AQUATIC MAMMALS 



to be present in the female a sex hormone with action of inhibiting in 

 this sex the developing of extraordinary neck size ; or vice versa. 



This "bull-neck" character in male otariids increases to a quite phe- 

 nomenal degree at the beginning of the breeding season, when they spend 

 more time on land. In fact, bull fur seals and Steller sea-lions, at least, 

 then spend many weeks on land without feeding or entering the water. 

 Unfortunately those in a position to do so have never reported the con- 

 ditions involved. Inferentially there is much fat deposited in the cervi- 

 cal region, this being increased as a reserve supply when the breeding 

 period approaches. O. J. Murie has told me that at the time of rut in 

 caribou (Rangifer) there is a definite thickening of certain of the cervical 

 muscles, which would be useful in the battles between males. This is 

 probably caused by a hormone released by the awakening sex-glands 

 and is entirely comparable with the lengthening of certain perineal 

 muscle fibers in females at the imminent approach of parturition. I deem 

 it likely that a similar thickening of cervical muscles of bull sea-lions 

 takes place at the beginning of the mating season. 



In young males and females the neck is very sinuous with remarkable 

 precision of movement, as all who have watched "trained seals" (usually 

 Zalopbus calif or nianus) will agree. This is inherent and assuredly of 

 fundamental importance to the economy of these mammals. It is not 

 merely developed by training, as indicated by the statement of Rowley 

 (1929) that when a stone is thrown at a sea-lion cow, "no matter how 

 violently nor how short the range, she will catch the stone with marvelous 

 accuracy in her mouth, often at the expense of breaking off her teeth". 

 Because of the shape and the fact that the propulsive mechanism is situ- 

 ated near the center of the mass the neck may act importantly as a rudder, 

 and yet it may be thrust in all directions for the capture of food without 

 disturbing equilibrium provided that at the same time the hinder end 

 is moved in proper compensation. If the aquatic stimulus were for a 

 shorter neck, however, terrestrial activity would be curtailed, and this 

 would result in still greater independence of the land. So the fact that 

 the long neck which is advantageous for swimming happens also to 

 help terrestrial locomotion should result in a tendency to slow up the 

 rate of aquatic specialization in other directions. 



As compared with the seal the cervical transverse processes of the sea- 

 lion are broader, following greater complexity of the longus colli. The 

 spinous processes are also better developed. The most striking additional 

 muscular differences are that the cephalohumeral and humerotrapezius 

 are better situated for twisting lateral movements of the neck and the 



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