4°4 DISCOVERY REPORTS 



It has already been shown that the duration of the gestation period is about n| months. A curve 

 showing the seasonal distribution of calving has, therefore, been constructed by advancing the curve 

 of conceptions by 3 weeks (Text-fig. 31). 



So far we have been concerned with the pairing season averaged from data collected over a number 

 of years. It would be interesting to know the possible variation in the timing of the pairing season from 

 year to year. 



Text-fig. 31. Monthly percentage frequency of pairing (full line) and calving (broken line) of 956 pregnant females. 



Table 11. Estimated frequency of conceptions in southern hemisphere fin whales, 

 based on 956 records of foetal length 



Conceptions 



Our material collected by biologists is inadequate for this purpose, but there is information on 

 foetal lengths of fin whales in the International Whaling Statistics covering the period 1925-58 and 

 comprising many thousands of records. These measurements are not as accurate as those made by 

 biologists which were used to elucidate the curve of foetal growth and the average pairing season. 

 As Brinkmann (1948) has pointed out, small foetuses tend to be missed so that in the earlier months 

 the mean foetal lengths given in the International Whaling Statistics are higher than expected in a 

 true sample. However, we are interested in relative differences from year to year and the absolute 

 lengths are not important in this connexion. 



In Text-fig. 32 the mean monthly foetal lengths given in the International Whaling Statistics have 

 been converted from feet to metres, and are plotted against the respective whaling seasons. The records 



