404 MECHANICAL AND ACOUSTICAL SENSES 



of terms such as dominants and subordinates may be as relevant as applying 

 such terms to cats and mice or foxes and rabbits. Future research will, it is 

 hoped, bring added understanding to this problem. 



The predatory nature of sharks relates well to their rapid response to 

 sounds of wounded, struggling fishes. Yet, predation must surely extend 

 beyond that limited source of food (Hobson 1963, Nelson et al. 1969). 

 Banner (1968) hypothesized that hydroacoustic stimuli associated with 

 normal feeding behavior in other fishes may also stimulate sharks to feed. 

 This could be an even more significant stimulus than that from struggling 

 prey. Such reasoning could explain why sharks are attracted to the natural 

 sounds of prey and nonprey alike (Banner 1972). This leads to the inevitable 

 conclusion that learning should play a role in the attraction process. Sharks 

 have amply demonstrated not only that they are able to learn rapidly and 

 retain a wide variety of tasks (e.g., Aronson et al. 1967, Graeber 1972, 

 Graeber and Ebbesson 1972, Gruber and Schneiderman 1975) but also 

 that individuals of different ages (based on size) within a given species often 

 react quite differently in the same situation. For example, small members of 

 many species are invariably more "curious," more "nervous," more unpre- 

 dictable in their movements, and far less cautious than that noted in larger 

 individuals (personal observation). This difference could be due to ontoge- 

 netic changes uninfluenced by experience, but under the conditions of relative 

 food scarcity encountered by many, if not all, species of sharks, it would 

 seem that the evolution of such carnivorous predators could ill afford food- 

 related activities to be totally uninfluenced by experience. 



One learning process commonly exhibited by sharks in the field is habitua- 

 tion to an attractive sound when trials are massed over a short period in the 

 absence of reinforcement (Myrberg et al. 1969, Nelson et al. 1969) (Fig. 4). 

 Nelson and Johnson (1972) suggested that this process might have affected 

 daily variations in the results of their study. Myrberg et al. (1969) found, 

 however, that at least in sharpnose sharks (Rhizoprionodon sp.— probably R. 

 porosus), prehabituation levels of response to sounds reappeared approx- 

 imately 1 hr after all sound transmission had stopped. 



Such a learning process is apparent in other contexts as well. For example, 

 Nelson and Johnson (1970) reported the following case: an 11-kg (25 lb) 

 grouper had been speared but escaped deep into a hole where neither divers 

 nor sharks could reach it. Within minutes, odors and sounds had attracted 

 several reef whitetips that excitedly circled the site and explored the various 

 holes leading to the fish. The sharks were unable to reach the wounded 

 animal, and their excitement soon waned. Shortly, all disappeared. Chum- 

 ming with fresh bait for the next hour at the site failed to attract a single 

 shark. The probable importance of "learning to ignore" in these animals 

 should not be regarded lightly by researchers, lest their hard-won data 

 contain a sizable artifact. 



The Antithesis of Approach— Withdrawal 



An interesting and apparently paradoxical effect on the behavior of the 

 few species of sharks examined thus far concerns the elicitation of a response 



