ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



69 



First, abrasion of the vibrissae takes place not only in the older animals that 

 feed on benthos, but also in very young calves which have no tusks and whose 

 diet consists entirely of milk. The vibrissae of the young calves evidently grow 

 continually. At the same time, they become appreciably shortened and rounded 

 soon after birth and, in two areas high on the anterior surface of the snout, 

 become worn down to the level of the skin. From observations of calves under 

 natural conditions, as well as in captivity, I concluded that the reduction in 

 length of their vibrissae occurs principally by abrasion during their search for the 

 mammary nipples. This search seems to be more by tactual than by visual 

 means: while brushing their mustache vigorously and forcefully over the 

 mammary area with a sort of "rooting" motion, the greatest frictional force is 

 applied to the dorsal edge of the snout. The abrasive surface, in this instance, is 

 the coarse skin and hair of the mother. The resulting areas of minimal vibrissa 

 length increase in extent with advancing age and usually coalesce medially 

 before the animals are 1 year old (Fig. 45). At that age, the infant walrus still is 

 dependent on a milk diet, though probably supplements it occasionally with 

 mollusks. When the young walrus is weaned, at 18 to 24 months, the area of 

 minimal length attains its maximal breadth, extending to the lateral edges of the 

 snout. This pattern is sustained to adulthood. 



Second, the vibrissae on the anterior surface of the snout are most intensively 

 abraded on their dorsal side. In the uppermost rows, the tips are steeply beveled; 

 vibrissae farther down show a progressively more oblique bevel (Fig. 47). Along 

 the upper edge of the snout, where the vibrissae are worn shorter than the 

 surface of the surrounding, highly cornified skin, their tips are abraded squarely 

 or slightly rounded. The few vibrissae posterior to the upper edge of the snout are 

 abraded mainly on their anterior surface; those farthest posterior tend to be the 

 longest and most obliquely beveled (Fig. 47). 



These findings indicate that the greatest and most direct frictional force, 

 perpendicular to the surface of the skin, is applied along the upper edge of the 

 snout. The incident angle of contact with abrasive materials is progressively 

 more acute and less intense ventrally and posteriorly from that area. Close 

 inspection of the abraded surfaces of the vibrissae indicates further that the 

 direction of motion of the abrasive particles contacting the front of the snout is 

 from dorsal to ventral, for example as might occur with a "rooting" motion of the 

 head — the same motion used by the calves when seeking the mammary nipples. 

 This kind of abrasion is the reverse of that predictable from digging with the 

 tusks. In digging, the relative motion of the abrasive particles would be from 

 ventral to dorsal and would tend to abrade the vibrissae more on their ventral 

 than on their dorsal surfaces. 



Vibrissal Function 



The dense population of mystacial vibrissae in the walrus identifies the broad 

 snout as an organ of extreme tactual importance. The theory expressed by 

 Sokolowsky (1908) and Brehm (1926:319) that these vibrissae serve as a kind of 

 food-sieve, in the manner of the baleen plates of mysticete whales, was rejected 

 by Mohr (1950), because the vibrissae are on the outside rather than the inside of 

 the mouth, and because they rarely attain sufficient length to cover the mouth. 



