AQUATIC MAMMALS 



employ a sculling motion, so as to reach out first on one side and then on 

 the other for undisturbed water, more readily than one with very broad 

 flukes. 



Breder (1926) has stated that in twelve high speed fish the width (or 

 height) of the tail averaged 21 per cent of the total length. The ror- 

 quals are the speediest of the large whales, and one which- 1 measured 

 had a length of 63 feet, with flukes 15 feet broad, so this same propor- 

 tion in this animal was almost 24 per cent. The tips of the flukes were 

 therefore of sufficient length to reach well laterad into undisturbed water 

 as the animal swam, and I have been assured by observers, including 

 trained naturalists, that in this sort of whale there is no lateral or scull- 

 ing motion to be noted when the animal is swimming. 



The shape of the flukes differs in various types of whales. Especially 

 in less speedy sorts the posterior border may be straight, or this edge of 

 either lobe convex in outline and with a median notch. In the rorquals, 

 however, the flukes are more falcate in shape, the tips extending farther 

 back and the posterior border being more suggestive of an S. Hence, 

 the tail is thus inclined to be forked. Apropos of this, Breder (1926) 

 has stated that fishes with squarish or spatulate tails are comparatively 

 slow but capable of extremely sudden short bursts of speed, while those 

 with deeply forked or lunate tails are capable of long continued swim- 

 ming at high velocity, the more lunate the tail the faster being the fish. 

 In discussing this fact Nichols (1915) has pointed out that during 

 speedy locomotion the water displaced by either side of the body of a 

 fish should, directly the fish has passed, meet again with a minimum of 

 disturbance by the median part of the vibrating tail. Breder considers 

 that in the main this is true, but that there are other factors involved 

 is shown by the fact that when he cut a prominent fork in the tail of a 

 fish normally having this member of spatulate shape, no greater speed 

 was attained nor was this reduced, but the motions of the body were dif- 

 ferent. It is therefore justifiable to presume that in the faster sorts of 

 whales such as the rorqual, the broad, falcate, slightly forked flukes are 

 of prime importance in attaining and maintaining speed, but that there 

 are also other factors involved, perhaps of equal import, including, at 

 least, shape of body and muscular conformation, In other words if a 

 gray whale could temporarily be equipped with the tail of a rorqual there 

 is no reason to believe that it could swim any better for the reason that 

 it probably has not the equipment to operate such a tail in the most ad- 

 vantageous manner. 



The structure of the whale's flukes is truly remarkable. The smooth 



[202] 



