ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



191 



diameter from 25 to 41 iim, and their nuclei were more diffused than in the 

 earlier specimens, with no distinct nucleolus (Fig. 116/). Diameters of lutein cells 

 in the February specimen were 20 to 31 /xm. In both of these specimens, the 

 cytoplasm had taken on a coarser, more granular appearance, and large vacuoles 

 were scarce. 



In specimens taken during late winter and spring, fibrous invasion of the 

 corpora was evident macroscopically, as was the reduction in size from the 

 autumn-winter maximum. I examined two such corpora histologically. The 

 first, from a specimen with a full-term fetus taken on 24 May, showed virtual 

 isolation of some lutein cells by the surrounding fibrous tissue (Fig. 116g). The 

 cells were large and more rounded than in winter specimens, with a coarse, 

 granular cytoplasm containing few vacuoles. The nucleus also was large and 

 round, and the nucleolus darkly stained and peripheral. Lutein cell diameters 

 ranged from 25 to 45 fim. The second corpus, from a specimen taken on 12 May 

 with a calf no more than a few hours old, was essentially the same as in the 24 May 

 specimen, except that it showed extensive thecal invasion, as well (Fig. 116^). 

 The corpora in two animals taken on 3 and 22 May, with calves judged to have 

 been at least 1 week old, were comparable histologically to the 12 May specimen. 



The largest follicles in the ovaries of postpartum females tended to be mostly 

 less than 5 mm in diameter during the first weeks after birth. By late May and 

 June, however, there was a trend of increase in follicle size, probably to a 

 maximum in July or August. Most of those larger follicles were degenerate; 

 apparently none had ruptured during this postpartum estrus. 



Time of Mating 



Several investigators have estimated that the mating of walruses occurs in the 

 spring, at various times from mid-April to July (Chapskii 1936; Collins 1940; 

 Brooks 1954; Mansfield 1958a; Krylov 1962). Those estimates were based mainly 

 on comparative embryonic development, rather than actual evidence of 

 ovulation, and were made with the assumption that in the walrus there is no 

 delayed implantation. More recendy, Krylov (1966^?) indicated that the mating 

 period is from March to July, but his basis for that statement was not made clear. 

 Subsequently, he (Krylov 1966c) declared that most of the mating occurs in May 

 and June. Still later, Krylov (1969) placed mating in late April to early June, in 

 the same period as birth. 



In the reproductive cycles of other pinnipeds, insofar as they are known (see 

 reviews by Harrison et al. 1952; Harrison 1969), the principal period of mating 

 seems always to be about 2 to 3 months long and to begin about the time of the 

 peak in size of the testes of the adult males. By analogy, the mating season of the 

 Pacific walrus would be in January to February, rather than in April to June. In 

 April to June, the testes of nearly all of the adult males already were in a state of 

 retrogression. 



As a whole, the findings presented here suggest that estrus in some walruses 

 may take place as early as December or as late as August. This should not be 

 regarded as indicative of a 9-month-long mating season, for the data from the 

 males indicate that mature adults are fertile mainly from December to March. 

 Hence, it is probable that most fertilizations take place in the winter period. Of 

 the 14 potentially estrous females taken in February and March, 10 (70%) 



