Читать книгу The Behavior of Animals - Группа авторов - Страница 71

In popular usage, the word motivation often refers only to internal causes of behavior. We speak of an animal’s search for food as motivated by hunger, but of chewing and swallowing as reflex actions to stimuli in the mouth. On close inspection, however, it turns out that a thoroughly sated animal will often spit out the same food it would have chewed if it were hungry; and hungry animals are clearly guided by environmental cues as they search for food. In fact, any behavior must be caused by some combination of both internal and external factors.

Many years ago, Lorenz proposed a motivational model of behavior that illustrates the interdependence of internal and external factors. This model is shown in Figure 3.2. According to Lorenz, each behavior pattern (motor mechanism) is associated with a reservoir that can hold a certain amount of energy. Whenever the behavior pattern occurs, energy is used up; but when the behavior pattern does not occur, energy can build up in the reservoir. The higher the level of energy, the more pressure it exerts on the valve. When the value opens, as a result of external stimulation, energy is released and the behavior occurs. Thus, in this model, a particular behavior pattern cannot occur without at least some internal causal factors as well as some external ones. Further, the model makes it clear that internal and external factors can substitute for each other in determining the intensity of a behavior pattern: a strong stimulus can compensate for weak internal factors and vice versa.

Figure 3.2 Lorenz’ model of motivation. The tap (T) supplies a constant flow of endogenous energy to the reservoir (R). The valve (V) represents the releasing mechanism and the spring (S) the inhibitory functions of the higher coordinating mechanisms. The scale pan (Sp) represents the perceptual part of the releasing mechanism, and the weight applied corresponds to the impinging stimulation. When the valve is open, energy flows out into the trough (Tr), which coordinates the pattern of muscle contractions. The intensity of the response can be read on the gauge (G). (From Lorenz 1950).

The fact that both internal and external factors are essential for any behavior to occur does not imply, of course, that one cannot study the effects of internal and external factors separately. The effects of varying various internal factors can be determined if the external situation is kept relatively constant, as can the effects of external factors if the internal state of the animal is held constant. A classic example of such a study is the one by Baerends et al. (1955) on the courtship behavior of the male guppy (Lebistes reticulatus). Courtship by the male comprises a number of behavior patterns, including posturing in front of the female, a special sigmoid posture, and copulation attempts. These authors were able to derive a scale of internal motivation using the relation of marking patterns on the body of the male to the number of copulation attempts; external stimulation was considered to be proportional to the size of a female. Figure 3.3 shows the results of an experiment in which females of different sizes were presented to males at different levels of internal motivation. The points plotted on the graph represent the relationship between the measures of internal and external stimulation at which particular patterns of behavior were observed. If it is assumed that the total motivation necessary for a specific behavior pattern to occur is always the same, the patterns “posturing,” “sigmoid intention,” and “sigmoid” can be seen to represent increasing values of courtship strength. The lines connecting these points of equal motivation have been called motivational isoclines by McFarland and Houston (1981).

Figure 3.3 Results of an experiment on guppy courtship. (a) Relationship between the intensity of the external stimulation, the intensity of the internal stimulation, and the kind and degree of development of the resulting activity. (b) “Calibration curve” for determining the place of the different marking patterns on the abscissa of (a). CA, copulation attempt; S, sigmoid posture; S_i, sigmoid intention; P_f, posturing in front of the female. (From Baerends et al. 1955).