248 DISCOVERY REPORTS 



as thick as 5 cm. ; it is usually less than 2-5 cm. In the involuted state it varies from 

 2-5 to 9-0 cm. and has not been found as thick as 10 cm. The two values for the lactating 

 gland are 16 and 18 cm., as thick as the lactating gland frequently is in Fin and Blue 

 whales. Even in the involuted state the average thickness would appear to be greater 

 than in Fin and Blue whales where it is 5-6 cm. 



The overall length of the gland in whale No. 2474 was 170 m. (thickness 16 cm.) 

 and it was lactating only moderately. The length of the gland in Fin and Blue whales 

 is given as about 2 m., so that, taking into account the greater length of those species, 

 it would appear that the mammary gland in the Sei whale is much larger in proportion 

 and will presumably give a proportionately larger quantity of milk. This may in part 

 account for the greater fatness of immature whales mentioned above. 



Gestation, breeding season and sexual cycle. Data relating to Sei whales at South 

 Georgia are very few except for the months of February, March and April. This does 

 not necessarily mean that Sei whales are not present on the South Georgian whaling 

 grounds except in these months, but that the larger species are then in reduced numbers, 

 because Sei whales are of no interest to whalers when larger whales are available. 

 Nevertheless, restricted though the data are, several facts of importance emerge from 

 a consideration of them. 



Table XX shows the numbers and condition of female Sei whales examined in those 

 months for the seasons 1927-31, together with a few records from South Africa for the 

 months of August, September and October. An inspection of these figures shows a very 

 striking decrease in proportion of pregnant whales and increase in proportion of resting 

 whales as the season at South Georgia advances. These changes are shown graphically 

 in Fig. 74, in which the percentages of the total catch formed by pregnant and resting 

 whales are plotted by months. In February pregnant whales form 57 per cent of the 

 catch and resting whales 24 per cent. In March the proportions are roughly equal, 

 being 44 and 40 per cent respectively, and in April resting whales form 78 per cent 

 of the catch and pregnant whales only 12 per cent. The conclusion to be drawn from 

 these figures is that there must be a steady flow of Sei whales passing through the South 

 Georgian whaling grounds in these months and that pregnant whales in the main 

 arrive first and are leaving throughout the early part of the year. Their place may 

 perhaps be taken by fresh arrivals of resting whales or their withdrawal alone may 

 increase the proportion of the latter. 



Table XX. Sei whale. Sexual condition of females examined 



