Feeding Behavior of Sharks — Hobson 
187 
1 1 times. As in Experiment VII, only grey sharks 
were involved and these approached in every 
case at an accelerated rate from downstream. By 
positioning the bait in mid-water we favored 
this species. A number of whitetips were seen 
about the bottom at various times during the 
trials. 
The baits introduced during this experiment 
were the first food given by us to the sharks in 
over a month, with the exception of a few un- 
injured living fish. This is an important con- 
sideration in this experiment, as any condition- 
ing of the shark population to feeding on our 
presentations would have resulted in a tendency 
to take our offerings indiscriminately. However, 
no conditioning was noted at this stage of 
experimentation. 
The failure of these sharks to show a pref- 
erence for the natural-appearing fish over the 
one which had been reduced to a simple object 
suggests that the visual appearance of a fish 
per se is not itself significant to these sharks 
when feeding. When the results of this experi- 
ment are considered along with the observations 
at Engebi already mentioned, in which the 
sharks struck at inedible objects in the imme- 
diate area of the baits, it appears that the sig- 
nificant visual cue in these cases was simply that 
of an object within the immediate area of the 
source of the olfactory stimulus. This evidence 
suggests that these sharks do not utilize a high 
degree of visual, acuity in their feeding activity. 
It follows, then, that if the visual stimulus is 
not moving, it must otherwise contrast with the 
background against which it is viewed in order 
to provide an effective visual cue. 
The following experiment involves the rela- 
tive effectiveness of two objects in attracting 
the attention of sharks, when these objects differ 
from one another in degree of contrast in bright- 
ness with their background. 
Experiment IX 
This experiment was designed to determine 
which of two baits, one white and the other 
black, would be taken first when both are pre- 
sented to these sharks together at the surface. 
Paired 3 -inch cubes of fish flesh (parrot fish, 
snapper, and grouper) were used as bait in this 
experiment, with members of each pair identical 
except that one retained its natural white color, 
while the other was dyed black with nigrosine 
dye. It had been previously determined that 
nigrosine dye was not sensed by blinded sharks 
in the tanks. 
The experiment was conducted at the raft 
over 40 ft of water when a current was run- 
ning. A wounded grouper was placed in the cage 
until a number of sharks had been drawn about 
the raft. The fish was then removed from the 
water, allowing the source-point of the olfac- 
tory stimulus to be carried off downstream and 
taking the sharks with it. When the sharks were 
about 20 yd downstream from the raft, the two 
baits were dropped into the water, spaced ap- 
proximately 10 ft apart. Movement of both baits 
was negligible as they slowly settled. At times 
the response of the sharks immediately followed 
the entry of the baits into the water, indicating 
that the splash had been sensed; at other times 
the response was noted after the baits began to 
sink, indicating a visual response. These re- 
sponses followed the introduction too closely to 
have been those of olfaction. In either case, the 
sharks came racing back upstream and in all 
cases the baits were taken before they had fallen 
10 ft. The bait taken first in each case was 
recorded, although the alternate bait was always 
taken almost simultaneously. As it was antici- 
pated that the amount of incident light present 
would influence the results, trials were con- 
ducted under different light conditions, with 
from 10 to 56 trials held each day for 4 days. 
The results under each of the different sets of 
conditions prevailing were as follows: ( 1 ) day- 
light with clear sky, in 96 trials black was taken 
79 times, or 82%; (2) daylight with overcast 
and drizzle, in 21 trials black was taken 15 
times, or 71%; (3) daylight with overcast and 
heavy rain, in 14 trials black was taken 8 times, 
or 57%; (4) after sunset, with clear sky but 
almost dark, in 41 trials black was taken 32 
times, or 78%. The total: for 172 trials under 
all conditions, black was taken 124 times or 
72%. 
As the sharks, all greys, raced back toward the 
test area they were viewing the baits against 
light surface water. The black bait, then, con- 
trasted with its background to a greater degree 
than did the white bait. This was as noticeable 
