AQUATIC MAMMALS 



hold its nose above the surface with slight effort because of the small 

 size of its head. A slight tilt in the cranial axis is all that is necessary. 

 If it had a blowhole upon the forehead it would be but two or three 

 inches from the present position of the nostrils and I judge that the 

 difficulty it might experience in keeping water from slopping into the 

 nose would be even greater than at present. Therefore small size should 

 be a retardant of nasal migration. On the other hand, a long-snouted, 

 large-headed beast with a body of ten or twelve feet would encounter a 

 different set of conditions. The snout would travel through a larger arc 

 for breathing, nostrils upon the forehead would be of more definite 

 advantage than in the otter for the reason that they could then be thrust 

 higher out of water with but slight exertion. The critical factors here 

 would be size of head and speed; but the forces at play are intricate. 

 Thus, in a mammal propelling itself from the rear the head (if the neck 

 be shortened as in the whales) is a critical part of the swimming ap- 

 paratus. If the head be large it cannot be swung about to thrust the 

 snout above the water without acting as an undesired rudder. Further, 

 if the neck be shortened to facilitate speed of locomotion the effort 

 to elevate the snout would be disproportionately great. To me these 

 facts point to the probability that when the whales were undergoing 

 the most marked migration of their nostrils they were of a size perhaps 

 comparable to a sea elephant or larger, with relatively large heads and 

 necks that had already become markedly shortened. The difficulty both 

 muscular and mechanical, of raising the snout while swimming at speed 

 obliged them to elevate and thus retract the nostrils to their full ability, 

 and this stimulus was largely critical in the migration of the latter to their 

 present position. 



The nostrils of the Phocidae, or those which I have observed, are 

 slightly elevated, and judging from other aquatic specializations of this 

 group they might be expected to show more marked modification. Ac- 

 cording to my experience the true seals are not very prone to breathe 

 while swimming at speed, but are more often in the habit of doing so 

 at rest, pausing after swimming beneath the surface quietly to renew 

 their air supply before starting off afresh. If this be really characteristic 

 and is persisted in they may never undergo more extensive migration of 

 the nares. 



The sea-lions (Otariidae) with which I am familiar habitually breathe 

 while swimming rapidly at the surface, but the alteration in the position 

 of the nares is even less marked than in the seals. For one thing there 

 seems to be a different set of stimuli here. Swimming with the anterior 



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