44© DISCOVERY REPORTS 



monthly frequency of ovulation. This would give an estimate of 92% for the incidence of ovulations 

 in September, which is probably nearer the true proportion. 



The life of the corpus luteum of the cycle as in other mammals is likely to be nearer half a month 

 than a month. Thus, Eckstein (1949, p. 400) remarks that: 'The life span of the c.l. in different 

 species is remarkably uniform and independent of body size. The range of variability encountered is 

 about 10-20 days (the upper limit being represented by the cow), but in the great majority of animals 

 it is probably of the order of only 10-15 days.' Harrison (1948, p. 323) also states that regression 

 begins 10-18 days after ovulation. There is no reason for supposing that pseudo-pregnancy is a feature 

 of the reproductive cycle of whales, and there are several good reasons for supposing that it is not. 

 While definite proof is lacking it is highly probable that the corpus luteum of ovulation in the whale 

 persists in a recognizably active state for a period of between 15 and 20 days. 



Table 23. Monthly percentage of adult females pregnant ; South Georgia 1925-31, 



pelagic 1932-52 



South Georgia Pelagic 



, * , , * , 



Month No. % No. % 



September 1 (ioo-o) — — 



October 40 85-0 — — 



November 86 77-9 — — 



December m 82-0 131 79-4 



January 134 70-1 336 64-3 



February 81 49-4 320 67-8 



March 65 32-3 239 51-9 



April 30 26-7 72 7-0 



May 2 (5°'°) — — 



550 64-9 1098 607 



We may now proceed to test the hypothesis concerning this post-lactation ovulation, using some 

 actual figures to compare the cumulative curve of ovulations with the increase in the population of 

 ' resting ' females in antarctic waters. The results of these calculations are presented in Table 24 and 

 illustrated in Text-fig. 49. 



As a basis for the calculations it has been assumed that the catch at South Georgia in the eight 

 seasons 1927-35 gives a reasonably accurate picture of the monthly variation in numbers of the 

 antarctic fin whale population. The figures for the pelagic catch cannot be used, because in recent 

 seasons there have been no catches before December or January ; and in earlier years the catches are 

 not considered to be representative of the population present, because of the selection of the more 

 valuable blue whale, so that the earlier months are under-represented (see Mackintosh, 1942). The 

 calculations have been repeated for the eight post-war seasons, 1948-54, 1955-57, at South Georgia 

 (the season 1954-55 being excluded, because it was clearly anomalous with peak catches of fin whales 

 in October and November). The results are similar to those for the earlier period. 



The monthly percentage of females in the catch is fairly constant, and on average slightly less than 

 50%, though it varies between 40-54% (from Mackintosh, 1942, table 25). The catch at South 

 Georgia in 1927-35 is here taken to represent the variation in numbers of females. The monthly 

 percentage of immature females (Table 24, column (2)) is taken from Mackintosh (1942, table 26b), 

 and an arbitrary figure allowed for September. This permits the approximate numbers of mature 

 females to be calculated (column (3)). The monthly percentage of non-pregnant mature females in 

 the catch (shown in column (4)), is obtained from Table 23, Text-fig. 48 a and enables the catch of 

 non-pregnant mature females to be estimated (Table 24, Text-fig. 486). The monthly percentage of 



