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Foraging as soon as possible: impulsiveness and self-control in animals






Self control has been defined historically as an intra-personal conflict: between reason and passion; between cognition and motivation; between higher and lower centres of the nervous system. Self control is said to be the dominance of the former member of each of these pairs over the latter; impulsiveness, the dominance of the latter over the former.

In the last three decades these terms of human psychology have become frequently used within two fields of animal studies, namely, in comparative psychology and behavioural ecology. In animal studies self control means choosing a larger delayed reward over a smaller immediate reward. The opposite of self control is impulsiveness.

A behavioural theory of self-control versus impulsiveness is mainly based on binary-choice studies which are called self-control experiments (Rachlin and Green, 1972; Ainslie, 1974; Mazur, 1987; Bateson and Kacelnik, 1997). Experimental studies are mainly based on the delayed response procedure as well as on the Skinnerian concept of schedule of reinforcement (see Chapter 6). The behavioural self-control experiments are anticipated by typical Skinnerian shaping (sometimes called the method of successive approximation). During this training stage of the experiment a subject must learn which reaction would be counted as “a choice”: for example, pecking, hopping forward from one place to another, touching a figure on a screen and so on.

In recent laboratory experiments on self-control animal and human subjects are given choices between a small reinforce available after a short delay and a larger reinforce available after a longer delay. Preference for the smaller more immediate reinforce is said to reflect sensitivity to reinforcement immediacy, sometimes labelled " impulsiveness, " whereas preference for the larger more delayed reinforce is said to reflect sensitivity to reinforce amount, labelled " self-control."

Fig. III-3 shows a single trial from such a study (Stephens, 2002). The subject waits for t seconds and then it is presented with two stimuli (say a red and a green pecking key for a pigeon). From previous experience it has learnt that the red key leads to a small - immediate consequence, and the green key leads to the large-delayed consequence. Typically, the experimenter designates a small proportion of the trials as forced-choice and no choice trials, in which only one option is available with the goal of ensuring that the subject has some experience with both options. After the animal makes a choice (say, it pecks the green key) it must wait for the programmed delay to expire before receiving the corresponding amount of food (in this example, it waits t1 seconds to obtain amount A1 after pecking the red key, but it waits t2 to obtain amount A2 after pecking the green key). After food delivery the animal must wait another t seconds before the next presentation. The investigator can vary the amounts and delays, and record the proportion of choices made to each alternative. Motivation is typically high: in most experiments with pigeons, for example, the birds are very hungry, as they are deprived of food until they are at 80% of their weight when allowed to eat during the experiments. The impressive strength of animal preference for immediacy has been demonstrated in many experiments: the first second of delay can cut value in half.

In principle, a technique of self-control experiments is a logically elegant way to ask an animal whether it is willing to wait, and under what conditions (Stephens and McLinn, 2003). Decision making demonstration in laboratory experiments should be derived from how animals solve their daily problems in real life and supplied by corresponding neural and sensory bases. No wonder that the self – control paradigm has become a useful tool for developing one of the fundamental concepts in behavioural ecology concerning how the timing and size of food gains affect an animal’s fitness, and how the animal’s actions determine magnitude and timing of fitness gains.

The main field within which parallels between self-control in comparative psychology and behavioural ecology work efficiently is foraging behaviour and corresponding decision making. Results from experimenters in laboratories indicate that animal feeding decisions are guided by short-time considerations. In other words, animals prefer small, quickly-delivered food rewards, even when they could do better (sometimes much better) in the long run by waiting for larger, more delayed rewards.

The difficulty in analysing data concerning impulsiveness and self-control is that animal psychologists and behavioural ecologists use different languages considering a problem of relevant choice in animal life. Some recent investigations have been aimed to integrate of these approaches.

Within a frame of behavioural ecology, foraging theory (Krebs et al., 1983; Stephens and Krebs, 1986) assumes a simple hierarchy of feeding decisions. It supposes that foragers first choose among habitats, then choose among patches within habitats, and finally choose among prey items within patches. These three decisions have usually been studied independently basing on well known ideas about diet choice and patch use (Pulliam, 1974; Charnov, 1976; Orians and Pearson, 1979). Stephens et al. (1986) have integrated these problems by considering patches as being nothing more than clumps of discrete prey, and they asked which prey should be attacked and in what order. As the authors noted, despite the simplicity of this approach, there is some confusion in the literature about how foragers should choose prey within patches. This problem directly concerns impulsiveness and self-control in animal life. Researchers consider the take-most-profitable rule which claims that the relative profitabilities of the items encountered are sufficient to predict preferences, and this includes long-term rate maximising, which is, in turn, based on the so called marginal- value theorem (Charnov, 1976), the best known model of optimal foraging (for details see: Krebs and Davies (ed.), 1997).

Animal psychologists have proposed a concept of preference which is nearly identical to the take-most-profitable rule and which they call momentary maximising (Staddon, 1983). This concept claims that an immediate reward is fundamentally more valuable than a delayed reward and that animals are momentary maximasers, but they sometimes take account of the long-term rate of gain.

