ECOLOGY AND BIOLOGY OF THE PACIFIG WALRUS 



47 



Table 6. Regional and age- and sex-related differences in thickness of 

 skin (epidermis and dermis) of some Pacific walruses. 



Skin thickness (mm) per region 



Neck Thorax Abdomen 



Sex 





Dorsal 



Ventral 



Dorsal 



Ventral 



Dorsal 



Ventral 



F 



Fetus 

 (Dec) 



5.0 



— 



- 



3.0 



— 



2.5 



M 



Fetus 

 (Dec) 



— 



— 



3.4 



— 



— 



— 



M 



Fetus 

 (Jan) 



8.0 



6.0 



7.0 



5.0 



6.5 



5.0 



? 



Fetus 

 (May) 



— 



— 



11.0^ 



— 



— 



— 



F 



1 week 



11.8 



— 



— 



— 



— 



— 



F 



2 weeks 









8.0 





8.0 



? 



3 weeks 









6.2 







F 



1 month 



9.0 



8.0 



8.0 



7.0 



8.0 



6.0 



F 



3 years 









12.7 







F 



10 years 







35.0 









F 



Adult 









17.1 







F 



19 years 









17.0 







F 



Old adult 













20.0 



M 



15 years 









27.0 







M 



20 years 









36.0 







M 



26 years 



40.0b 













^Data from G. C. Ray (personal communication). 

 ^Thickness between bosses; thickness of bosses was 50 mm. 



(Fig. 19). Krylov (1967) observed that the earliest appearance of these bosses is in 

 males 7 years old, and that they are consistently present in males more than 16 

 years old. 



Functional Aspects of Skin 



The unusual thickness, strength, and durability of the walrus' skin are func- 

 tions mainly of the reticular layer of the dermis, in which thick bundles of 

 collagen fibrils form a particularly dense network (Sokolov 1960). It seems likely 

 that the main function of this pachyderma is as a protective armor against the 

 jabs and thrusts by tusks of other walruses. Probably for that reason, the skin 

 tends to be thickest on the dorsal than on the ventral surface of the body, 

 contrary to the "axiom" of greater skin thickness ventrally in pinnipeds, expressed 

 by Sokolov (1960) and King (1964). 



In adult males, the normal "armor" is supplemented by secondary thickening 

 in the bosses, mainly on the neck and shoulders. These bosses generally have been 

 regarded as a secondary sexual character that develops as the males approach 

 maturity, presumably by hormonal stimulus (Chapskii 1936; Nikulin 1941; 

 Rrooks 1954; Sokolov 1960). Their development in free-living walruses clearly 



