46 



NORTH AMERICAN FAUNA 74 



Fig. 31. Pigment-free areas of the skin on the right foreflipper of a Pacific walrus fetus, 

 about 6 months post-implantation. Dorsal (left) and ventral (right) xiews. (Photos bv 

 F. H. Fay) 



from superficial wounds. They were most apparent on subadult males. 



The skin is greatly wrinkled and folded (Fig. 29), except on individuals that 

 are very fat. These folds develop early in the fetus (Chapskii 1936), as if in 

 anticipation of later thickening of the skin. I presume that their pattern remains 

 more or less constant for the lifetime of the animal, and that it is unique on each 

 individual. 



The thickness of the skin (epidermis and dermis) in the specimens that I exam- 

 ined seemed greatest on the neck and shoulders, greater on the dorsal than on the 

 ventral surfaces, and greater in males than in females. These differences were 

 apparent even in small fetuses and were most marked in the adults (Table 6). 



The skin is thinnest on some parts of the face and attains its greatest thickness 

 on the neck (up to 40 mm) and shoulders of adult males, where it is supplemented 

 by rounded bosses or "lumps" about 1 cm thicker than the surrounding skin 

 (Fig. 32). These bosses, mostly about 5 to 8 cm in diameter, tend to be hairless 

 and scarred on their distal surface and to have numerous minor cuts and 

 abrasions there. Their internal structure is not remarkably different from that of 

 the surrounding skin, except for extraordinary thickening of the reticular layer of 

 the dermis and minor amounts of scar tissue in the outermost, hairless parts. The 

 total dermal-epidermal thickness of the skin in these bosses may be up to 7 cm 

 according to Brooks (1954), though it is usually about 5 cm. 



Freiman (1941) observ^ed that the bosses were absent in males whose zoological 

 length was less than 300 cm, and that they usually were well developed on males 

 whose zoological length was 340 cm or greater. Males less than 300 cm long 

 (standard length < 279 cm) are mostly less than 10 years old; those with a length 

 of 340 cm or more (standard length >316 cm) are mostly 15 years old or older 



