Background: From iridescent butterfly wings to brightly colored bird feathers to jet-black mammal hair, animals
are capable of producing tissues of nearly every shade of the rainbow. The colors they produce in their tissues result from two categories of mechanisms: pigments and structural colors. Pigments are molecules the differentially absorb and emit wavelengths of visible light. Some pigments (such as melanins) can be produced by most animals, whereas others (such as carotenoids) cannot be produced by vertebrates and must instead be absorbed from their diet. Because carotenoids play a role in scavenging dangerous free radicals such as antioxidants that have been linked to disease and aging, these pigments have direct links to health and immune function. For this reason, these dietary pigments are thought to play a key role in honest signaling in many animals, transmitting information related to immune status, foraging ability, and so forth. In contrast to carotenoids, melanins are among the most abundant pigments in animals, found in everything from feathers to skin to hair. The two main forms of melanin are eumelanin, which produces brown and black coloration, and pheomalanin, which produces red and yellow coloration. Human hair, for example, varies in coloration at least partly because of different proportions of eumelanin and pheomelanin. Melanin pigments play a key role in many vertebrate and invertebrate badges of status, which are signals of dominance or fighting ability. In contrast to pigments that absorb and emit light, structural colors are produced by light interacting physically with the nanometer-scale arrangement of tissues and air. The different colors that we observe in the iridescent scales of some butterflies or in the plumage of some birds are the result of how these structures are layered and how those layers interact with light. This is why, for example, if you look at a hummingbird from different angles, the colors you see can be very different. The production costs of structural colors are thought to be lower than those of pigment-based colors, though there have been surprisingly few students on this topic. Over the past few decades, spectrometric color quantification has become a standard tool in behavioral biology, both in the field in the lab. This has enabled researchers to examine color in a diversity of freeliving species, lab organisms, and even museum specimens. Most behavioral biologists use a reflectance spectrometer to measure the reflectance of colorful tissues. Ina matter of seconds, precise measurements can be made of the complete visible light spectrum from wavelengths of 300 to 700 nanometers, which include the ultraviolet wavelengths that many animals - thought not humans - can see. Reflectance spectra can then be decomposed using statistical analyses to determine which individuals, tissue types, or body regions are more colorful than others. In this way, coloration can be quantified consistently, inexpensively, and perhaps most important, repeatedly.
a. Given what you've just read about the different mechanisms of animal coloration, which types of colors are more likely to be used in signaling to (1) mates, (2) rivals, or (3) predators? Think about the production costs of making the different types of colors and how they may influence the evolution of color signal.
b. Do you think that some types of colors are more likely to be used in honest signals than others? To answer this question, think about how individuals might be able to cheat in the production of signals using the different kinds of mechanisms.
c. In a number of bird species characterized by mostly white bodies, their flight feathers are distinctly black or dark brown. What pigmentation type is likely responsible for these dark feathers? Why might it be adaptive to have the darker feathers as flight feathers as opposed to, say, chest feathers? How would you test your hypothesis?