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COPYRIGHT 2005 American Institute of Biological Sciences
Coloration is a diagnostic tool for identifying mammals, but inquiry into its function has lain dormant for almost a century. Recently, the topic has been revived and modern phylogenetic methods have been applied to large data sets, allowing researchers to assess, for the first time, the relative importance of three classic hypotheses for the function of coloration in mammals: concealment, communication, and regulation of physiological processes. Camouflage appears to be the single most important evolutionary force in explaining overall coloration in mammals, whereas patches of colored fur are used for intraspecific signaling. Sexual selection is associated with flamboyant ornamentation in a minority of primates and other restricted mammalian taxa, but to a far lesser extent than in birds. Interspecific signaling among mammals includes aposematic coloration, exaggeration of signals to deter pursuit, and lures for misdirecting predatory attack. Physiological causes of coloration, including melanism, are evident but poorly researched. The relative importance of evolutionary forces responsible for external coloration varies greatly between vertebrate taxa, but the reasons for this variation are not yet understood.
Keywords: comparative method, color, functional hypotheses, mammals, signals
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One of the first things children learn about nature is that certain large mammals have characteristic fur colors: The giraffe is reticulated (i.e., its reddish-brown coat is divided by a network of fine white lines into large geometric shapes), whereas the skunk and giant panda are black and white (figure 1a). When children ask why, adults recite reasons that were formulated more than a century ago, when naturalists speculated about the survival value of pelage and skin colors that they saw in specimens brought back from collecting expeditions (Wallace 1889, Poulton 1890). Parents' dated or incomplete answers (camouflage, advertisement, or "I don't know") stem not from their own ignorance but, sadly, from the fact that the field has advanced so little in 100 years. Naturalists' anecdotes about mammalian coloration were never put to experimental test, and the generality of these ideas--most of them formulated on the basis of only one or a handful of species---remained unexplored until very recently, except for one monumental treatise (Cott 1940). Now, however, we are in a better position to answer children's awkward questions with a modicum of authority.
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The most salient point about the evolution of animal coloration is that different species and different parts of the body are subject to different selective pressures (Hingston 1933, Cott 1940). Classically, these can be divided into concealment, communication, and regulation of physiological processes.
My purpose here is to review new evidence for each of these evolutionary pressures that may have helped to form skin and pelage coloration in mammals and to attempt to assess their relative frequency in nature.
Concealment
Animals can remain concealed when their overall coloration (box 1) resembles or matches the natural background of their environment (Endler 1978). This phenomenon, also known as general color resemblance, includes crypsis (a type of camouflage), in which overall body color resembles the general color of the habitat, or pattern blending, in which color patterns on the body match patterns of light and dark in the environment. Background matching may change seasonally (termed variable background matching) or with age. Concealment may also be achieved through disruptive coloration (also termed obliterative shading) by contrasting colors or irregular marks that break up the body's outline (Merilaita 1998). Finally, animals may attain concealment if they have a lighter ventral surface, because this may counteract the sun's effects--lightening the dorsum and shading the ventrum--when it shines from above (Thayer 1909, Kiltie 1988).
Box 1. The measurement of color. The artist Albert Munsell developed a system for measuring color. He divided hue into 10 classes, red, yellow, green, blue, purple, and their intermediates; he then divided saturation, also known as chroma or intensity, into 6 uniform steps from to 5; finally, he divided tone, or brightness, into 10 intervals ranging from black (0) to white (10). These scores can be measured using a reflectance spectrophotometer, or they can be compared to color chips in a standard reference collection. This has become standard practice in avian studies (Hill 2002), but it is rarely used for mammals (but see Sumner and Mollon 2003). Instead, color is still scored subjectively, classifying first the overall coloration of the coat as patterned or uniform (usually ignoring variation, e.g., lumping black, dark brown, dark gray-black, and brown-black under "dark") and then the markings on specific body parts, usually extremities, such as ears, tails and legs. Markings are defined as an area of color contrasting with the rest of the body or with the nearest area of the body. Thus, a white tail tip on a white animal would not be recorded as such, but a white tail tip on a black animal or one with a black tail would constitute a marking (Ortolani and Caro 1996). Unfortunately, for most taxa, it is difficult to relate coloration or markings to crypsis (camouflage) or conspicuousness because animals that are easy to notice close up may be difficult to see a long way off; zebras are highly conspicuous nearby but surprisingly difficult to see at a distance (figure 1b). Second, the contrast between an animal and its background depends on ambient illumination and spectral reflectance to the background; thus, an animal may be cryptic at one time of day but not later on (Burtt 1981), or against one background but not another (Endler 1990). Third, an animal may be conspicuous to humans but not to nonprimate animals, because primates have three types of color-sensitive retinal cones, whereas carnivore predators possess only two; or they may be cryptic to humans but conspicuous to birds, which have four types of cones, the additional one of which is sensitive to ultraviolet light. Most mammalian studies throw caution to the wind and assume that the human visible spectrum is representative of all the visible spectra possessed by conspecifics and predators in an animal's environment.
Uniform coloration. There is overwhelming evidence of mammals' pelage coloration matching their backgrounds, both between and within species. Across species, at least five different coat colors appear to match the typical background on which they are found among carnivores, artiodactyls, and lagomorphs, the three orders in which statistically and phylogenetically controlled comparisons have been made to date (table 1). Thus, species that are white or become white in winter are found in arctic and tundra biomes (figure 1c), pale species in desert and open environments, red and gray species in rocky habitats, and dark species in closed environments and in dense or tropical forests. Unfortunately, these robust associations do not make a clear-cut case for concealment, because coats of different color...
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