Feeding Behavior of Sharks — Hobson 
185 
in very well with the bottom on which it was 
resting. The cessation of movement by this 
grouper upon the approach of the sharks may 
have been a well-established protective behavior 
pattern. We will consider this incident further 
in the next section. During a later period of the 
study, a grouper of the same species and ap- 
proximately the same size was caught at the raft 
where the underwater action was witnessed from 
the observation cage. The grouper wrapped the 
line about a piece of coral and by the time the 
line was freed and the struggling fish hauled 
toward the surface, a single 5 -ft grey came rush- 
ing into the area from downstream. The shark 
went directly for the struggling fish, took the 
tail section into its mouth, and with a few 
vigorous lateral shakes of its head came away 
with the after portion of the fish. 
There is no doubt that rapid erratic move- 
ments are a prime releaser of attack patterns in 
these sharks, particularly in the presence of spe- 
cific olfactory cues. Many authors have noted an 
excited state in sharks resulting from the sensing 
of movement (for example Limbaugh, 1958). 
As both a mechanical disturbance and a visual 
stimulus are usually produced by a moving ob- 
ject underwater, it is very often difficult to say 
with certainty which is more significant as a 
releaser of attack in a given situation. 
On one occasion, while fishing for sharks, a 
3 -ft grey was hooked in the presence of five 
larger greys. As the shark struggled vigorously 
on the line, the other five sharks became highly 
active and appeared to be chasing the hooked 
animal. Initially we supposed that an attempt 
was being made to attack the hooked shark, but 
upon landing this individual after several min- 
utes of activity we noted no evidence of injury. 
If the larger sharks had been attempting to feed 
on the smaller hooked shark there is no doubt 
that they could have done so. It is probable that 
the activity of the hooked shark excited the 
others which were then simply following the 
focal point of this activity. Inasmuch as other 
species of fish, similarly hooked and struggling, 
were immediately taken by the grey shark there 
is a suggestion here that this species of shark is 
inhibited in some way from attacking members 
of its own species. This possibility is further 
supported from observations made while fishing 
for snapper ( Lutjanus bohar) at Palmyra. Grey 
sharks drawn to the fishing area showed no in- 
terest in the many free-swimming snapper, but 
would immediately attack a snapper which be- 
came hooked and began to struggle on the line. 
Contrariwise, although the sharks would swim 
rapidly about one of their own species which 
was hooked and struggling, no attacks were 
observed. 
RESPONSE TO VISUAL STIMULI 
Contrary to many reports on the subject (for 
example Halstead, 1958) vision was found to 
play a major role in the feeding activity of the 
sharks in the Eniwetok lagoon. We briefly men- 
tioned vision above in regard to instances in- 
volving movement. The following experiments 
were designed to further clarify the role of 
vision in cases where there was little or no 
movement of the bait. 
Experiment VII 
The object of this experiment was to deter- 
mine the role of vision in these sharks when 
they are approaching a motionless bait up an 
olfactory corridor. 
Two baits of similar appearance were pre- 
pared for each trial, one of which was a 4-inch 
cube of grouper flesh while the other was a 
4-inch cube of wood. After we were confident 
that no sharks were visible in the area, the two 
baits were presented together in a strong cur- 
rent, suspended 3 ft apart, midway between the 
surface and the bottom. Observations were made 
from the cage, and a record was kept of which 
bait was hit first, as well as of the nature and 
direction of approach of the sharks. 
Experiment III had shown that these sharks 
are capable of orienting on an olfactory stimulus 
in a current and swimming directly to its source. 
In every case during the present experiment the 
sharks appeared from downstream, swimming 
at an accelerated rate, presumably following an 
olfactory trail emitted by the grouper flesh. If 
the sharks had continued to orient exclusively 
on the olfactory stimulus all the way to the bait, 
then the fish would presumably have been taken 
in all trials. This, however, did not occur. In 20 
trials conducted over a period of 2 days, the fish 
was struck first 11 times while the block of 
wood was hit first 9 times. As no preference was 
