30 



NORTH AMERICAN FAUNA 74 



Subsequently, Chapskii (1941) and Mohr (1952) observed that thick deposits of 

 cementum on the cheek teeth of walruses were layered in such a way that they 

 seemed to offer promise as indicators of age. Brooks (1954) and Fay (1955) 

 independently reached the same conclusion and established that there is a 

 positive relation between the number of cementum layers in the teeth and the 

 state of physical development of the individual. Investigating this relation 

 further, Mansfield (1958g), Krylov (1963, 1965a), and Burns (1965) concluded 

 that the number of major layers in the cementum on the cheek teeth could be 

 used to determine precisely the age of each individual. 



The validity of the foregoing conclusion has been confirmed by my findings in 

 free-living animals and in a series of known-age animals that were reared in 

 captivity (see Dentition). In the known-age animals, the number of completely 

 formed cementum layers usually was equal to or slightly less than the actual age 

 in years of the animal (Table 2). In most instances, the layers were not as clearly 

 expressed or as regular in thickness as those in the teeth of free-living walruses, 

 but they were comparable otherwise in structure and appearance. 



All of the ages reported here were determined from counts of those annual 

 layers of cementum, as seen in medial longitudinal sections of an incisor, lower 

 canine, or postcanine tooth from each animal. By sectioning entire dentitions 

 from several specimens, I determined that the number of layers is the same in 

 each of the teeth (except the tusks) of a given individual. The number of 

 cementum layers in the tusks usually is much lower than that in the other teeth, 

 due to greater loss of dental tissue from abrasion. 



Body Size and Weight 



Length of Body 



The methods for measuring body size in pinnipeds have not yet been 

 standardized on an international basis, and unless one is acquainted with the 

 different national standards, interpretation and comparison of measurements 

 can be confusing and misleading. I measured lengths of dead Pacific walruses in 

 the field in two ways: 



1. Along the body surface from the anteriormost tip of the snout to the 

 posteriormost tip of the tail flesh, with the animal lying on its back. This 

 measurement is called "curvilinear length" by American biologists (Scheffer 

 1967Z7). It is similar to the "zoological length" (often designated as Lc) used by 

 Soviet biologists, which is measured with the animal lying on its belly (Gromov 

 et al. 1963). Both of these measurements are imprecise, but their use often is 

 expedient in the field. Although I have no comparative data from animals in 

 which both the cur\ ilinear and the zoological length were measured, I have 

 assumed that they are approximately equal. 



2. The straight-line distance between the tip of the snout and the tip of the tail 

 flesh, with the animal lying flat on its back and the neck extended. This is the 

 "standaid length" measurement routinely taken by American mammalogists 

 (Scheffer 1967i») and is essentially the straight length of the articulated skull and 

 axial skeleton, plus a small amount of flesh at each end. It is greater than the 

 "horizontal length" (Lev) taken by Soviet biologists, which is measured in a 



