460 DISCOVERY REPORTS 



rate of accumulation. We are now in a position to give an estimate of the average annual rate of 

 accumulation of corpora, and to compare ages determined in this way with ages determined by other 

 independent methods. 



Types of corpora albicantia 



Three age groups of corpora albicantia have been identified on the basis of gross anatomical appearance, 

 and confirmed by histological examination (p. 366). The mean diameters of these groups in order of 

 increasing age are 4-01 cm., 2-94 cm., and 2-01 cm. These three groups represent stages of regression, 

 and in the ovaries there are on average about 1-54 of the youngest corpora albicantia and 3-22 of the 

 next age group. The number in the oldest group is cumulative, and bears a direct relation to the 

 total number of corpora in the ovaries. That is to say, the first two groups represent stages of 

 regression, while the third group represents the accumulation of fully regressed corpora albicantia 

 which increase in numbers with age. 



If the first two groups represent successive stages of regression, their relative frequency of occur- 

 rence should represent the relative length of time taken for each stage of regression. It is found that 

 the ratio of ' young ' to ' medium ' corpora albicantia is almost exactly 1 : 2 and it appears that together 

 they occupy about three years. The ' young ' corpora albicantia, therefore, represent about one year's 

 accumulation, and ' medium ' corpora two years. The average annual rate of accumulation of corpora, 

 therefore, appears to be about 1-5-1-6, but a correction (explained on p. 380) is necessary because 

 the process of regression lasts longer in older animals, so that there are more ' young ' and ' medium ' 

 corpora in the higher corpora groups, that is at higher ages. This suggests that 1 -4-1-5 is a more 

 accurate value for the annual increment of corpora. 1 It is found that newly mature females ovulate 

 on average about 1-42 times before becoming pregnant. 



The sexual cycle 



Evidence has been presented in this paper which strongly suggests that the female fin whale has a 

 seasonally monoestrous type of sexual cycle. It has been shown that in the majority of females there 

 are three ovulatory periods, two of which (a post-partum oestrus and a post-resting oestrus) usually 

 occur in low latitudes in the southern winter, during or after a northward migration ; and the other 

 (a post-lactation oestrus), usually occurs in higher latitudes in the southern summer, during or after 

 a southward migration. With the present data it is not possible to show conclusively that these ovula- 

 tory periods are invariably monoestrous, but the material available strongly suggests that this is so. 

 The evidence for newly mature females is perhaps the most conclusive. Even if female fin whales 

 are not invariably monoestrous there is certainly a very strong tendency towards the monoestrous 

 condition, similar perhaps to that which has been demonstrated, for example, in certain species of 

 Equidae and Bovidae (Eckstein and Zuckerman, 1956, pp. 238, 245). 



In this connexion it is of interest that Chittleborough (1954) has shown just such a strong tendency 

 to monoestrus in the humpback whale during the breeding season. Thus, he finds that the mean 

 number of ovulations for a female humpback whale during its ovulatory period (in Australian waters) 

 is only slightly over one. It is possible that the monoestrous condition in the fin whale is more apparent 

 than real, and that there is in fact a polyoestrous cycle in which the first ovulation is almost invariably 

 successful. However, in an ovulatory period, such as the post-lactation oestrus of the fin whale, when 

 males are out of breeding condition, the polyoestrous cycles should not be suppressed. The evidence 

 on this point, both in females at puberty and in adult females, is strongly in favour of monoestrous 

 cycles in fin whales at each of the three ovulatory periods. This evidence precludes more than a slight 

 tendency towards polyoestry. 



1 From the evidence of the corpora albicantia the possibility that the rate is only 0-7-0-75 cannot be excluded (see p. 385). 



