THE AGE OF FEMALE BLUE WHALES 



243 



With a population of whales which is changing its composition from month to month, 

 and which in any case may not be entirely representative of the whole stock, it is extremely 

 difficult to arrive at certain conclusions on statistical grounds. But the scarcity of 

 lactating whales in the early part of the summer, and the increasing proportion of resting 

 whales as the summer wears on, suggest that lactating whales are to some extent absent 

 and not sufficiently represented on the whaling grounds. If they are absent in a sub- 

 stantial proportion the pregnancy percentage would not be as high as 58, that is com- 

 puting the pregnant whales as a percentage of the adult stock, in contrast to the adult 

 catch. The pregnancy figure might in fact be low enough to warrant the conclusion that 

 a considerable proportion of females are pregnant only once in three years. 



If, however, we assume that a large proportion of females are absent, then we should 

 find males correspondingly more numerous than females. This is not found to be the 

 case, and it may therefore be argued that a large proportion of the adult females are not 

 absent from the whaling grounds. But it may equally be argued that the lactating females 

 are absent, and accompanied by an equal number of males. This hypothesis, while 

 unlikely, is by no means impossible. There is little evidence either for or against family 

 life among Blue whales. 



The matter must at present be left in some doubt. It may be regarded as probable 

 that while some females become pregnant every two years, an unknown proportion 

 become pregnant only once in every three years. 



AGE DETERMINATION 



We can now attempt to collate material from various sources in an endeavour to find 

 a method of determining the age of female Blue whales by reference to the number of 

 corpora lutea in the ovaries. It will be as well to recapitulate at this point what is known 

 about the life history of the Blue whale from conception to sexual maturity. Mackintosh 

 and Wheeler have shown that among Blue whales pairing takes place over an extended 

 period in the southern winter, but that most pairing is in July (pp. 420-7). Evidence for 

 this conclusion is a synthesis of indications from foetal growth statistics and data on the 

 activity of the reproductive organs at different times of the year. From the foetal growth 

 curve mentioned above and from records of the smallest calves taken and the largest 

 foetuses found, it is concluded that birth takes place when the foetus reaches approxi- 

 mately 23 ft., on the average in April or May, after a gestation period of slightly over 