Logue (1988) has suggested functional explanations for cross-species differences in self control. One hypothesis could be called the metabolic hypothesis. It says that impulsiveness is adaptive for creatures with faster metabolisms - they need food now rather than later. The other hypothesis could be called the ecological hypothesis. This says that self control or its lack has been shaped by the ecological conditions under which animals evolved.

It could be unadaptive for highly food-deprived animals to wait for a larger amount of food instead of choosing a smaller amount of food that is available sooner. Then one would predict that species with higher metabolic rates - animals for which there is a greater cost to waiting for food-should be less likely to show self-control for food. This has indeed been shown in a comparison among three species: pigeons (with the highest metabolic rate and the most impulsiveness), rats (with the next highest metabolic rate and somewhat less impulsiveness), and humans (with the lowest metabolic rate and the least impulsiveness; Tobin and Logue, 1994). In a further test involving macaque monkeys, it was therefore predicted that the macaques would show more self-control than rats, but less than humans, due to their metabolic rates falling between those of rats and humans. However, instead, the macaques showed more self-control than any other species tested (Tobin et al., 1996). This may be due to the particular ecological niche in which these monkeys live. Wheatley (1980) has described the natural environment of this type of monkeys as consisting of a constant year-round climate and fruiting of food trees. Similarly, Menzel and Draper (1965) found that chimpanzees would often pass up food that was easily accessible if there was a high probability that they would be able to find a greater amount of food at another location.

The world is a noisy place, as Stephens (2002) gives this, and short-sighted choice rules can lead to better long-term results in animal life because they provide a cleaner discrimination of delayed alternatives. This, in particular, enables us to suggest that in many situations the non-human subjects display impulsiveness not because they are incapable of self-control in principle but because testing procedures are not adequate.

Classic behaviouristic procedures sometimes allow revealing more or less adequately chosen strategies based on self-control. A scheme of experiments was suggested by Rachlin and co-authors (Rachlin and Green, 1972; Siegel and Rachlin, 1995). This scheme is as following (see Fig.III-4). In the first series of trials a pigeon is trained to peck a button to obtain food from a hopper. Then the pigeon is offered a choice between two buttons to peck. A single peck on the green button leads to an immediate reward of 2-seconds of access to food; a single peck on the red button leads to a 4- second blackout (a delay) followed by a reward of 4-seconds access to food. The first reward is called the smaller-sooner reward, SS, whereas the second reward is the lager-later reward, LL. The “price” of SS is one peck, while the “price” of LL is 1 peck plus a wait of 4 seconds. When offered such a choice, pigeons invariably choose SS and thus display impulsiveness.

Stephens and Anderson (2001) consider data of self-control experiments from the point of behavioural ecology and they analyse theoretically and experimentally the parallel between the patch exploitation by a foraging animal and its policy to make a choice in self-control experiments.

When a forager exploits a food patch it makes a decision that is analogous to that in a self-control experiment, because it must decide whether to spend a short time in the patch obtaining a small amount, or spend a longer time, obtaining more. Travel time from one patch to another is analogous to the Inter-Trial Interval (ITI) in self control experiments. Empirically it has been demonstrated in many behavioural experiments that ITI shows very little effect. This dramatically contradicts to some data of behavioural ecologists. Patch experiments consistently show animals spending longer to extract more when travel times are long (Stephens and Krebs, 1986). This empirical disagreement stimulated the study of Stephens and Anderson (2001) devoted to testing foragers in equivalent patch and self-control contexts.

The subjects were six adult blue jays which were tested in two economically equivalent situations. The first situation followed the behaviouristic paradigm: jays made a binary choice between small-immediate and large-delayed options. The second situation was modelled on patch-use problems derived from behavioural ecology: the jays made a leave-stay decision in which “leaving” led to small amount (two food pellets) in a short time, and “staying” led to larger amount (four food pellets) in a longer time. Each context was tested at three different ITI’s (30 s, 60 s, and 90 s).

The self-control situation was modelled as a typical self-control experiment. During the ITI, no stimuli were present and birds waited on the “rear” perch. After the ITI expired, the computer switched on a pair of stimulus lights. The bird then chose a side by hopping forward. When the bird made its choice, the computer switched off the unchosen light signal. The pellets were dispensed after the programmed delay elapsed if the bird was on the “food” perch.

The “patch situation” was modelled differently. Only the ITI phase was identical to that in self-control experiments. After the ITI expired, the computer presented the stimulus colour associated with the small amount on a randomly chosen side of the apparatus. Again, the bird must have been on the rear perch for this presentation to occur. When the bird hopped forward, the “small colour” was washed out and the programmed delay began. When the programmed small delay expired, the computer dispensed two pellets and the colour associated with the large (“large colour”) was displayed for 1 s and then washed out; the programmed delay to large (t 2 – t 1) began when the first two pellets were dispensed. If the bird was on the front perch when the programmed delay expired, then the remaining two pellets were dispensed. At any time after the initial hop forward, a hop on the rear perch cancelled the trial and started a new ITI, that is, the bird was free to “leave the patch” at any time.

