Learning, like consciousness, is something that everybody can recognize and no one can define without provoking controversy. Perhaps this is why some important books dedicate hundreds of pages to learning without defining it (e.g., Mackintosh, 1974; Marler and Terrace, 1984). In one unusually candid book, the indexed page that promised a definition of learning proved to be completely blank. That stimulated me to make my own definition, something that is easier for a person who is not an expert in the field: learning is a change in the nervous system manifested as altered behavior due to experience (based on discussions in Marler and Terrace, 1984; Bell, 1991; Mackintosh, 1974, 1983; Papaj, 1994). Most people, including most biologists, probably underestimate the importance of learning in the biology of nonhuman animals. But there have been important exceptions, for example, in the writings of Baldwin (1902), Hinde (1959), Partridge (1983), Roper (1983a,b), Slater (1983,1986), Shettleworth (1984), Davey (1989), Wcislo (1989), Real (1993, 1994), Dyer (1994), Morse (1980), Marler (1998), and others (see Marler and Terrace, 1984). Some form of learning, whether habituation, associative learning (Pavlovian conditioning, in which a reward or punishment is associated with some cue such a color, odor, or sound), aversive learning, or trial and error learning (operant conditioning, in which a rewarded behavior is repeated or a punished one stopped), seems to occur in all animal groups where there is enough versatility in movement to allow it to be recognized. The venerable animal psychology text by Maier and Schneirla (1935 [1964]) gives many interesting examples from a time when researchers sought to demonstrate learning in a wide variety of organisms. They found it even in protists. In more recent research in areas such as foraging behavior and kin recognition (e.g., see Heinrich, 1979; Fletcher and Michener, 1987), learning has proven to be important but is a sidelight to research more concerned with optimization and adaptation. So learning itself has not always received the attention it deserves as a phenomenon of general evolutionary interest.
Do gastrointestinal taste receptors contribute to associative learning and foraging behavior?1
