THE REPRODUCTIVE CYCLE 403 



Table 10. Growth in length of southern hemisphere fin-whale embryos 



used to determine the pairing season, individual variations will have the effect of slightly extending it. 

 It should be noted that this effect tends to cancel out the effect produced by the under-representation 

 of very small embryos and very large foetuses in the sample. 



It has now been shown that, for the purpose of establishing the frequency of conceptions in the fin 

 whale, the foetuses for which data are available (956 in number) may be taken to be a representative 

 sample. For the reasons given above any errors are likely to be small. 



The pairing season and the season of parturition 



The evidence which has been considered above strongly suggests that the majority of pairings in the 

 southern hemisphere fin whale take place during the period April-July. It is now possible to obtain 

 a more precise idea of the relative frequency of pairing by examining the foetal length data, using the 

 method employed by Mackintosh and Wheeler (1929, p. 427, fig. 146). 



First, the 956 foetal lengths were plotted according to the dates on which they were taken. The mean 

 curve of growth in foetal length (Table 10, Text-fig. 30) was then drawn and similar curves constructed 

 at lateral intervals of one month. The foetuses which lie between two of these growth curves can then 

 be referred to the monthly periods in which they were conceived. The effect on the results of uneven 

 sampling and individual variation in the rate of growth have already been discussed (p. 402) and it was 

 concluded that these influences tended to neutralize each other. 



All the foetal length records have been referred to the various monthly periods of conception, for 

 example 12 June- 11 July, and the frequency distribution of conceptions, determined in this way, is 

 shown in Table 11. The actual frequencies have been converted to percentage frequencies and are 

 plotted graphically in Text-fig. 31. It will be seen that although 12 June is estimated to be the mean 

 date of conception, the distribution of pairings about this mean date is not symmetrical, but skewed, 

 so that the modal frequency occurs earlier than the mean. This curve of the frequency of pairing is 

 in close agreement with the conclusions reached from consideration of the male sexual cycle, and of 

 the ovarian cycle of females near puberty. It is estimated that 77% of foetuses are conceived in the 

 4 monthly periods between April and August; only about 14% of all foetuses are conceived in the 

 7 monthly periods from September to April, and only about 6% in the 5 months between October 

 and March. This demonstrates that there is a limited season in the female extending over 4-5 months 

 during which most conceptions occur, but it will be noted that some foetuses may be conceived in 

 every month of the year except February. 



