408 MECHANICAL AND ACOUSTICAL SENSES 



for fishes, regardless of group. It may also mean, however, that the test 

 animals were able to assess rapidly the relative value of specific stimuli in 

 specific contexts. Thus, under the artificial conditions often present in 

 "biological engineering" designs, fishes may well show no adaptive response 

 because the context neither calls for it nor requires it. An appropriate con- 

 text involving sounds of predators may well bring about unmistakable avoid- 

 ance or withdrawal responses (e.g., Moulton 1960, Steinberg et al. 1965). 

 Further study will surely expand on this point, but our findings with silky 

 and lemon sharks suggest that appropriate sounds may indeed be aversive in 

 the biologically adequate context. 



Other workers mention similar withdrawal responses by sharks con- 

 fronted by sudden sounds, such as yelling underwater (Eibl-Eibesfeldt and 

 Hass 1959). Yet, there are also reports to the contrary (e.g., Hobson 1963). 

 Although these differences may have been due to species differences or to 

 variations in underlying motivation, possibly the latter case either involved 

 sound levels below the hearing thresholds of the oncoming sharks or they did 

 not cause a sufficient change in ongoing events to cause the animal to change 

 course. 



Apparently sound level per se cannot be the entire answer, since one 

 sound at a given level will result in rapid withdrawal by sharks, while another 

 at that same or even slightly higher level will attract them right up to the 

 source. The real key to the problem may actually be the progressive increase 

 in loudness as perceived by an approaching shark. Attraction may be initia- 

 ted and maintained by moving toward a given sound whose level increases 

 smoothly (relative to some unknown reference). Withdrawal, on the other 

 hand, may be initiated and maintained by a sound whose structure results in 

 sudden, increased levels, these levels differing greatly (relative to that same 

 reference) from that just previously experienced (or expected) during ap- 

 proach. This implies that the shark is aware of a normal increase in level as 

 it approaches the source. This formulation can be fitted into the well-known 

 Biphasic Theory of approach/withdrawal processes as proposed by Schneirla 

 (1959, 1965). Although the theory had originally been used to explain the 

 organization of processes underlying early behavioral development in verte- 

 brates, its author and others have subsequently attempted to apply it to 

 other stages as well. The theory maintains that intensity of stimulation 

 determines the operative process, i.e., approach or withdrawal. Undoubtedly, 

 intensity plays an important role in attraction and withdrawal behavior in 

 sharks but that factor per se seems less important than the nature of the in- 

 crease in intensity during approach. The apparent importance for sharks of 

 the latter factor suggests that its role should be evaluated in other groups of 

 animals. 



Directional Hearing 



The rapid and directed orientation of free-ranging sharks to distant sound 

 sources had been, until recently, the single exception to certain "rules" 

 formulated by van Bergeijk (1964, 1967) in his popular theory of acoustic 



