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This introductory chapter talks about how every creature is guided by its eyes as it carries out its accustomed behaviors. Each animal's eyes allows it to execute the behavior necessary for its survival. This study of how visual systems function to meet the ecological needs of animals is called visual ecology. Researchers who work at various levels of inquiry, from genes to behavior, call themselves visual ecologists, but all are primarily concerned with how animals use vision for natural tasks and behaviors. Although the outcomes of visual ecological research may well have implications for health or may be applicable for use in engineering or technology, the research itself centers on the animal of interest and on how it employs its visual system to meet its own ecological needs.

This chapter explains how humans, and nearly all animals on Earth, witness astonishing variation in their optical environment. Brightness changes by many orders of magnitude each day, and colors also shift dramatically. Those animals that enter forests and especially the water experience even larger changes. Given this, it is surprising that nearly all the natural light on Earth ultimately comes from two sources, the sun and bioluminescence. A final source of light that is potentially relevant to vision is mechanoluminescence. In this process, light is produced by mechanical processes, including deformation (piezoluminescence), fracturing (triboluminescence), and crystallization (crystalloluminescence). The latter two have been suggested as being at least partially responsible for ambient light at deep-sea vents.

This chapter discusses how darkness provides excellent advantages for a wide variety of animals, for the simple reason that vision—a primary sense for predators and foragers alike—becomes severely disabled when faced with a paucity of light. Thus, in a fiercely competitive rainforest, the cover of night provides respite from visually dependent predators and competitors, a fact that has encouraged the evolution of nocturnal activity in many different taxa. In the endlessly dim world of the deep ocean, the cover of darkness is instead permanent, and vision is relentlessly pressed at the limits of the physically possible. In some species the eyes have evolved extreme adaptations for extracting the most fleeting of visual cues. Others have given up the fight altogether, their eyes having regressed to mere vestiges.

This chapter addresses why animals have evolved different numbers, ranges, and placements of spectral channels in their color-vision systems. It also examines the factors, such as water transmission, visual task, phylogeny, and activity patterns, that drive the evolution of such diverse modes of seeing color. Even in the absence of any color sense, trees are still visible, as most of the information in natural scenes can be gained from achromatic cues alone. Color vision, however, gives an animal more information, allowing it to make quicker and more informed decisions. The chapter attempts to disentangle man's experience of color from that of other animals to provide an objective measure of what color vision is and how evolution has molded its variety of forms.

This chapter focuses on visual pigments and photoreceptors. In living things, photoreception inevitably begins with a photochemical event—a molecule intercepts a photon of light and is somehow changed. Various molecules, generally known as photopigments, perform this function in animals and plants. The molecules involved in vision are called visual pigments. In all animals, vision ultimately depends on a single family of proteins that all have descended from one common ancestor—these are the opsins. The chapter cites the hydrothermal vent crab as a good example of how changes of visual pigments appearing in various developmental states reflect ecological adaptation. The animal's life stages require visual systems sampling opposite ends of the visual spectrum.

This chapter studies the eye designs of the animal kingdom. Today, there are ten generally recognized optical eye types that have evolved in various branches of the animal kingdom. Whereas vertebrates possess only one of them, invertebrates possess all ten, from simple assemblies of photoreceptors that underlie phototaxis to advanced compound and camera eyes that support a sophisticated range of visual behaviors. Some invertebrates even possess several eyes of more than one type. The chapter identifies some of these eye types in the context of sensitivity and resolution, namely, pigment-pit eyes, compound eyes, and camera eyes. The last of these are characteristic of the vertebrates, although they are also commonplace among the invertebrates. The remaining nine eye types are found only within the invertebrates.

This chapter analyzes the optical building blocks of eyes. Irrespective of their optical specializations, all eyes have one thing in common: they collect and absorb light arriving from different places in the environment, thus giving animals information about the relative distribution of light and dark in the surrounding world, including the contrasts and positions of objects. This information is used to support a variety of visual tasks, such as identifying and avoiding predators, detecting and pursuing prey or conspecifics, and orientating and navigating within the habitat. Although some animals use their eyes to perform more or less all of these tasks, others do not. All visual systems evolved within one of two main categories, being either general purpose or special purpose.