2^6 DISCOVERY REPORTS 



oestrus, therefore, the majority (12 out of 17 = 70 per cent) of these Sei whales became 

 pregnant at the first ovulation, but thereafter 

 Fig. 81 suggests that there is an average of three 

 dioestrous cycles at each oestrus. 



The breeding season appears to be determined 

 by the physiological condition of the females only 

 because, as has been shown above, no marked 

 seasonal activity of the male structures has been 

 found. It is probable that oestrus lasts for some 

 time and is made up of any number up to 4 di- 

 oestrous cycles, until fertilization takes place. The 

 unfruitful cycles are presumably due to absence 

 of males from the neighbourhood of the female 

 at the time of oestrus, though they may no doubt 

 be due to unsuccessful matings, or other un- 

 known factors. 



The sexual cycle of the female Sei whale may 

 therefore be summarized diagrammatically as in 

 Fig. 82 which represents two years of the whale's 

 life. 



Number of Corpora Lutea 



Fig. 81. Sei whale. Females. Frequency 

 of corpus luteum numbers. 



Dioestrus 



after the 



first 



sexual 



season. 



ru5 



Ovulation 



Maq-Aug. 

 Season A. 



V MaL|-Aug. Season A. 



Pregnancy to 



MaLj-Aug. Season B. 



Lower Latitudes 



Southward Migration 

 Higher Latitudes 

 Northward Migration 



Parturition Mat] -Aug. Season B. Lower Latitudes 



Maq- Aug. Season B. 

 Lactation to 



Nov -Jan. Season B. 



Nov -Jan. Season B. 

 Anoestrus to 



Mau-Aug. Season C. 



Southward Migration 



Higher Latitudes 

 Northward Migration 

 . Lower Latitudes 



Fig. 82. Sei whale. Females. Oestrous cycle. 



