80 BRAIN MECHANISMS AND LEARNING 



species at least, an internal change directly reduces the appetitive following- 

 drive with age. Conversely, once the following-behaviour has had a 

 chance to establish itself, it will continue towards those models which 

 have become associated with the achievement of the consummatory 

 situation resulting from successful following, and may be yet further 

 generalized to others. 



It is clear that the broad nature of the stimulus to which following 

 animals respond represents a difference only of degree from other instinc- 

 tive responses, especially those of young animals. Not only does the 

 following-tendency itself become primed through exercise and so more 

 firmly established, but the following gives opportunitv for the animal to 

 become conditioned in various ways to the characteristics of the object 

 which has elicited the response in the first place. 



Imprinting can occur in response not only to visual stimuli but to 

 auditory ones also. Thus an animal with an initially weak tendency to 

 respond to both objects and sounds within a wide range may learn to pay 

 attention to, and ultimately to follow, certain sounds which have been 

 associated with the visual stimuli first releasing the following response. 

 This may work both ways. Auditory stimuli may be conditioned to 

 visual characteristics of the object, and vice versa. The recent studies of Dr 

 Klopfer at Madinglcy and in the United States have given particularly 

 good examples of this. In the first place certain surface-nesting species of 

 waterfowl, chiefly mallard and redhead [Aytliya aimricana), were examined 

 and it was found that they were able tc^ learn appropriate sound-signals 

 involved in the maintenance of the brood-parent relationship as a con- 

 sequence of visual imprinting. No unlearned preferences for any particular 

 auditory signals seem to exist; they tended to approach most rhythmic 

 repetitive signals without distinguishing between them. Nor could 

 auditory imprinting to a particular one occur in the absence of visual 

 stimulation. When Dr Klopfer came to investigate the wood duck (^4/.v 

 sponsa) which is a hole-nesting species, the situation was found to be 

 reversed: they did not initially tend to approach the repetitive signals, but 

 exposure to a sound signal alone could produce a subsequent preference 

 for that sound, thus demonstrating auditory imprinting. Further, the 

 imprinted response was not reinforced or altered in any way by following 

 or by a visual stimulus. This fact can be explained by the nesting habit of 

 wood ducks, which require that the young duckling's first response to the 

 mother be based on auditory stinauli. In surface-nesting birds, visual cues 

 can play a larger role from the start, and thus assume paramount import- 

 ance. The quackless muscovy duck [Cuiriiia niosclmra) was shown to be 



