The Eastern Indigo Snake ( Drymarchon couperi), a large, mostly iridescent black serpent found in open pine habitats of the southeastern United States, may represent one of the rare snake species in which sexes differ in color pattern. This contrasts with snakes, which rarely exhibit sexual dichromatism and, in the best studied cases of this feature, do not support sexual selection as the driving force generating sexual dichromatism. agamids: anoles:, chameleons: collared lizards: ). In particular, many groups of lizards are primarily visually-oriented, utilize color signals, and exhibit sexual dichromatism (i.e. For example, among snakes, a diverse group that includes some 3000 species that are nested within the group Squamata, display color variation among individuals –, but some aspects of color variation appears to be more restricted than it is among the nearly 6,000 species of lizards comprising the rest of the group Squamata. Studies of vertebrate groups in which color variation is rare may provide novel insights into the evolutionary development and adaptive significance of these traits. A first step in understanding how color may be used by individuals is to determine the extent to which color correlates with sex identification or relatedness. However, possession of conspicuous color is often at odds with predator avoidance, implying that the benefits of producing colors that attract mates outweigh the survival costs of being more conspicuous to predators or outweigh costs of producing color pigments or structures.
In many vertebrates, color is used to signal sexual identity and mate quality, and, thus, color is used to select mates –. Sexual dichromatism (when males and females within a species differ in color traits) is a common result of sexual selection for color traits. For example, sexual selection may lead to bright conspicuous coloration, but natural selection via predator avoidance would favor cryptic colors. Studying species that exhibit bright colors within lineages that are not known for such coloration will contribute greatly to our understanding of the evolutionary and ecological factors that drive these differences.Īnimal color signals undergo selection through many different evolutionary processes that may be at odds with one another. We suggest that future studies should focus on determining whether color variation in these snakes is determined by maternal effects or genetic components and if color is influenced by natural selection or neutral evolutionary processes. We found that Eastern Indigo Snakes do not exhibit sexual dichromatism, but their coloration does vary among clutches therefore, the pattern of sexual selection leading to sexual dichromatism observed in many squamates does not appear to play a role in the evolution and maintenance of color variation in Eastern Indigo Snakes. We used color spectrometry to characterize important aspects of color in two ways: by examining overall color differences across the entire color spectrum and by comparing differences within the ultraviolet, yellow, and red colorbands. We tested these alternative hypotheses by examining whether coloration of the sublabial, submaxillary, and ventral scales of this species differed by sex or among clutches. Alternatively, color might vary among individuals because it is controlled by genes that are under natural selection or neutral evolution. Past researchers have proposed that the color variation on the head and chin is associated with the sex of the individual. Despite significant conservations efforts for this species, little is known about its biology in the field. Eastern Indigo Snakes ( Drymarchon couperi), a federally threatened species, have coloration on the sides of the head and the chin that can vary from black to red or cream. Various forms of natural selection, such as mimicry, crypsis, and species recognition, as well as sexual selection, can influence the evolution of color. Many species exhibit variation in the color of their scales, feathers, or fur.
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