BREEDING AND GROWTH 



443 



length groups is that the young whale migrates northwards in the next winter, grows a 

 little more and returns to the south in the second summer where it reaches a length of 



21 m. At the end of this second summer it returns again to the north to grow up to 



22 or 23 m. It is at this length (speaking still of male Blue whales) that sexual maturity 

 is reached. Although there are very few whales among the winter catches at Saldanha 

 Bay approximating to the length at which maturity is reached, it is still probable 

 that this condition is attained before the commencement of the southward migration, 

 for it appears that this locality is not the ordinary haunt of whales which leave the south 

 in the winter. 



29- 



28- 



27- 



26- 



25- 



24- 



23- 



22- 



21 



20 



1 



18' 



17 



16 



15 



li 



i:j 



12 



IJ- 



10- 



■Biue 



r^ik^- 



-FEMALES: SEXUAL MATURITY AT 20 M 



.males: sexual maturity at 19-lM. 



WEANING AT 120 M -i' 



JUNJUL 



Fig. 155. Fin whales. Estimated mean curve of growth from conception to sexual maturity. 

 Thus if the whale is born at the beginning of May, then the indications are that 

 sexual maturity is reached after a period of slightly more than two years. The general 

 significance of this surprisingly rapid growth will be discussed later, and in the meantime 

 we may complete the curve of growth for male Blue whales up to sexual maturity and 

 slightly beyond that point, as it is to be supposed that the newly adult whale now grows 

 up towards the third length group (24 m.) on its third visit to the south. This is shown 

 in Fig. 154 in which the curve in Fig. 149 is continued and that of the female Blue 

 whales added. It is suggested that the differences in length between males and 

 females at sexual maturity and at maximum growth first appears some time after 

 weaning. The smallest whales of both sexes caught at South Georgia are of about 

 the same length. 



2t-3 



