How animals learn has long been the domain of psychology, where learning processes have been described with the objective of defining underlying cognitive mechanisms. More recently, psychologists have joined with ecologists to ask why animals learn in the way that they do. This functional approach to how animals learn is readily introduced with respect to the basic categories of nonassociative and associative learning. Nonassociative learning is exemplified by habituation, defined as the waning of a response to a stimulus upon repeated presentation of that stimulus. Habituation permits an animal to suppress wasteful neural processing of and behavioral responses to stimuli that are irrelevant to fitness. Associative learning entails pairing a stimulus with another stimulus (S—S), or with a motor response (S—R), in space and time such that the response to the first stimulus is altered as a consequence of the pairing. Associative learning enables animals to describe the correlative structure of information in their environment and to adopt behavior that takes advantage of that description. Animals learn by association to orient toward stimuli predicting fitness gains (e.g., acquisition of food or mates) and away from stimuli predicting fitness losses (e.g., effects of heat, toxins, or natural enemies).
The use, or not, of social learning offers another example of a functional approach to learning. Social learning, long known for vertebrates but recently described in insects and other invertebrates, is a form of learning that is facilitated by interactions with other individuals, usually of the same species. Social learning is diverse in terms of mechanisms. One form, local enhancement, occurs when an individual is attracted to a locale where another individual is present, and consequently learns something in that locale. Another form, observational conditioning, occurs when an observer learns a stimulus-reward association by watching a demonstrator experience that association. For example, nectar-foraging bees might learn to prefer a certain floral color by watching other bees visit and extract nectar from flowers of that color. Acquisition of a fear of snakes in monkeys who watch conspecific respond fearfully to snakes has been categorized as observational conditioning. Imitation involves learning in which a pattern of behavior engaged in by an individual is copied. Song learning in birds is viewed by some as a special case of imitation. True imitation that involves imitation of a novel behavior, is rare in animals and difficult to document, but noteworthy because it allows for the rapid spread in a population of truly novel behavior. Chimpanzees learn to use tools to obtain food by imitating other, usually older individuals.
Social learning has costs and benefits that overlap partly, but not entirely with those of individual (asocial) learning. An animal might therefore adjust its use ofsocial or asocial learning as the costs and benefits of each change. This appears to be the case, at least with respect to food-foraging behavior in birds. When a task associated with finding food is made more difficult in terms of individual learning, birds are more likely to rely on social learning.
Memory processes are similarly open to functional explanations. Forgetting is intuitively considered to reflect a constraint on the durability of memory. However, forgetting might be adaptive if it permits animals to discard obsolete information in a way that facilitates the acquisition, storage, and use of new, more relevant information. Forgetting of obsolete information may also reduce 'operating costs' associated with maintaining memory.
A variety of learning processes described mainly in the laboratory may have functional significance in nature. Generalization, a ubiquitous phenomenon in which animals trained to a stimulus S+ respond also to stimuli that resemble S+ along some perceptual dimension, was once assumed to reflect a constraint on learning capacity. From an ecological perspective in which variability in the environment is viewed as a given with which all organisms contend, generalization seems functional. A forager that learns to search only for an S+ associated with a food reward and ignores related stimuli may overlook perfectly suitable food items. Generalization is thus a mechanism for coping with stochastic variation in environmental stimuli.
An adaptive argument can also be made for a phenomenon in learning studies known as blocking. Blocking occurs when an animal that first learns to respond to a stimulus (A+), and is then reinforced on A and a novel stimulus, B, presented together ([AB]+), subsequently fails to show an enhanced response to B alone. Learning of stimulus B has been 'blocked' by coupling it with the previously learned stimulus A. Blocking illustrates a kind of economy in learning: for a stimulus to be learned, it must convey new information. Given a cost to storing information, blocking is functional because it minimizes the storing of redundant information.
Functional explanations like those above have been put forward for a variety of other phenomena including attentional shifts, latent inhibition, peak shift, and effects of local context on learning.
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