AQUATIC MAMMALS 



quick to discover the approach of a polar bear or other enemy, and 

 that not only the killer whale but the cachalot is in the habit of thrusting 

 the head above the water for the purpose of ascertaining what may be 

 of interest above the surface. Hence it is evident that there yet remain 

 for settlement several fundamental questions regarding the optics of 

 aquatic mammals. 



ACOUSTIC SENSE 



There is a tendency for the elimination of the external ear in aquatic 

 mammals, partly following the law that ultimately the aquatic life will 

 eliminate superfluous prominences upon the body, and partly because 

 of the ultimate disuse of the pinna as an accumulator of atmospheric 

 sound waves. In the case of those sorts which inhabit streams this re- 

 duction of the pinna may be barely or not at all appreciable, partly be- 

 cause they are not very highly modified aquatically if they are still 

 stream dwellers, and partly for the reason that although hearing has 

 ceased to be an aid in capturing aquatic prey, they must still be on the 

 alert for terrestrial foes. Also it must not be forgotten that mammals 

 of this category are almost always densely furred, and that in these the 

 ears are usually hidden in the pelage, constituting elimination of pinna 

 as far as concerns the external contour. But this need not be of aquatic 

 significance since many terrestrial representatives of these mammals 

 (among Insectivora and Rodentia) exhibit the same condition. 



The platypus, anomalous in so many ways, has rather peculiar ears. 

 Although it lacks the pinna, the musculature enables it to "cock" the 

 orifice forward, as is shown to excellent advantage in plate 6 of Bur- 

 rell's (1927) book. This authority also states that the auditory "ori- 

 fice lies at the posterior end of a facial furrow, the eye lying at the an- 

 terior end, while the furrow is incompletely divided into two by an 

 oblique fold of skin. The edges of this furrow act as a long pair of 

 lids, by means of which both eye and ear may be tightly closed at the 

 will of the animal. The aural aperture may also be dilated and con- 

 tracted while the eyes are open." 



The aquatic Talpidae Desmana and Galemys also lack the pinna, 

 which has no aquatic import, for they belong to the earless family of 

 moles. Nectogale is the only other insectivore which is said to lack a 

 pinna. And Anotomys is the only such rodent, but I question this 

 without verification (I have not seen this genus) . Among pelagic 

 mammals the Cetacea, Sirenia, Phocidae, and Odobenidae have no ex- 

 ternal ears. 



[68] 



