Feeding Behavior of Sharks — H obson 
181 
It might be suggested that the olfactory stim- 
ulus had not itself been directional, but that 
the sharks, having been alerted by the olfactory 
cue, had simply turned upstream, and oriented 
to the current. However, while a rheotaxic re- 
sponse may have influenced the shark in its ini- 
tial decision as to which way to go in the cor- 
ridor, this observer does not feel that the re- 
sponse observed could have been directed by 
such a cue. Among other considerations, In an 
approach directed solely by current the shark 
would have at least briefly overshot the source 
of the olfactory stimulus. This would have re- 
sulted in a brief but certainly noticeable period 
of uncertainty as the shark turned back to pick 
up the olfactory cue once again. Nothing of this 
sort was seen. Furthermore, the following of an 
olfactory corridor was observed in the absence 
of current and will be described shortly. 
Occasionally the extract elicited a response 
from other fish in the area which could have 
provided an approaching shark with supplemen- 
mentary cues. However, in only 2 of 27 ob- 
served approaches might this source of error 
have affected the results. 
When the current subsided, the approach of 
the sharks immediately became less direct — 
illustrating the importance of the current in 
maintaining the definition of the corridor. No 
new sharks appeared during slack water and 
those already present milled about continuously 
within 30 yd of the cave. Approaches to the 
cave during slack water were made in a random 
manner and from all directions. At this time it 
was apparent that the material was diffusing 
out in all directions from its source and was at 
the same time being retained in the immediate 
area. 
The whitetip did not seem to be as respon- 
sive to the introductions as were the blacktip 
and the grey. On several occasions whitetips 
swam directly to the cave in the manner noted 
in the grey and blacktip. However, whitetips 
just as often swam past the hole without any 
noticeable response to the extract. As apparent 
unresponsiveness has been noted as characteris- 
tic of the whitetip, it is difficult to draw any 
direct comparisons between this species and the 
grey and blacktip from the observed behavior. 
Whitetips do have the ability to follow an 
olfactory corridor. The following quotation from 
the field notebook describes an incident which 
clearly illustrates this and also the formation of 
a corridor in the absence of current: 
A large parrot fish (Scaridae, 10 lb.) was speared in 
about 20 ft. of water. The fish tore itself from the 
spear and took shelter in a large coral head. Within 1 
min. a 5 ft. whitetip appeared. It became obvious that 
the shark had sensed the presence of the wounded fish 
as it poked about the holes of the coral head and then 
swam into one of them. The chase which followed 
was witnessed from the surface. The two fish could 
periodically be seen through one or another of the 
many holes which honeycombed the coral head. First 
the parrot fish would flash by and then the whitetip 
in pursuit. The coral head contained an extensive net- 
work of caves and the chase seemed to take advantage 
of most of them. Suddenly the parrot fish emerged 
from a hole. Apparently it had temporarily eluded the 
whitetip because there was no immediate sign of the 
shark. The parrot fish swam off rapidly on a straight 
course for about 30 yd. where it made a 90° turn and 
continued on the new course, in a straight line, until 
it was out of sight. The whitetip emerged from the 
hole just seconds after the parrot fish, but already its 
prey was out of sight. The whitetip circled briefly, 
then started out along the same path taken by the par- 
rot fish. When it arrived at the point of the 90° turn 
it continued on a few yards, but quickly slowed and 
turned around. After another brief period of circling 
the shark picked up the second leg of the trail and 
followed it straight out of sight. 
In this instance, the corridor was formed by 
olfactory substances given off by the moving 
wounded fish. There was no noticeable current 
at the time. 
The sharks involved in Experiment II had 
apparently been initially alerted by an olfactory 
stimulus emitted by a fish which, while in dis- 
tress, was uninjured. 
The ability of these sharks to detect the pres- 
ence of an unwounded fish in a state of stress 
through an olfactory cue was noted by Tester 
(1963) early in the program. This point has 
been neglected since the observations of Shel- 
don (1911), in which a dogfish was noted to 
locate an undamaged crab wrapped in eelgrass. 
However, in Sheldon’s experiment the attract- 
ing stimulus (or stimuli) may have been a 
movement or sound made by the crab instead 
of, or in addition to, an olfactory stimulus. Any 
conclusions derived from Experiment II are 
open to the same criticism. Another source of 
criticism might be the assumption that a fish 
with a line passing through its maxillary mem- 
brane may be considered uninjured. The ex- 