Self-control experiments demonstrated strong preferences for immediacy, and the observed outcome agreed with short-term rate maximising. In the patch situations, the results agreed more closely with long-term rate maximising, but the picture was complex. When the delay-to small was 60 s, the ITI had no effect on preference, but the jays were more likely to choose the large outcome in the patch context. When the delay-to small was 5 s, preference for large was greatly reduced. The most striking feature of these data, as the authors describe this, is an interaction between context and ITI. In patch, preference for large increased with ITI, while in self-control preference for large decreased with ITI.

Stephens and Anderson (2001) concluded that the birds use the same short-term rules in both situations, and this short-term rule approximates long-term maximising in the patch situations, but not in the self-control situations. This means that long-term rate rules and short-term rate rules accomplish the same thing in patch use, and possibly other situations. It is argued that natural selection may have favoured short-term rules because they have long – term consequences under many natural foraging circumstances.

A general question is still open, that is, how currency and context combine to shape decision rules.

Some answers could be obtained from experiments with honey bees. Stephens et al. (1986) asked a question: are forager’s choices influenced only by immediate, relative payoffs, as the take-most-profitable rule (and momentary maximising) predict, or do the values of the forager’s options elsewhere affect preference, as the theory of optimal foraging predicts? This is the question of distinguishing between momentary rate maximising and long-term-average rate maximising. In experiments with honeybees the researchers arranged pairs of artificial flowers such that a close flower was less profitable and energetically less rewarding than a distant flower. Theoretic models allow constructing predictions as follows. The behaviour predicted by the take-most-profitable rule is to pass by the close flower (since it always is less profitable) and to attack the distant flower. The theory of optimal foraging predicts that “attacking close then distant” should occur at low habitat rates of energy intake and that “attacking only distant” should occur at high habitat rates of energy intake. Investigators can therefore use the first flower visited to distinguish between the models.

In the experiment individually marked bees were trained to visit a long alleyway covered by nylon mesh. Bees extracted “nectar” from artificial flowers of two kinds: “open-faced” and “doughnut”. “Doughnuts” markedly increased the time taken to extract nectar.

Manipulations with habitat richness, flower quality, and several other characteristics led authors to the conclusion that honeybees act like efficient shoppers in a market whose choices depend not only on what they see in the market, but also on what other markets have for sale.

Honeybees do not simply take the most profitable flower available. The relative values (measured in time and energy) of the flowers offered affect honeybee’s flower choice, but flower choice is also affected by how well the bees can do elsewhere in their habitat. The bees choose immediate gain by taking the close unprofitable flower when the habitat rate of energy gain was low, but they passed up the close unprofitable flower when the habitat rate of energy gain was high. The data obtained support long-term-average-rate maximisation over momentary maximisation.

These findings are supplemented by recent study of Cheng et al. (2002). Three experiments based on different procedures showed that bees exhibit relatively high level of self control. The experimental group was offered the classic choices of self control on the first day. They preferred a large delayed reward over a small immediate reward. On the second day, they chose between a large non-delayed reward and small non-delayed reward. The seeming improvement from day 1 to day 2 is most likely due to practice at the task. That is because a similar pattern was found for bees offered both choices with no delay throughout (control). They too improved on day 2, to a similar extent. The results contradict the metabolic hypothesis, but a much clearer formulation of the ecological hypothesis is needed. As it stands at the moment, the formulation is too vague to allow precise quantitative predictions.

Honey bees have helped investigators to accept an idea that momentary maximising cannot be a complete description of behaviour. From the human end, researchers have shown theoretically that taking the most profitable is sometimes the long-term rate maximising behaviour.

 

CONCLUDING COMMENTS

 

There is much work to be done to make our understanding of how the brain deals with past and future events more complete. We know now that very complex processes take place in the brain as well as in the adjacent organism when a gained new skill becomes a memory. This occurs, for example, when a one day chick firstly distinguishes between edible and inedible items. At this stage of our knowledge, we may not consider a brain that learns and remembers as a “black box” but rather as a “Pandora’s box”. An individual carries a brain so that the brain serves as a source of reward and punishment decoding memories whether appropriately or not. It seems that correlation between power in memory and intelligence is more pronounced in non human animals than in our species. Animals do not use memory for eidetic tricks but rather for surviving and improving their lives. Natural selection should lead to differences in specialised learning and memory abilities between animals facing different vital problems such as decision making, food searching and cashing, landmark storage, information processing, and social navigation. At the same time, there are no strong correlations between adaptive specialisation, types of nervous systems, and memory capacities in living things. Exceptionally robust memory systems can be implemented on eccentric brains lacking both cortex and hippocampus. All these enable researchers to broaden comparative investigation on learning and memory within wide variety of species in order to find a thin balance between memory and intelligence in nature and thus better understand the advantages and disadvantages of brain design in our own species.

 


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