ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



41 



300r 



3 200 



I 100 



O O 



8 



,0-8-J-O. 



J-J- 



0 2 4 6 8 10 12 14 16 18 20 >20 

 Age (years) 



Fig. 27. Dry weight of calcified tissue in os clitorides of Pacific walruses in relation to 

 age. Each symbol represents one specimen. 



of data, are sigmoid, and the point of inflection is in the 7th or 8th year. The 

 upper limit of uterine size (average, 22 cm long; 4 cm diameter) seems to be 

 attained usually between the 10th and 12th years and remains more or less 

 constant thereafter. At all stages of development, the diameter of the nonpreg- 

 nant uterine horns is about one-sixth of the length. 



Relation of Physical to Reproductive Development 



In the development of body size and weight, as well as of the reproductive 

 organs, the Pacific walrus shows comparable patterns, each of which indicates 

 that full physical and reproductive maturity are attained both actually and rela- 

 tively later in life in this mammal than in other pinnipeds. Physical development 

 of males is not completed until about 15 years of age, after some 38 to 40 % of the 

 maximum life span is past. Although they seem to be reproductively fertile 5 to 7 

 years earlier than that (Krylov 1967, 1970), they probably are not able to 

 compete for mates until physically mature. The height of the females' physical 

 and reproductive development seems to be reached at about 10 years, although 

 they are capable of reproducing 4 to 5 years earlier (Krylov 1966^?, 1970). Like 

 many other mammals (e.g., see Montagu 1957; Andersen et al. 1962; Belling 

 1963; Parsons 1964), their ability to produce offspring is not optimal until they 

 attain full physical maturity. This is an important point to bear in mind, not 

 only from a comparative aspect, but because it has practical relevance to man- 

 agement and manipulation of walrus populations. 



The body weights of the males represented in Fig. 21 may be typical only of 

 animals in lean condition. All but three were taken between 9 March and 

 29 August; the other three (indicated by "W") were taken in late January- to mid- 

 February. According to Vibe (1956) and my Eskimo informants, the males tend 



