The Behavioral Significance of Color

Edward H. Burtt, Jr.

• Publisher: Routledge
• Year: 2018
• ISBN:
  9781351270427
• Edition: 1
• Topic: Psicología
• Subtopic: Historia y teoría en psicología

Prior to publication the study of animal coloration was plagued by fanciful speculations, post hoc explanations and untestable hypotheses. This title, originally published in 1979, draws together widely scattered research into the coloration of animals; formulates predictive hypotheses to account for color; documents the accuracy of many of these hypotheses; and suggests directions for future research. The book grew out of a symposium, The Behavioral Significance of Color at the 1977 meeting of the Animal Behavior Society, and presents evidence concerning patterns of coloration and their influence on animal behaviour and interaction

Physical principles of radiation are discussed in Chapter 1, followed, in subsequent chapters, by an examination of the physiological functions of animal coloration (e.g. thermoregulation, hydroregulation, abrasion-resistance, extraretinal photoreception). Treatment of coloration that affects the animal's visibility to other animals opens with a masterful overview of theories of color vision and its occurrence throughout the animal kingdom. Chapter 6 explores the role of color vision and fruit color in the selection of food by wild primates with comments on the coevolution of fruiting trees and their primate customers. Dr Jack P. Hailman addresses the elusive concept of conspicuousness. He summarizes a strategy for calculating conspicuous coloration based on measurements in natural habitats. Experiments, naturalistic observations and anecdotes of optical communication are exceedingly numerous. Chapters 8 and 9 review these data and suggest general principles of inter- and intraspecific optical communication. Each chapter is enhanced by the critical evaluations of Drs. C. Richard Tracy and W. J. Hamilton III. In closing, the editor discusses coloration as it affects an animal's own vision (e.g., black eyelines to reduce glare).

Most significantly the book emphasizes the need for a balanced, scientifically rigorous approach to the question of evolution of animal coloration. It is an important source for anyone contemplating or currently involved in research in this field of investigation.

Part 1

Physical Principles

Chapter 1

Physics of Light: An Introduction for Light-Minded Ethologists

B. Dennis Sustare

Introduction

Electromagnetic Radiation

Waves or Particles?

Mathematical Description

What is Color?

Geometrical Optics

Reflection

Refraction

Focal Points

Diffraction

Sources of Electromagnetic Radiation

Properties of Black Bodies

Emissivity

The Measurement of Radiation

Inverse-Square Law

Radiometry

Photometry

Sunlight

Solar Spectra

Sunlight’s Earthly Fates

Polarization

Interference

Water and Light

Zoochromes and Phytochromes

Bioluminescence

Recommended Reading

INTRODUCTION

Energy is absorbed, reflected, and radiated by all plants and animals and most animals use some of the information carried by radiant energy to modify their subsequent behavior. Regardless of the behavior examined, the unceasing flow of radiant energy affects the behavioral system. Hence ethologists must renew or make their acquaintance with the physical properties of radiation. For the sake of those whose memories are rusty, I briefly survey some features of the physics of electromagnetic radiation.

ELECTROMAGNETIC RADIATION

Waves or Particles?

Electromagnetic radiation can be thought of as little packages of energy, each package being a discrete unit. The amount of energy in a package characterizes the type of radiation. Along the bottom scale in Figure 1, energy is plotted in attojoules. As a reminder, a joule is the energy required to apply a force of one newton over a distance of one meter, a sizable amount of energy. Because the metric prefix atto- corresponds to a factor of 10⁻¹⁸, an attojoule is a very tiny unit of energy. You can see in Figure 1 that there is little energy in the packages of a radio wave; infrared, visible light (what we can see), and ultraviolet show increasing amounts of energy/package respectively. If the figure continued to the left, more highly energetic radiation would be displayed, for example, X rays.

Energy/package is only one way to characterize electromagnetic radiation. Under some experimental conditions, electromagnetic radiation acts like a stream of particles; under other conditions radiation acts like waves. The more energetic each package of radiation, the more it acts like a particle. High-energy radiation can be more accurately located in time and space and tends to penetrate substances better than low-energy radiation. Low-energy radiation is very wavelike; it bends around objects easily, and is hard to locate in time and space. However, regardless of the energy content, any radiation may demonstrate wave properties, such as the ability to interfere with other radiation in the same way that water waves or sound waves interfere, adding in some places, canceling in other places. Radiation tends to seem more wavelike when it is being transmitted through space, and more particle-like when it interacts with materials.

Fig. 1. Lower scale: Energy of photons, measured in attojoules. Spectral equivalences are shown above the scale (only shown to 0.8 aJ). Upper scales: Correspondence between frequency and wavelength of photons that have the amount of energy shown on the bottom scale. Only that portion of the spectrum up to 1200 THz is shown.

