by crabs of the genera Hyas, Stenorhynchus, etc. Such animals are concealed in any environment. If sedentary, like the former example, they are covered up with local materials; if wandering, like the latter, they have the instinct to reclothe. Allocryptic methods may also be used for aggressive purposes, as the ant-lion larva, almost buried in sand, or the large frog Ceratophrys, which covers its back with earth when waiting for its prey. Another form of allocryptic defence is found in the use of the colour of the food in the digestive organs showing through the transparent body, and in certain cases the adventitious colour may be dissolved in the blood or secreted in superficial cells of the body: thus certain insects make use of the chlorophyll of their food (Poulton, Proc. Roy. Soc. liv. 417). The most perfect cryptic powers are possessed by those animals in which the individuals can change their colours into any tint which would be appropriate to a normal environment. This power is widely prevalent in fish, and also occurs in Amphibia and Reptilia (the chameleon affording a well-known example). Analogous powers exist in certain Crustacea and Cephalopoda. All these rapid changes of colour are due to changes in shape or position of superficial pigment cells controlled by the nervous system. That the latter is itself stimulated by light through the medium of the eye and optic nerve has been proved in many cases. Animals with a short life-history passed in a single environment, which, however, may be very different in the case of different individuals, may have a different form of variable cryptic colouring, namely, the power of adapting their colour once for all (many pupae), or once or twice (many larvae). In these cases the effect appears to be produced through the nervous system, although the stimulus of light probably acts on the skin and not through the eyes. Particoloured surfaces do not produce particoloured pupae, probably because the antagonistic stimuli neutralize each other in the central nervous system, which then disposes the superficial colours so that a neutral or intermediate effect is produced over the whole surface (Poulton, Trans. Ent. Soc. Lond., 1892, p. 293).
Cryptic colouring may incidentally produce superficial resemblances between animals; thus desert forms concealed in the same way may gain a likeness to each other, and in the same way special resemblances, e.g. to lichen, bark, grasses, pine-needles, etc., may sometimes lead to a tolerably close similarity between the animals which are thus concealed. Such, likeness may be called syncryptic or common protective (or aggressive) resemblance, and it is to be distinguished from mimicry and common warning colours, in which the likeness is not incidental, but an end in itself. Syncryptic resemblances have much in common with those incidentally caused by functional adaptation, such as the mole-like forms produced in the burrowing Insectivora, Rodentia and Marsupialia. Such likeness may be called syntechnic resemblance, incidentally produced by dynamic similarity, just as syncryptic resemblance is produced by static similarity.
Animal colouration has been a topic of interest and research in biology for well over a century. Colours may be cryptic (functioning as an adaptation allowing the prevention of prey detection; aposematic (functioning as a warning of unprofitability) or may be the result of sexual selection. Colouration may also be function in mimicry of other organisms. The subject may be investigated in terms of both the chemical and physical basis of the colours (proximate cause) and the evolution of colouration (ultimate cause).
Camouflage is generally viewed as a result of natural selection, and involves an organism's colour blending in with its biotic (e.g. moss) or abiotic (e.g. sand) surroundings. Camouflage is often accompanied by behavioural adaptations that make the most of it, such as landing on areas of similar colour, and aligning the body correctly. It may involve costs as well as benefits, such as the cost of finding a suitable resting spot. Colour may change during the seasons, during an organism's life cycle, or even over very brief intervals, such as with the chameleon. Polymorphism may also occur, allowing individuals of the same species to have different camouflage, and making prey detection more difficult for predators. Organisms living in the same environment may come to have similar colouration through convergent evolution. Colours are an aspect of only one of the senses, and although the visual system is most important for humans, some animals cannot even see (such as those living in caves, underground, in the deep sea, or those active at night) and their colour may be of little or no adaptive value. These organisms rely primarily on other senses, such as olfaction and hearing, and even electroreception.Contents.
A camouflaged Jumping spider can easily capture prey.
