ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



67 



Full-term fetus 



Calf 3 weeks 



1 year 



2 years 



Adult 



Fig. 45. Isograms of length (mm) of 

 mystacial vibrissae in the anterior sur- 

 face of the snout of Pacific walruses, 

 in relation to age. Lengths were meas- 

 ured from the surface of the surround- 

 ing skin. Crosshatched areas indicate 

 lengths less than 1 mm. 



stouter and paler but usually not any longer than those of the 1-year-old young. 

 The largest vibrissae in a 4-year-old male had basal diameters of 2.2 to 2.5 mm; 

 those of an adult female were 2.5 to 2.9 mm. 



The topographic variation in length of vibrissae on the anterior surface of the 

 snout in several walruses of different ages (Fig. 45) shows that in each specimen, 

 the shortest hairs were situated near the upper edge of the snout and the longest 

 hairs near the ventrolateral edges. This pattern was virtually the same in all 

 individuals more than 2 years old, though the actual lengths varied widely. The 

 length of each vibrissa is a function of its rate of growth and rate of decrement by 

 abrasion. The result of growth alone is seen in the fetus, where length increases 

 continually, except for breakage of the tips. It is seen also in certain individuals 

 raised in captivity, an outstanding example of which was shown by Mohr (1950: 

 Fig, 2). Such animals often develop very long vibrissae, unlike those of walruses 

 under natural conditions, but the basic pattern of relative lengths is nonetheless 

 similar (Fig. 46). 



