Sensory systems provide animals with essentially all of the information they have about their externalenvironment—as well as most of the information they have about their internal environment. Forexample, consider the nocturnal encounter between a hunting bat and a moth in the accompanyingphoto. The bat finds its prey by echolocation: It emits ultrasonic cries (sounds at frequencies above thelimit of human hearing) and hears the echoes of its cries that bounce off objects in the environment, ratherlike human detection of submarines by sonar. Moths, however, have evolved auditory organs that are verysensitive to bat cries. A moth will fly away from faint bat cries (at distances beyond the range of the bat’sechodetection), but in response to louder cries (when the bat is close enough to get clear echoes) the mothwill either take erratic and apparently random evasive action or power-dive to the ground.Clearly it is of selective advantage for bats to have evolved auditory systems that allow them to useecholocation to detect and capture insects in the dark, while also detecting and avoiding obstacles. Likewiseit is of selective advantage for moths to detect and evade hunting bats. We will examine aspects ofauditory sensory systems in both bats and moths later in this chapter, along with other sensory systemsthat provide animals with information about their environments.Sensory systems of all kinds depend on specialized sensory receptor cells that respond to stimuli,either environmental stimuli or stimuli arising inside the body. Different sensory cells respond to differentstimuli, and they vary greatly in sensitivity (the ability to distinguish among stimuli of different intensity)and specificity (the ability to distinguish among stimuli of different types). Auditory reception in bats andmoths is both sensitive and specific, having been shaped by long periods of natural selection. But all cellsare somewhat responsive to aspects of their environment and thus subserve some functions that can beconsidered sensory. For example, bacteria and protists respond to light and to chemical gradients. Cellularresponses of this kind presumably preceded the evolution of specialized sensory neurons. A sensoryreceptor cell is a cell that is specialized to transform the energy of a stimulus into an electrical signal. Thekind of stimulus that excites different receptor cells maybe chemical, mechanical, or electromagnetic. A stimulusis a form of external energy (external to the cell) to whicha sensory receptor cell can respond.Humans have studied sensory functions since theancient Greeks, and currently new experimental approachesare revolutionizing our understanding of howsensory systems work. Recording techniques such aspatch-clamp recording (see Figure 12.17) have greatlyincreased the ease of registering the responses of sensorycells. Imaging methods have allowed the examination ofactivity of many neurons at once. Molecular techniquesSensory ProcessesCHAPTER 14A nocturnal encounter between a moth and a hunting batBats echolocate to orient and to capture insects, and mothshave evolved auditory systems that help them evade batpredators. In this case a moth hears the bat’s cries and is ableto evade the bat by diving toward the ground. Sensory systemsmay be energetically expensive, but the information theyprovide is often crucial for survival.