,28 DISCOVERY REPORTS 



the total period spent above water (p. 1 16) ; the rest of the time seems to be devoted to getting below 



again, which usually takes much longer than coming up. 



A whale at the surface probably has two primary mechanical disadvantages to contend with, the 

 surface drag and the insufficient purchase which a thin surface layer affords to the action of the flukes. 



A whale at rest, or swimming slowly at the surface, in a more or less horizontal posture (Fig. 4), is 

 probably at this disadvantage, and submergence by means of the forward motion usually observed 

 probably means that submergence is most easily effected gradually. 



The impetus acquired by whales blowing more quickly would thus appear to have a purpose in 

 preventing a situation from arising which would be mechanically weak and would retard submergence. 

 The movements of such whales beneath the surface appear to be as follows. 



The appearance of the head and neck when the whale has broken surface, together with such a view 

 of the body as may be gained while still beneath the water, suggests that the whale has, at first, a 

 straightened back (Fig. 5 a). Towards the end of the action, on the other hand, the caudal peduncle 

 is strongly curved and the body is presumably arched (Fig. 5^). 



The whale breaks surface with a high forward velocity which then decelerates until eventually the 

 exposed portions at the hind-end have very little forward motion, but instead a marked vertical 

 component. 



b a 



Fig. 4. Diagrams illustrating the posture of Fin whales engaged in breaking surface and submerging leisurely. 



The shape of the animal appears to the writer to be particularly suited to this act of submergence. 

 An outstanding characteristic of the Fin whale's back anterior to the dorsal fin is its breadth, and the 

 head especially is flattened. This is a fact that has frequently been noted, though its function appears to 

 have been overlooked (Scammon, 1874, p. 34; Millais, 1906, p. 243). The dorsal fin itself and the 

 caudal peduncle behind it, as remarked by Collett (1886, p. 247), Olsen (1913, p. 1076), Gray (1936, 

 p. 199) and others, is sharply acuminate. Gray has pointed out the value of such a peduncle to the 

 normal swimming actions of fast-swimming forms, but among rorquals it seems to have a further 

 advantage. It will be seen that when the whale approaches the surface its action conforms most closely 

 to the normal swimming — the body is straight and it travels forwards with high velocity — but that 

 when leaving the surface, the body loses forward movement, arches and takes on vertical movement 

 in a dorsoventral plane for which, in section, the fin and peduncle are well shaped. 



In considering the implications of these movements, the following observations are also of assistance. 



The oily looking patch left on the surface where the whale submerges, indicates that the last beat 

 of the flukes is in an upward direction (Fig. 5^-;). Such action would of course be necessary to drive 

 the whale downwards. But it follows that the penultimate beat is in a downward direction (Fig. 5e-/). 

 A downward beat of the tail flukes would tend to lift the posterior part of the trunk unless the latter 

 were able to carry out some other compensatory movement. In this connexion, the observations shown 

 in Fig. i^d-g are probably significant since they show that while the body is arching, the back rises 

 out of the water. Compare with this also the actions of the whales noted on 3 1 January and 7 February 

 which dived suddenly while still beneath the surface with the hind-end of the body as they did so ; 

 one thrusting the dorsal fin and the other, both fin and peduncle out of the water (p. 124). 



It would appear that the movement of the whale after it has blown, is to dip the head and the tail 

 (Fig. 5<i, e). The action of the muscles near the neck and shoulder have been noted, and at this time 

 the whale is still travelling forwards. The arching of the body in the region of the dorsal fin shows that 



