174 
PACIFIC SCIENCE, Vol. XVII, April 1963 
Fig. 3- The whitetip shark, Triaenodon obesus. (Photo by E. Hobson.) 
the other sharks always followed in close pur- 
suit. Although the flight of prey was commonly 
noted to release attack in the grey, the aggres- 
sive behavior of these other sharks in this case 
was believed to have been directed against the 
bait rather than the fleeing shark. 
There was never any mistaking an alerted 
grey or blacktip from one engaged in normal 
patrolling activity. Thus we had no difficulty 
distinguishing a grey or blacktip entering the 
test area in response to a stimulus situation we 
had presented from one incidentally passing 
through the area. The movements of the alerted 
greys and blacktips were markedly accelerated 
and the grey in particular seemed tense and 
highly responsive to subsequent stimulation. 
Movements of the grey immediately before at- 
tack were markedly abrupt; its body often ap- 
peared stiff, with back slightly arched and head 
extending straight out and slightly upward. The 
pectoral fins were characteristically pointed no- 
ticeably downward. Attack was prefaced in 
many cases by such anticipatory movements as 
a lateral shaking of the head (noted also by 
Eibl-Eibesfeldt and Hass, 1959) and a move- 
ment of the jaws as in biting (Hobson et al, 
1961 ). 
Although it was not always so easy to make 
this distinction with the seemingly unrespon- 
sive whitetip, these sharks were undoubtedly 
more responsive than they appeared to be. It is 
likely that their reactions were simply more 
subtle than those of the more excitable greys 
and blacktips. Whitetips usually appeared in 
an area shortly after divers had undertaken 
various types of underwater activity. However, 
after their appearance the whitetips would swim 
about without any apparent interest in the pro- 
ceedings. Nevertheless, the consistency of these 
appearances indicated that the sharks sensed ac- 
tivity and were interested. 
DISTRIBUTION OF SPECIES WITHIN LAGOON 
Blacktips were the most commonly observed 
shark over the sand and coral rubble flats lying 
under approximately 1-40 ft of water at the 
perimeter of the lagoon. These flats extend out 
from shore for distances ranging from approxi- 
mately 50 yd to several miles before the bottom 
falls off sharply into the depths. Coral growth 
in this area is generally restricted to large iso- 
lated heads which, in many cases, reach the 
surface of the water at low tide. Although black- 
tips exceeding 6 ft in length were seen, speci- 
mens of more than 4 ft were not common. Small 
blacktips were very common on the seaward 
reef flats when the sea covered these flats at high 
tide. The seaward flats were largely exposed at 
low tide. 
The whitetip also frequented the shallow 
waters of the lagoon, although unlike the black- 
tip, which foraged widely over the flats, the 
whitetip centered its activity among the coral 
heads and about the coral-rock ledges which 
border the seaward passages. Unlike the other 
two species, the whitetip was commonly seen 
resting motionless on the bottom, often under 