High-energy radiation acts as though its waves were very close together, with a high repetition rate, or frequency, and a very short distance between adjacent wave crests. On the upper scales of Figure 1 are the frequencies and wavelengths corresponding to the energies plotted on the lower scale. Frequency is measured in hertz, one hertz being one wave crest/second; a terahertz is 10¹² hertz. Wavelength, the distance between adjacent wave crests, is measured in nanometers in Figure 1, one nanometer equaling 10⁻⁹ meter. Interestingly, visible light displays a good mix of wave and particle properties, given the scale and sensitivity of our usual measuring devices.

Note that the perceived color of a beam of light is a function of the beam’s energy. Radiation at the low-energy end of the visible light segment appears red. As we progress toward higher energies, the light appears to pass through the colors of the rainbow: red, orange, yellow, green, blue, and violet, the last being at the high-energy end of the visible range.

Mathematical Description

There is a direct relation between the frequency of the radiation and the energy in one radiation package, called a quantum or photon. Note that frequency and energy are plotted arithmetically in Figure 1; they differ only by a constant factor:

where U represents the energy of a single quantum and v is the frequency of that quantum. If energy is measured in joules, and frequency in hertz, the proportionality constant is h, Planck’s constant (6.6256 x 10⁻³⁴ J.s).

There is also a regular relationship between the wavelength and the frequency of radiation. The speed of an object is the distance it travels per unit time. If you multiply the wavelength, a distance, by the frequency, the reciprocal of time, the result is the speed of light (or any other electromagnetic radiation):

where λ is the symbol for wavelength and c for speed of light. One useful fact about electromagnetic radiation is that all quanta travel at the same speed in a total vacuum (approximated by outer space), no matter what their energy content; speed is independent of the wavelength or frequency of the radiation. This speed is very close to 3 x 10⁸ m.s⁻¹. You may find the wavelength (A) by dividing the speed (c) by the frequency (ν); similarly, divide the speed (c) by the wavelength (λ) to find frequency (ν); but be careful that your units are consistent.

WHAT IS COLOR?

What happens when light strikes something? (The following discussion deals with the behavior of a beam of many quanta or packages, not with the behavior of a single quantum.) In general, three things may happen to the light.

It may bounce off the surface (reflection), it may be absorbed and transfer its energy to the absorbing material, or it may pass entirely through the substance (transmission). Often all three processes occur, with some quanta meeting each fate according to the relationship:

where I is the number of incident quanta, R the number of reflected quanta, A the number of absorbed quanta, and T the number of transmitted quanta.

The perceived color of an object depends on the frequencies of visible light remaining after absorption. If the object reflects frequencies that we would call green, then we call the object green. Similarly, if we look at an object by transmitted light, we would call it green if it transmits green light, i.e., light in the frequency range of about 540–600 THz (about 0.36–0.4 aJ or 500–555 nm).

The color of an object is determined by the light that is not absorbed by the object. If you shine a green light on an object that absorbs green light, it appears black. The process of absorption is complicated, and is not discussed in detail; but, briefly, absorption of light by the molecules of the substance causes those molecules to become excited, i.e., to have more energy. The molecules may give up this energy by reradiating it (but at a lower frequency, and hence with less energy, than the radiation absorbed—a phenomenon called fluorescence), or by bumping into a nearby molecule and giving up some of the energy to the bumped molecule, or by flying off from the surface or otherwise converting the energy into work performed.

GEOMETRICAL OPTICS

A quantum (package of radiation) is in motion during its entire existence and tends to maintain its direction, all other things being equal, in a manner similar to the inertia of objects in motion. Those “other things” add many complications that are of little or no importance to ethologists, so I will not discuss such other things as strong gravitational fields, or what “maintain its direction” really means. The movement of radiation in straight lines allows some useful approximations, called geometrical optics, to be developed.

Fig. 2. Reflection: The angle of incidence equals the angle of reflection; here, it is measured relative to the surface plane rather than to the perpendicular with the surface.

Reflection

If light strikes a mirror and bounces off again, the angle between the reflected beam and the mirror is the same as that between the impinging beam and the mirror (Fig. 2). These angles are customarily measured with respect to the perpendicular to the mirror surface rather than as in the figure, but you will recall from your high school geometry that this makes no difference in the equality of angles entering and leaving.

Refraction

When light passes from one medium into another, it changes direction, bending closer to the perpendicular when it enters a denser medium (Fig. 3). This process is called refraction. The relation between the angles is not as simple as with reflection.

Equation (1.4) states that the sine of one angle is a constant (n) times the sine of the other angle. What is this...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Contributing Authors
• Introduction: Edward H. Burtt, Jr.
• PART I: PHYSICAL PRINCIPLES
• PART II: PHYSIOLOGICAL FUNCTIONS OF ANIMAL COLORATION
• PART III: PHOTORECEPTION
• PART IV: COLORATION FOR COMMUNICATION
• Index