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Lecture 11

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Department
Biology
Course
BIOL 225
Professor
Ian Ferguson
Semester
Winter

Description
Pigments are coloured chemicals (such as melanin) in animal tissues. For example, the Arctic fox has a white coat in winter (containing little pigment), and a brown coat in summer (containing more pigment). Many animals, includingmammals, birds, and amphibians, are unable to synthesize most of the pigments that colour their fur or feathers, other than the brown or black melanins that give many mammals their earth tones. For example, the bright yellow of an American goldfinch, the startling orange of a juvenile red-spotted newt, the deep red of acardinal and the pink of a flamingo are all produced by carotenoid pigments synthesized by plants. In the case of the flamingo, the bird eats pink shrimps, which are themselves unable to synthesize carotenoids. The shrimps derive their body colour from microscopic red algae, which like most plants are able to create their own pigments, including both carotenoids and (green) chlorophyll. Animals that eat green plants do not become green, however, as chlorophyll does not survive digestion. Chromatophores are special pigment-containing cells that can change their size, thus varying the colour and pattern of the animal. The voluntary control of chromatophores is known as metachrosis. For example, cuttlefish and chameleons can rapidly change their appearance, both for camouflage and for signalling, as Aristotle first noted over 2000 years ago: The octopus ... seeks its prey by so changing its colour as to render it like the colour of the stones adjacent to it; it does so also when alarmed. —Aristotle When cephalopod molluscs like squid and cuttlefish find themselves against a light background, they contract many of their chromatophores, concentrating the pigment into a smaller area, resulting in a pattern of tiny, dense, but widely-spaced dots, appearing light. When they enter a darker environment, they allow their chromatophores to expand, creating a pattern of larger dark spots, and making their bodies appear dark. Amphibians such as frogs have three kinds of star- shaped chromatophore cells in separate layers of their skin. The top layer contains 'xanthophores' with orange, red, or yellow pigments; the middle layer contain
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