The Goldenrod Crab Spider (Misumena vatia) has the capacity of changing colour by secreting a liquid yellow pigment into the outer cell layer of the body.
Main article: Camouflage
Cryptic colouration has evolved in many species that have been subjected to the pressures of predation and also in predatory species. Such colours help predators (aggressive resemblance or anticryptic colouring) and prey (protective resemblance or procryptic colouring). Protective resemblance is far commoner among animals than aggressive resemblance, in correspondence with the fact that predaceous forms are as a rule much larger and much less numerous than their prey. In the case of insectivorous vertebrates and their prey such differences exist in an exaggerated form. Cryptic colouring, whether used for defence or attack, may be either general or special. In general resemblance the animal, in consequence of its colouring, produces the same effect as its environment, but the conditions do not require any special adaptation of shape and outline. General resemblance is especially common among the animals inhabiting some uniformly coloured expanse of the Earth's surface, such as an ocean or a desert. In the former, animals of all shapes are frequently protected by their transparent blue colour; on the latter, equally diverse forms are defended by their sandy appearance. The effect of a uniform appearance may be produced by a combination of tints in startling contrast. Thus the black and white stripes of the zebra blend together at a little distance, and their proportion is such as exactly to match the pale tint which arid ground possesses when seen by moonlight (F Galton, South Africa, London, 1889).
Special resemblance is far commoner than general, and is the form which is usually met with on the diversified surface of the earth, on the shores, and in shallow water, as well as on the floating masses of algae on the surface of the ocean, such as the Sargasso Sea. In these environments the cryptic colouring of animals is usually aided by special modifications of shape, and by the instinct which leads them to assume particular attitudes. Complete stillness and the assumption of a certain attitude play an essential part in general resemblance on land; but in special resemblance the attitude is often highly specialized, and perhaps more important than any other element in the complex method by which concealment is effected. In special resemblance the combination of colouring, shape and attitude is such as to produce a more or less exact resemblance to some one of the objects in the environment, such as a leaf or twig, a patch of lichen, or flake of bark. In all cases the resemblance is to some object which is of no interest to the enemy or prey respectively. The animal is not hidden from view by becoming indistinguishable from its background, as in the cases of general resemblance, but it is mistaken for some well-known object.
In the past these effects were explained as a result of the direct influence of the environment upon the individual (GLL Georges-Louis Leclerc, Comte de Buffon), or by the inherited effects of effort and the use and disuse of parts (JEP Jean-Baptiste Lamarck), but natural selection, which can accumulate any and every variation which tends towards survival, has been the accepted explanation now for almost a century. A few of the chief types of methods by which concealment is effected may be briefly described. The colours of large numbers of vertebrate animals are darkest on the back, and become gradually lighter on the sides, passing into white on the belly. Abbott Handerson Thayer (The Auk, vol. xiii., 1896) has suggested that this gradation obliterates the appearance of solidity, which is due to shadow.
The colour-harmony, which is also essential to concealment, is produced because the back is of the same tint as the environment (e.g. earth) bathed in the cold blue-white of the sky, while the belly, being cold blue-white bathed in shadow and yellow earth reflections, produces the same effect. Thayer has made models (in the natural history museums at London, Oxford and Cambridge) which support his interpretation in a very convincing manner. This method of neutralizing shadow for the purpose of concealment by increased lightness of tint was first suggested by EB Poulton in the case of a larva (Trans. Ent. Soc. Loud., 1887, p. 294) and a pupa (Trans. Ent. Soc. Loud., 1888, pp. 596, 597), but he did not appreciate the great importance of the principle. In an analogous method an animal in front of a background of dark shadow may have part of its body obliterated by the existence of a dark tint, the remainder resembling, e.g., a part of a leaf (W Müller, Zool. Jahr. JW Spengel, Jena, 1886).
