Much of the behavior of many animals is highly stereotypic (it is performed in the same way every time) and species-specific (there is little variation in the way different individuals of the same species perform it). We can identify species of spiders, for example, by their web designs (Figure 52.1). Web spinning requires thousands of movements performed in just the right sequence, and for a given species, most of that sequence is performed the same way every time. Different spider species spin webs of different designs, using different sequences of movements.
52.1 Spider Web Designs Are Species-Specific Each spider performs a stereotypic sequence of movements typical of its species that results in a species-specific web design.
Web spinning by spiders is also an example of a complex behavior that requires no learning or prior experience. In many species of spiders, when juvenile spiders hatch, their mother is already dead, and they disperse immediately (remember Charlotte's Web by E. B. White?). They have no experience of their mother's web. Yet when they construct their own webs, they do it perfectly, without the benefit of experience or a model to copy. In fact, their web-spinning behavior is resistant to modification by learning. When confronted experimentally with obstacles to web construction, young spiders appear incapable of learning how to modify the design of their webs.
Many classic studies of stereotypic and species-specific behavior were performed by scientists who studied the behavior of animals in nature. These scientists became known as ethologists. A parallel field of animal behavior studies, called comparative psychology, focused on learning by animals— mostly rats and pigeons—in laboratory situations. In contrast, the early ethologists asked to what extent behaviors are determined by genetic inheritance and to what extent they are modifiable by experience.
Experiments can determine whether a behavior is inherited
Two experimental approaches have been used extensively by ethologists to determine whether behavior is inherited. In a deprivation experiment, an animal is reared so that it is deprived of all experience relevant to the behavior under study. If it still exhibits the behavior, the behavior is presumed to be inherited. In one such experiment, a tree squirrel was reared in isolation, on a liquid diet, and in a cage without soil or other par-ticulate matter. When the young squirrel was given a nut, it put the nut in its mouth and ran around the cage. Eventually it made stereotypic digging movements in the corner of its cage, placed the nut in the imaginary hole, went through the motions of refilling the hole, and ended by tamping the nonexistent soil with its nose. The squirrel had never handled a food object and had never experienced soil, yet the stereotypic behavior of a squirrel burying a nut was fully expressed.
In a hybridization experiment, closely related species are interbred and the behavior of their offspring is observed. Many closely related species show distinct differences in certain kinds of behavior. When such species can be interbred, it is possible to see whether their offspring have inherited elements of the behavior of one or both parents.
Konrad Lorenz, a pioneer in the field of ethology, conducted hybridization experiments on ducks to investigate the hereditary basis of their elaborate courtship displays. Dabbling duck species such as mallards, teals, pintails, and gad-walls are closely related to one another and can interbreed, but because of the specificity of their courtship displays, they rarely do so in nature. Each male duck performs a carefully choreographed water ballet that is typical of his species (Figure 52.2). A female is not likely to accept his advances unless the entire display is successfully and correctly completed.
When Lorenz crossbred dabbling duck species, he found that the hybrid offspring expressed some elements of the courtship displays of each parent species, but expressed them in new combinations. Of particular interest was his observation that the hybrids sometimes showed display components that were not in the repertoire of either parent species, but were characteristic of the displays of other species. Lorenz's hybridization studies clearly demonstrated that the stereo-
52.2 The Mallard Courtship Ballet The courtship display of the male mallard duck contains ten elements.The displays of closely related duck species contain some of the same ten elements, but have other elements not displayed by mallards.The elements of the courtship display and their sequence are species-specific and act to prevent hybridization.
1. Tail shake I 2. Head flick I 3. Tail shake typic motor patterns of the courtship displays were inherited. The observation that females were not interested in males performing hybrid displays was evidence that natural selection was shaping these genetically determined behaviors.
If a behavior is not expressed during a deprivation experiment, it may nonetheless have genetic determinants. The right conditions may not have been available to stimulate the behavior during the experiment. The squirrel described above, for example, had to be given a nut for its digging and burying behaviors to be triggered. Specific stimuli are required to elicit the expression of many inherited behaviors. Konrad Lorenz and Niko Tinbergen conducted classic studies of the nature of the stimuli that elicit such behaviors and called such stimuli releasers.
Releasers are usually a specific component of the sensory information available to an animal. Adult male European robins, for example, have red feathers on their breasts, which serve as releasers of aggressive behavior in other males. During the breeding season, the sight of an adult male robin stimulates another male robin to sing, perform aggressive displays, and attack the intruder if he does not heed these warnings and retreat. An immature male robin, whose feathers are all brown, does not elicit this aggressive behavior. A tuft of red feathers on a stick, however, is a sufficient releaser for male aggressive behavior, even though it looks nothing like a real robin.
Tinbergen and A. C. Perdeck carefully examined the re-leasers involved in the interactions between herring gulls and their chicks during feeding. An adult herring gull has a red dot at the end of its bill (Figure 52.3). When the gull returns to its nest with food in its stomach, its chicks beg for food by pecking at the red dot, thereby stimulating the adult to regurgitate the food for the chicks to eat. Tinbergen and Perdeck hypothesized that the red dot was a releaser for the chicks' begging behavior.
To test their hypothesis, Tinbergen and Perdeck made paper cutout models of gull heads and bills, varying the colors and the shapes. Then they rated each model according to how many begging pecks it received from naive, newly hatched gull chicks (see Figure 52.3). The shape or color of the model head made no difference. In fact, a head was not even necessary; the chicks responded just as well to models of bills alone—as long as they included the red dot. The
4. Bill shake
6. Tail shake
4. Bill shake
6. Tail shake
Question: What characteristics of the herring gull parent release begging responses from their chicks?
METHOD Paper cut-out models of gull heads with many variations were presented to chicks and their begging pecks in response to each model were counted.
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