Читать книгу The Behavior of Animals - Группа авторов - Страница 75

Stimuli can control behavior in many ways: they can release, direct, inhibit, and prime behavior. Chapter 2 discussed many examples of stimuli that release and direct various behavior patterns. Some stimuli can have exactly the opposite effect: rather than facilitate behavior, they inhibit it. A good example is provided by the nest-building behavior of many species of birds. Birds typically build their nests using specific behavior patterns. The stimuli that release and direct their behavior have been studied in several cases and conform to the general principles already discussed. However, at a certain point the birds stop building and no longer react to the twigs, lichens, or feathers with which they construct their nest. There are several possible reasons why they stop, but one reason is that the stimuli provided by the completed nest inhibit further nest building. This can be seen when a bird takes over a complete nest from the previous season and shows very little nest-building behavior. Other birds, in the same internal state, that have not found an old nest show a great deal of nest-building behavior (Thorpe 1956).

Another example of the inhibitory effects of stimuli is seen in the courtship behavior of the three-spined stickleback (Gasterosteus aculeatus), a small fish. Male sticklebacks set up territories in small streams early in the spring, build a nest of bits of plant material, and will generally court any female that may pass through their territory. Courtship includes a zigzag dance by the male, appropriate posturing by the female, leading to and showing of the nest entrance by the male, following and entering the nest by the female, laying eggs, and finally fertilization (see Figure 3.4). The female swims away and the male then courts another female. The male could continue courting egg-laden females for many days, but usually he does not. Experiments in which eggs were removed from or added to the nest have shown that visual stimuli from the eggs inhibit sexual activity: if eggs are removed from the nest, the male will continue courting females, but if eggs are added to the nest he will cease courting, regardless of the number of eggs he has fertilized (Sevenster-Bol 1962). This is an especially interesting example because the same visual stimulus that inhibits sexual activity has an activating effect on the parental behavior (fanning the eggs) of the same male.

Figure 3.4 Courtship and mating behavior of the three-spined stickleback. The male is on the left and the female, with a swollen belly, is on the right. A typical courtship sequence is indicated below the diagram. (From Tinbergen 1951).

Stimuli not only control behavior by their presence, but in many cases continue to affect behavior even after they have physically disappeared. When a stimulus has arousing effects on behavior that outlast its presence, priming is said to occur. Aggressive behavior in the male Siamese fighting fish (Betta splendens) provides a good example (Hogan & Bols 1980). This fish shows vigorous aggressive display and fighting toward other males of its species (including its own mirror image). If a fish is allowed to fight with its mirror image for a few seconds and the mirror is then removed, it is very likely to attack a thermometer introduced into the aquarium. If the thermometer had been introduced before the mirror was presented, the fish very likely would have ignored it. Thus, the sight of a conspecific not only releases aggressive behavior, it must also change the internal state of the fish for some time after the conspecific disappears. We can say that the stimulus primes the mechanism that coordinates aggressive behavior or, more simply, that it primes aggression. Similar priming effects have been demonstrated with food and water in rats and hamsters, and with brain stimulation in several species (see Hogan & Roper 1978). An especially elegant mathematical analysis of priming in cichlid fish and crickets is presented by Heiligenberg (1974).

These examples of priming all occur during the time span of a few minutes. Some stimuli prime behavior over a much longer period. Stimuli from the eggs of the stickleback inhibit sexual behavior, as we have just seen, but they also prime parental behavior. Male sticklebacks fan the eggs in their nest by moving their fins in a characteristic manner, which directs a current of water into the nest and serves to remove debris and provide oxygen to the developing embryos. The amount of fanning increases over the 7 days it takes for the eggs to hatch. It has been shown that CO₂, which is produced by the eggs, is one of the stimuli releasing fanning, and the amount of CO₂ produced is greater from older eggs. Thus, one might expect that the increased fanning is a direct effect of CO₂ concentration. This supposition was tested in an experiment by Van Iersel (1953). He replaced the old eggs on day 4 with newly laid eggs from another nest. There was a slight drop in fanning with the new eggs, but fanning remained much higher than the original day-1 level. Further, the peak of fanning activity was reached the day the original eggs would have hatched. This means that the stimuli from the eggs must prime a coordinating mechanism and that the state of the coordinating mechanism is no longer completely dependent on stimulation from the eggs after 3 or 4 days.

A similar example is provided by the development of ovulation in doves. A female ring dove (Streptopelia risoria) will normally lay an egg if she is paired with an acceptable male for about seven days. If the male is removed after 2 or 3 days, the developing egg regresses and is not laid. However, if the male is allowed to remain with the female for 5 days before he is removed, the majority of females will lay an egg 2 days later. Experiments by Lehrman (1965) and his colleagues demonstrated that it is the stimuli from the courting male that prime the mechanism responsible for ovulation.

Longer-term effects of stimuli can be seen in the yearly cycle of gonad growth and regression in some birds and fish as a result of changes in day length. And changes in day length can also stimulate a host of other physiological changes including those that prepare migratory birds for their long-distance flight (e.g., Piersma & Van Gils 2011) or various mammals for hibernation in the winter (Nelson 2016).