A stimulus is any phenomenon that directly influences the activity or growth of a living organism. Phenomenon, meaning any observable fact or event, is a broad term and appropriately so, since stimuli can be of so many varieties. Chemicals, heat, light, pressure, and gravity all can serve as stimuli, as indeed can any environmental change. Nor are environmental changes limited to the organism's external environment. In some cases its internal environment can act as a stimulus, as when an animal reaches the age of courtship and mating and responds automatically to changes in its body.
All creatures, even humans, are capable of automatic responses to stimuli. When a person inhales dust, pepper, or something to which he or she is allergic, a sneeze follows. The person may suppress the sneeze (which is not a good practice, since it puts a strain on blood vessels in the head), but this does not stop the body from responding automatically to the irritating stimulus by initiating a sneeze. Similarly, plants respond automatically to light and other stimuli in a range of behaviors known collectively as tropisms, which we explore later in this essay.
Not all responses to stimuli are automatic, however. Certainly not all behavior on the part of higher animals is automatic, though, as we have noted, even humans are capable of some automatic responses. In general, behavior can be categorized as either innate (inborn) or learned, but the distinction is frequently unclear. In many cases it is safe to say that behavior present at birth is innate, but this does not mean that behavior that manifests later in life is learned. (Later in this essay we look at an example of this behavior as it relates to chickens and pecking.)
Behavior is considered innate when it is present and complete without any experience whereby it was learned. At the age of about four weeks, human babies, even blind ones, smile spontaneously at a pleasing stimulus. Like all innate behavior, babies' smiling is stereotyped, or always the same, and therefore quite predictable. Plants, protista (single-cell organisms), and animals that lack a well-developed nervous system rely on innate behavior. Higher animals, on the other hand, use both innate and learned behavior. A fish is born knowing how to swim, whereas a human or a giraffe must learn how to walk.
Ethology is the study of animal behavior, including its mechanisms and evolution. The science dates back to the British naturalist Charles Darwin (1809-1882), who applied it in his research concerning evolution by means of natural selection (see Evolution). Darwin presented many examples to illustrate the fact that, in addition to other characteristics of an organism, such as its morphologic features or shape, behavior is an adaptation to environmental demands and can increase the chances of species survival.
The true foundations of ethology, however, lie in the work of two men during the period between 1930 and 1950: the Austrian zoologist Konrad Lorenz (1903-1989) and the Dutch ethologist Nikolaas Tinbergen (1907-1988). Together with the Austrian zoologist Karl von Frisch (1886-1982), most noted for his study of bee communication and sensory perception, the two men shared the 1973 Nobel Prize in physiology or medicine.
Lorenz and Tinbergen, who together are credited as founders of scientific ethology, contributed individually to the discipline and, during the mid-twentieth century, worked together on a theory that animals develop formalized, rigid sequences of action in response to specific stimuli. According to Lorenz and Tinbergen, animals show fixed-action patterns (FAPs) of behavior which are strong responses to particular stimuli. Later in this essay, we look at examples of FAPs in action. In addition, Lorenz put forward the highly influential theory of imprinting, discussed briefly in this essay and in more detail elsewhere (see Instinct and Learning).
The development of ethology by Lorenz and Tin-bergen occurred against the backdrop of the rise of the behaviorist school in the realms of philosophy, psychology, and the biological sciences. This school of thought had its roots in the late nineteenth century, with the writings of a number of philosophers and psychologists as well as practical scientists, such as the Russian physiolo-gist Ivan Pavlov (1849-1936). Pavlov showed that an animal can be trained to respond to a particular stimulus even when that stimulus is removed, so long as the stimulus has been associated with a secondary one.
Pavlov began his now famous set of experiments by placing powdered meat in a dog's mouth and observing that saliva flowed into the mouth as a reflex reaction to the introduction of the meat. He then began ringing a bell before he gave the dog its food. After doing this several times, he discovered that the dog salivated merely at the sound of the bell. Many experiments of this type demonstrated that an innate behavior can be modified, and thus was born the scientific concept of conditioning, or learning by association with particular stimuli.
The variety of conditioning applied by Pavlov, known as classical conditioning, calls for pairing a stimulus that elicits a specific response with one that does not, until the second stimulus elicits a response like the first. Classical conditioning is contrasted with operant conditioning, which involves administering or withholding reinforcements (that is, rewards) based on the performance of a targeted response.
During operant conditioning, a random behavior is rewarded and subsequently retained by an animal. According to operant conditioning theory, if we want to train a dog to sit on command, all we have to do is wait until the dog sits and then say, "Sit," and give the dog a biscuit. After a few repetitions, the dog will sit on command because the reward apparently reinforces the behavior and fosters its repetition.
Human parents apply operant conditioning when they admonish their offspring with such phrases as "You can't watch TV until you've cleaned your room." Likewise, young chimpanzees learn through a form of operant conditioning. By observing their parents, young chimps learn how to strip a twig and then use it to pick up termites (a tasty treat to a chimpanzee) from rotten logs. Their behavior thus is rewarded, an example of the way that operant conditioning enables animals to add new, noninherited forms of behavior to their range of skills.
Though the theory of operant conditioning goes back to the work of the American psychologist Edward L. Thorndike (1874-1949), by far its most famous proponent was another American psychologist, B. F. Skinner (1904-1990). In applying operant conditioning to human beings, Skinner and his followers took the theory to extremes, maintaining that humans have no ideas of their own, only conditioned responses to stimuli. Love, courage, faith, and all the other emotions and attitudes that people hold in high esteem are, according to this school of thought, simply a matter of learned responses, rather like a parrot making human-like sounds to earn treats. This extreme form of behaviorism is no longer held in high regard within the scientific or medical communities.