A camouflaged Orange Oak Leaf butterfly (centre)
This method of rendering invisible any part which would interfere with the resemblance is well known in mimicry. A common aid to concealment is the adoption by different individuals of two or more different appearances, each of which resembles some special object to which an enemy is indifferent. Thus the leaf-like butterflies (Kallima) present various types of colour and pattern on the under side of the wings, each of which closely resembles some well-known appearance presented by a dead leaf; and the common British yellow under-wing moth (Tryphaena pronuba) is similarly polymorphic on the upper side of its upper wings, which are exposed as it suddenly drops among dead leaves. Caterpillars and pupae are also commonly dimorphic, green and brown. Such differences as these extend the area which an enemy is compelled to search in order to make a living. In many cases the cryptic colouring changes appropriately during the course of an individual life, either seasonally, as in the ptarmigan or Alpine Hare, or according as the individual enters a new environment in the course of its growth (such as larva, pupa, imago, etc.). In insects with more than one brood in the year, seasonal dimorphism is often seen, and the differences are sometimes appropriate to the altered condition of the environment as the seasons change. The causes of change in these and Arctic animals are insufficiently worked out: in both sets there are observations or experiments which indicate changes from within the organism, merely following the seasons and not caused by them, and other observations or experiments which prove that certain species are susceptible to the changing external influences. In certain species concealment is effected by the use of adventitious objects, which are employed as a covering. Examples of this allocryptic defence are found in the tubes of the caddis fly larvae (Trichoptera), or the objects made use of by crabs of the genera Hyas, Stenorhynchus, etc. Such animals are concealed in any environment. If sedentary, like the former example, they are covered up with local materials; if wandering, like the latter, they have the instinct to reclothe. Allocryptic methods may also be used for aggressive purposes, as the ant-lion larva, almost buried in sand, or the large frog Ceratophrys, which covers its back with earth when waiting for its prey. Another form of allocryptic defence is found in the use of the colour of the food in the digestive organs showing through the transparent body, and in certain cases the adventitious colour may be dissolved in the blood or secreted in superficial cells of the body: thus certain insects make use of the chlorophyll of their food (Poulton, Proc. Roy. Soc. liv. 417). The most perfect cryptic powers are possessed by those animals in which the individuals can change their colours into any tint which would be appropriate to a normal environment. This power is widely prevalent in fish, and also occurs in Amphibia and Reptilia (the chameleon affording a well-known example). Analogous powers exist in certain Crustacea and Cephalopoda. All these rapid changes of colour are due to changes in shape or position of superficial pigment cells controlled by the nervous system. That the latter is itself stimulated by light through the medium of the eye and optic nerve has been proved in many cases. Animals with a short life-history passed in a single environment, which, however, may be very different in the case of different individuals, may have a different form of variable cryptic colouring, namely, the power of adapting their colour once for all (many pupae), or once or twice (many larvae). In these cases the effect appears to be produced through the nervous system, although the stimulus of light probably acts on the skin and not through the eyes. Particoloured surfaces do not produce particoloured pupae, probably because the antagonistic stimuli neutralize each other in the central nervous system, which then disposes the superficial colours so that a neutral or intermediate effect is produced over the whole surface (Poulton, Trans. Ent. Soc. Lond., 1892, p. 293).
Cryptic colouring may incidentally produce superficial resemblances between animals; thus desert forms concealed in the same way may gain a likeness to each other, and in the same way special resemblances, e.g. to lichen, bark, grasses, pine-needles, etc., may sometimes lead to a tolerably close similarity between the animals which are thus concealed. Such, likeness may be called syncryptic or common protective (or aggressive) resemblance, and it is to be distinguished from mimicry and common warning colours, in which the likeness is not incidental, but an end in itself. Syncryptic resemblances have much in common with those incidentally caused by functional adaptation, such as the mole-like forms produced in the burrowing Insectivora, Rodentia and Marsupialia. Such likeness may be called syntechnic resemblance, incidentally produced by dynamic similarity, just as syncryptic resemblance is produced by static similarity.
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