80- 



-c 60- 

 "c 



OJ 



o 



(_ 



(U 



t^ ZO 



Immature 



Pregnant 



Resting 



THE SEI WHALE 249 



Fig. 75 is a scatter diagram of the lengths of foetuses plotted for the months under 

 consideration and shows the progress of pregnancy in the gravid whales at South 

 Georgia during the time when they are leaving 

 the grounds in ever greater numbers. Returning 

 now to Table XX it will be seen that in South 

 Africa in August one whale is recorded as ovu- 

 lating and in September one was found with a 

 very early (2 mm.) pregnancy. Table XXI shows 

 the series of dated records of 151 foetal lengths 

 taken from the statistics in the British Museum, 

 and Table XXII shows the records of foetuses 

 collected by the Expedition. Fig. 76 is a scatter 

 diagram constructed from all these data com- 

 bined. The average foetal length, when plotted 

 by half-monthly periods, produces the curve 

 shown in Fig. 77, which indicates a length of 

 gestation of about 12 months, if the young are 

 born at a length of about 4-5 m., as shown by 

 the lengths of the largest foetuses recorded. 

 Taking the data in Table XX, together with 

 Figs. 76 and 77, the conclusion drawn is that 

 pairing in the main takes place in the months of 



May, June, July and August in lower latitudes, that there is a southern migration 

 during the summer months, followed by a return migration to warmer waters in March 



Feb 



Mar 



Apr 



Fig. 74. Sei whale. Proportions of immature, 

 resting and pregnant whales. 



(U ^^ 



E 



jr 

 4- 

 txjO 



c 



• •. 



• • 



• • 



I 





Februari 



March 



April 



Fig. 75. Sei whale. Foetal lengths for February to April. South Georgia. 



9-2 



