426 



DISCOVERY REPORTS 



Table 17. Estimated frequency of conceptions in multiparous southern hemisphere 

 fin whales, based on 694 records of foetal length 



Conceptions 



MONTHS 



Text-fig. 43. Monthly percentage frequency of pairing (full line) and calving (broken line) of 694 

 multiparous females (with 3 or more corpora in the ovaries). 



and shown graphically in Text-fig. 43. The peak pairing season is from April to July, as opposed to 

 May to September in primiparous females, and the mean date of conception for multiparous females 

 (20 June), is a month in advance of the estimated mean date of conception for primiparous females 

 (21/22 July). The multiparous curve is skewed with a long 'tail' from July to December, whereas 

 the primiparous curve is much more symmetrical about the mean, so it is better to compare the median 

 dates. The median pairing of multiparous females (8 June) is then seen to be about 6 weeks in advance 

 of the median date for primiparous females (21 July). It should also be pointed out that the beginning 

 of the steep rise in the multiparous curve of conceptions is actually about half a month later, that is to 

 say in May, if half-monthly periods are considered. Of the 164 foetuses estimated to have been con- 

 ceived between 12 April and 1 1 May, 98 were conceived in the second half of the period, so that only 

 about 90 foetuses were conceived earlier than the beginning of May. These amount to about 13% 

 of all multiparous conceptions. 



Evidence given above suggests that the majority of females, if not all, experience a monoestrous 

 cycle in their first sexual season. There is unfortunately very little information on this point for 

 multiparous females. F. D. Ommanney and A. H. Laurie, who examined whales at Durban, South 



