AQUATIC MAMMALS 



lished, but is a question of some controversy. Their precise pattern 

 need not concern us here, but it may be mentioned that the dental arma- 

 ture of the sea-lion appears to be extraordinarily powerful for the use 

 to which it is put in coping with fish, large ones being torn asunder 

 by a powerful twist of the head rather than by shearing with the teeth 

 proper, and small ones swallowed entire. The strong canines may be 

 chiefly for the purpose of fighting among themselves. 



Among aquatic mammals there are several exceedingly interesting 

 modifications of the front teeth. In the fetal narwhal there is found 

 within a socket in either premaxillary a small, nonp rejecting tusk. In 

 males the right tusk does not ordinarily develop further, but the left 

 one grows, projecting straight forward from and through the upper 

 lip, and attains a length of eight feet or even more. It is twisted, with 

 spiral grooves running in a sinistral direction. Occasionally the right 

 tusk also develops and this too shows a sinistral twist. Females do not 

 normally have a projecting tusk, though very rarely one develops to 

 some extent. No definite use is known for the narwhal tusk. It has 

 been claimed that the males sometimes fence with them in play or 

 in battle, and the theory has been advanced that they are used for 

 digging in the sea bottom, but they grow too long to be handily used 

 for such purpose. The proper explanation probably is that the tusk 

 at first developed to a moderate length of say, one foot, for the purpose 

 of fulfilling a definite need, such as rooting in the mud or as an ice 

 pick. But tusks, antlers, and appendages of this sort are exceedingly 

 prone to acquire undue evolutional velocity and to develop beyond the 

 point of real use, ultimately becoming a hindrance, and hastening ex- 

 tinction through overspecialization (as some extinct elephants, the 

 saber-toothed tiger, Irish stag, etc.). It therefore seems probable that 

 the tusk of the narwhal is too long for any useful purpose and that it 

 is more of a handicap than a help. The enormous tusks of the male 

 Pacific walrus belong in the same category. Originally the canines 

 were undoubtedly used for prying up clams and developed in response 

 to this stimulus until they were ideally fitted for this purpose, with a 

 length of several inches. But the development was not checked and 

 ran wild, resulting in a length and bulk of tusk in the male that must 

 surely prove most unwieldly, and if continued, threatens the extinction 

 of this pinniped through constriction of the mouth to a degree which 

 will hinder the ingress of food. The female walrus is provided with 

 tusks also, but these are of but moderate length. 



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