210 



NORTH AMERICAN FAUNA 74 



O O O 00 0 



A-g8"g»oo«(fc»o I I 1 ^ L-1 \ \ L_J I ' _ ' I ' I I A ' I 



0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 



Age (years) 



Fig. 125. Comparison of observed with expected numbers of corpora albicantia in 

 Pacific walruses in relation to age. Each circle represents data from one specimen, the 

 age class means of which (dots) are connected by the dashed line. Smoothed curv e (solid 

 line) is the predicted age class mean from the age-specific ovulation rate of Table 34. 

 assuming that o\'ulation at age tg results in a corpus albicans at age tj. (From Kr\"lo\' 

 1966fc and F. H. Fay. unpublished data). 



the corpora do not persist indefinitely; some degenerate and disappear after a 

 period of time (Mansfield 1958a; Krylov 1966Z7). This is illustrated in Fig. 125. 

 where counts of the corpora in the ovaries of 325 indi\'iduals are plotted against 

 the age of the animals. The distribution of data points indicates a tendenc\- for 

 steady increase in the numbers of corpora per indi\1dual up to about age 20. 

 followed by a marked decrease thereafter, perhaps to zero by age 30. The steady- 

 increase in the early years apparently is the result of long-term retention of most 

 of the corpora; the decline in numbers in old age suggests that all corpora 

 eventually degenerate to the degree that they are no longer distinguishable. 



The close correspondence of the empirical mean numbers of corpora per 

 individual per age class (dashed line. Fig. 125) with the estimated means derived 

 from Table 34 (smoothed curve. Fig. 125) suggests that, in most individuals 

 between the ages of 6 and 12 years, all of the corpora albicantia are retained and 

 are distinguishable visually. Thereafter, they seem to disappear at the rate of 

 about one per 2 \'ears up to about age 25. after which the>" disappear more 

 rapidly. 



The onh' reliable bases for assessing relative success in gestation are the 

 presence of an implanted embryo or fetus in association with a corpus luteum 



