Читать книгу The Behavior of Animals - Группа авторов - Страница 77

In living organisms, the nervous system is continuously active, and this has many consequences for the occurrence of behavior. Adrian et al. (1931) was the first to demonstrate spontaneous firing of an isolated neuron, and Von Holst (1935) showed that such nervous activity underlay the endogenous patterning of neural impulses responsible for swimming movements in fish. That behavior can occur spontaneously, i.e., without any apparent external cause, was an idea that was long resisted by many behavioral scientists. As we have seen (Chapter 1), there is a long history of behavioral scientists fighting against mentalistic concepts such as consciousness or intentions as causes of behavior. However, it has gradually become clear that intrinsic causes can be studied scientifically and that any explanation of behavior that only takes the effects of external stimuli into account will be incomplete.

One of the earliest attempts to incorporate intrinsic causes into the scientific study of behavior was made by Skinner (1938). His concept of the operant is a motor unit of behavior that occurs originally due to unspecified, intrinsic causes. It is only as a result of conditioning that the operant comes to be controlled by specific stimuli. The motivational model of Lorenz was another attempt (Figure 3.2). Lorenz postulated that motivational energy builds up as a function of time. His model predicts that the probability that a particular behavior pattern will occur increases with the time since its last occurrence. One might imagine that as the pressure in the reservoir increases, it becomes more and more difficult to prevent the energy from escaping through the valve. In fact, behavior does sometimes occur in the absence of any apparent external stimulus. Such behavior has been called a vacuum activity . Lorenz described the behavior of a captive starling that performed vacuum insect hunting. This bird would repeatedly watch, catch, kill, and swallow an imaginary insect.

Lorenz’s model implies a continuously active nervous system kept in check by various kinds of inhibition. A particularly striking example concerns the copulatory behavior of the male praying mantis (Mantis religiosa). Mantids are solitary insects that sit motionless most of the time waiting in ambush for passing insects. Movement of an object at the correct distance and up to the mantis’s own size releases a rapid strike. Any insect caught will be eaten, even if it is a member of the same species. This cannibalistic behavior might be expected to interfere with successful sex, because the male mantis must necessarily approach the female if copulation is to occur. Sometimes a female apparently fails to detect an approaching male and he is able to mount and copulate without mishap, but very often the male is caught and the female then begins to eat him. Now an amazing thing happens. While the female is devouring the male’s head, the rest of his body manages to move round and mount the female, and successful copulation occurs.

In a series of behavioral and neurophysiological experiments, Roeder (1967) showed that surgical decapitation of a male, even before sexual maturity, releases intense sexual behavior patterns. He was then able to demonstrate that a particular part of the mantis’s brain, the subesophageal ganglion, normally sends inhibitory impulses to the neurons responsible for sexual behavior. By surgically isolating these neurons from all neural input, he showed that the neural activity responsible for sexual activity is truly endogenous.

A more recent example of endogenous control is dustbathing behavior in fowl. Most animals possess behavior patterns that can be used for cleaning themselves or for keeping their muscles, skin, or feathers in good condition. These patterns range from simply stretching or rubbing up against some object to complex integrated sequences of behavior used for grooming in many species, such as dustbathing in fowl. This behavior comprises a sequence of coordinated movements of the wings, feet, head, and body of the bird that serve to spread dust through the feathers. One might suppose that this behavior is primarily a reaction to dirt or parasites in the feathers. However, a series of experiments, testing young chicks after periods of dust deprivation, has provided strong evidence that dustbathing is primarily endogenously controlled. In one experiment it was possible to test genetically featherless chicks (Figure 3.5); these chicks also showed a strong correlation between length of dust deprivation and amount of dustbathing (Vestergaard et al. 1999).

Figure 3.5 Genetically featherless chicks dustbathing in sand. Courtesy of Klaus Vestergaard.

The intrinsic factors just discussed are all related to the motivation of specific behavior patterns. One additional intrinsic factor is the pacemaker or oscillator cells that are thought to be responsible for biological clocks (see Chapter 4). These clocks do not control any specific behavior pattern, but rather modulate the behavior mechanisms that control many different types of behavior. Dustbathing in chickens provides an example. A bout of dustbathing can last for half an hour and usually occurs in the middle of the day (Vestergaard 1982). Experiments have confirmed that an internal clock is an important causal factor in the timing of dustbathing (Hogan & Van Boxel 1993). The timing of human sleep is also an important example of oscillator control of behavior (Borbély et al. 2001).

There are many other examples of oscillator control of behavior, but most of the experimental work has investigated the oscillators responsible for daily (circadian) rhythms, often at a neurophysiological or genetic level. There has also been considerable work on the oscillators controlling interval and hourglass timers (Buhusi & Meck 2005). Timing mechanisms and biological rhythms are discussed further in Chapter 4.