188 



NORTH AMERICAN FAUNA 74 



50 



40 - 



E 



E 



i 30 

 E 



.5 20 



Q 



10 



o o 

 Proestrus and 

 - Estrus '"^ 

 o 



o 



Corpus 



o Luteum 



Pregnant 



o 



°o 

 o 



o o 

 o 



Postpartum 



Largest 

 Follicle 



J Utli 



i**Y I I 



SONDJ FMAMJJASONDJFMAMJJA 



Month 



Fig. 115. Diameters of the largest healthy (dots) or degenerate (triangles) follicles 

 and of the corpora lutea (circles) in ovaries of adult walruses, in relation to date of collec- 

 tion. Each point represents one specimen. Curve was fitted visually. (From J.J. Burns, 

 F. H. Fay, and A. W. Mansfield, unpublished data). 



Ovarian Cycle 



One of my objectives in this study was to obtain monthly series of ovaries from 

 adult females, in order to determine the time when most ova are shed and 

 fertilized (i.e., the mating season). The data obtained, together with those 

 provided by Mansfield (1958^) and by J. J. Burns (personal communication), are 

 represented graphically in Fig. 115. 



Follicle size in the nonpregnant, potentially estrous animals taken in 

 September to December suggests a slight trend of increase during that period, 

 but only one animal on 6 December had a healthy follicle 11 mm in diameter 

 that appeared to be ripening. Regrettably, very few potentially estrous 

 specimens were obtained in mid-December to April, and those few did not show 

 any ripe follicles. The occurrence of new corpora lutea of pregnancy as early as 

 mid-February and March, however, indicates that ovulation had begun during 

 the winter. Ripe or ripening follicles were present in some individuals as late as 

 July and August, long after most of the metestrous females had a well-developed 

 corpus luteum and were in their second trimester of pregnancy. 



