Chapter 4 Summary

Light is a form of electromagnetic radiation that, unlike sound, can travel in a vacuum. It consists of an oscillating electric field and a magnetic field at right angles to each other and to the direction of travel. Electromagnetic radiation exhibits the wave properties of reflection, refraction, diffraction, spreading loss, and attenuation. Its frequency is inversely proportional to its wavelength, but it can also be viewed as a rapid stream of tiny energy packets. The natural electromagnetic radiation spectrum spans a huge range of wavelengths. Low-frequency (long-wavelength) radiation in the radio range bends around most objects in the environment and acts like a wave. High-frequency (short-wavelength) radiation ionizes and scatters molecules and therefore acts more like a flux of massless particles.
Visible light is a very narrow range of electromagnetic radiation in the middle-frequency range. Only in this range can electromagnetic radiation interact constructively with organic matter to facilitate visual communication. Lower frequencies possess too little energy to be detected by organisms except as heat, higher frequency radiation possesses too much energy to be absorbed, and biological tissues are essentially transparent to both extremes. Nearly everything absorbs ultraviolet and infrared waves. The intermediate energy of visible light can be partially absorbed by organic molecules without damaging them, and this energy may then be either reflected or coupled to a detector. Due to atmospheric absorption of the higher and lower frequencies, most of the radiation from the sun that reaches the Earth’s surface is in this narrow visible range.
The speed of light transmission is slowed to varying degrees when it travels through different media because the molecules pose an electric counterforce. Transmission through air is only slightly reduced because molecular density is low and irregular. Rayleigh scattering selectively scatters shorter wavelengths, which makes the sky appear blue. When light waves encounter a boundary with a liquid or solid medium, they interact with the surface layers of molecules and will undergo some combination of reflection, transmission, and absorption. If the medium molecules do not possess a resonance frequency close to the frequency of the light waves and are uniformly arrayed, the waves will be mostly transmitted through the medium layer in a series of reradiations, but travel speed will be slowed. Such a medium—like water or glass—is transparent. If the medium molecules have a resonance frequency in the visible light range, these frequencies will be absorbed, the energy will be dissipated as heat, and neither transmission nor reflection will occur. If the medium is very heterogeneous in density, light waves will be scattered and reflected at the discontinuous boundaries.
Most animals make use of reflected light from the sun to produce visual signals, while only a few produce their own light. Regardless of the source of light, all visual signals can be characterized by the following four properties: (1) intensity or brightness of the signal; (2) spectral composition or color of the signal, including hue and saturation; (3) spatial characteristics of the signal; and (4) temporal variability in intensity, color, and spatial properties.
Pigments are chemical compounds whose molecules undergo selective absorption of certain wavelengths within the visible light range via electron orbital shifts and scatter the remaining wavelengths. The receiver perceives the color of the scattered light. All pigments are organic compounds with chains or networks of conjugated double bonds. Most absorb the higher-energy short wavelengths in the visible range and therefore appear yellow, orange, or red. Pigments include carotenoids, melanins, porphyrins, pteridines, and purines. Some, like guanine, are crystalline in form and reflect and scatter all wavelengths, thus producing white. A few more complex pigment molecules produce green and blue-green colors. Melanin is a very large molecule that absorbs nearly all wavelengths and appears black or brown.
Ultraviolet, violet, and blue colors are usually produced by nanometer-scale structures in the integument that produce coherent scattering of a narrow range of wavelengths. Most green colors and some yellow-to-red colors are also generated with this mechanism. Coherent scattering can be achieved in several different ways, including the alternation of thin layers of high- and low-refractive-index materials (quarter-wave stack), diffraction gratings, and two- and three-dimensional ordered arrays of scatterers with diameters about half the size of a wavelength of light. Layered structures and gratings can generate very bright iridescent colors. Diffuse white color can be generated with a structure of larger, irregular scatterers that produces incoherent scattering.
Bioluminescence is the strategy of chemically self-generated light that is most effective in dark environments and most commonly found in the marine environment. There are only about half a dozen chemical mechanisms for producing light. The basic process involves a luciferin substrate that is combined with molecular oxygen in the presence of a luciferase catalyst to produce an excited-state peroxy-luciferin intermediate, which quickly breaks down while releasing a photon of light. Bioluminescence appears to first arise in response to nonsignaling selective pressures for avoiding or facilitating predation, and then may be co-opted for communication functions.
Visual signals often make use of several coupling mechanisms to generate brighter and more saturated color signals. Two or three color-producing mechanisms may be combined to achieve this objective. In particular, pigment layers on the surface need to have a structural reflecting or scattering layer underneath to direct the nonabsorbed wavelengths back out. Similarly, many of the structural color mechanisms, including thin layers, multilayer stacks, diffraction gratings, and multidimensional arrays, often incorporate a layer of melanin pigment to absorb wavelengths outside of the tuned reflection structure. Some animals have evolved anticoupling mechanisms to render them transparent.
Animals modify color signals using temporal and spatial variation to produce more distinctive or conspicuous visual signals. One strategy is the development of color patterns, which often involves laying down geometric patterns of melanin and one or two other colors. Another strategy is to evolve color-changing mechanisms, such as covering and uncovering color patches with another body part, or evolving chromatophore cells or organs that can move pigment molecules and tissues. Finally, color patches are often coupled with postures and movement displays that feature the appendages or body parts bearing the color.