288 CHEMICAL SENSES 



a renewed "hunting" response could be elicited when the odor from "fright- 

 ened" or "excited" fish was introduced into the sharks' environment. The 

 author ascribed this renewal of response to stimulation by a "new odor," 

 released by fish under stress but not by quiescent individuals. 



Injury of the fish's skin was not necessary for the release of the odor in 

 question, the source of which, therefore, was not likely to be found in the 

 "body juices." Tester raised the question of whether the attractant, ap- 

 parently emanating from fish under stress, might be chemically related to 

 "Schreckstoff," released through the injured skin of minnows and other 

 cyprinids (von Frisch 1941), and identified as a pterinlike substance by 

 Huttel (1941), Pfeiffer and Lemke (1973), and Pfeiffer (1975). Hobson 

 (1963) confirmed the attractiveness to sharks in Eniwetok lagoon of water in 

 which uninjured but "agitated" groupers had been kept. A siphon releasing 

 that water was located from a distance by whitetip (Triaenodon obesus) and 

 grey (Carcharhinus menisorrah) sharks which followed the odor "trail" up- 

 stream. However, neither Tester nor Hobson has demonstrated the release by 

 fish under stress of a specific odor substance that would allow sharks to 

 distinguish such individuals from "relaxed" fish. Furthermore, it would seem 

 difficult to attribute an adaptive advantage to the ability to discriminate 

 between quiescent prey and prey under stress. Nevertheless, the problem is 

 intriguing and deserves a careful experimental analysis of behavior in re- 

 sponse to body odor from quiescent prey and that under stress. 



Hobson (1963), in a general study of the feeding behavior of the grey 

 shark (Carcharhinus menisorrah), blacktip shark (C. melanopterus), and 

 whitetip shark (Triaenodon obesus), dedicated several field experiments to 

 the role of olfaction in that behavior. In assessing that role, the criterion 

 chosen was the relative time required to locate an uninjured fish, struggling 

 from a line, from downstream or upstream directions. The rationale was 

 that, if the directional cues were olfactory, approach from the downstream 

 direction should predominate and be faster than that from upstream and 

 random directions. In 9 out of 10 trials, involving 16 grey and 2 whitetip 

 sharks, the approaches were indeed from the downstream direction. As 

 Parker had done before him, Hobson raised the important question of 

 "whether sharks can follow an olfactory cue directly to its source in the 

 absence of other cues." Based on Parker's (1914), Tester's (1963), and his 

 own results, Hobson (p. 179) postulated that sharks are "capable of follow- 

 ing an olfactory trail in running water, particularly when the current is 

 strong and the trail narrow, thus forming what would essentially be an 

 olfactory corridor." He also concluded that olfactory stimuli release explora- 

 tory behavior in grey sharks but that feeding requires an additional stimulus, 

 such as vision. 



Hodgson and Mathewson (1971) (and Mathewson and Hodgson 1972) 

 further pursued the analysis of orientation behavior in response to chemical 

 stimulation in lemon sharks (Negaprion brevirostris) and nurse sharks (Gin- 

 glymostoma cirratum). They reexamined the long-standing question of 

 whether "true gradient searching" is used to orient toward the highest con- 

 centration of odor substances or whether these stimuli only trigger anemo- or 



