Visual Target Discrimination in Sharks — Tester and Kato 
467 
ing which time it received 282 reinforcements 
in 9 sessions. Although it showed many signs 
of discrimination during training periods, tests 
failed to verify the positive conclusion. 
GREY vs. green: One blacktip (B3) and a 
grey shark (Gl) failed to discriminate this 
combination, but a second blacktip (B8) 
showed some indications of discrimination. 
B3 received 299 reinforcements (15 sessions 
in 2 days), while Gl received 424 (11 sessions 
in 2 days). 
B8 showed several signs of discrimination 
during training (18 sessions in 3 days, with 417 
reinforcements). However, tests failed to con- 
firm a positive conclusion. 
grey vs. yellow: A grey shark (G2) pre- 
sented with this problem failed to discriminate 
whereas a blacktip (B8) showed dear signs of 
discrimination. 
G2 underwent 11 training sessions in 2 days, 
receiving 483 reinforcements without showing 
any signs of learning. 
B8 showed that it could discriminate from 
the first of 6 training sessions (total of 155 re- 
inforcements, 99 min) conducted in 1 day, and 
in a test conducted the following day. The 
rapid rate of learning suggests that stimulus 
generalization had occurred. B8 had been 
trained to the grey-green combination prior to 
training against yellow. 
grey vs. red: Of three sharks presented with 
this problem, a grey (G2) and a blacktip (B7) 
showed only inconsistent signs of discrimina- 
tion, but a second blacktip (B8) definitely 
made the discrimination. 
G2 underwent 12 training sessions in 2 days, 
receiving 434 reinforcements. Occasional signs 
of discrimination were shown during training 
and concluding tests, but no decision was pos- 
sible because of inconsistent behavior. 
After an initial 2 days of training (13 ses- 
sions, 362 reinforcements) B7 showed some 
signs of discrimination. However, tests failed 
to confirm the conclusion. An additional day of 
training with 166 reinforcements in 7 sessions 
failed to produce more definite signs of dis- 
crimination. 
In contrast, B8 showed definitely that it 
could discriminate after 86 reinforcements in 
5 sessions. An additional 6 sessions (108 min, 
189 reinforcements) and concluding tests left 
no doubt of discrimination. 
Brightness Discrimination 
Experiments on differential brightness were 
conducted with two blacktips (B3 and B8) and 
one grey shark (G2). One blacktip (B3) 
demonstrated the ability to distinguish between 
shades of grey differing by 2 Munsell units, 
and the grey shark discriminated a difference 
of 3 Munsell units. 
When trained to distinguish between N5/ 
and N6/, B3 showed no signs of discrimination 
after 6 sessions over 2 days (117 reinforce- 
ments). In 6 sessions of the following day (171 
reinforcements) it was then successfully trained 
to distinguish between N4/ and N8/. A test 
confirmed the positive conclusion. Another test, 
conducted after an additional 4 training ses- 
sions (42 reinforcements) left no doubt of 
discrimination. When N6/ was substituted for 
N8/ following the last test, it was found that 
stimulus generalization had occurred, and B3 
reacted to N6/. However, it did not respond 
similarly to N5/ which was also substituted for 
M8/,' giving a difference of only 1 Munsell 
unit. 
B8 showed no signs of discrimination be- 
tween N5/ and N8/ after 9 sessions (2 days) 
and 196 reinforcements. 
G2 showed inconsistent signs of discrimina- 
tion between N5/ and N8/ during 12 training 
sessions in 2 days, involving 473 reinforce- 
ments. Tests confirmed that it could discrimi- 
nate. Substitution of N6/ for N8/ produced 
some signs of discrimination, but no firm con- 
clusion was possible. 
DISCUSSION 
Training Technique and Learning 
The principal aim of our training technique 
was to induce sharks to avoid the shocking area 
when they had learned to discriminate the nega- 
tive target, thus producing a quantitative meas- 
ure of response based on the number of passes 
into the neutral and negative zones. In pre- 
liminary experiments conducted in 1959, some 
of which involved training tanks and tech- 
