THE AGE OF FEMALE BLUE WHALES 



239 



6. Correlation of ankylosis of the vertebral epiphyses with corpora lutea numbers 

 shows that physical maturity coincides with the accumulation of eleven to twelve 

 corpora lutea in the ovaries. Blue females become physically mature at a wide range of 

 lengths. Length is no guide to age except in a very rough way up to 85 ft. Growth in 

 general may cease at 86 ft. (although females are occasionally found 99 ft. long). 



THE RATE OF BREEDING 



A knowledge of the breeding cycle is essential to any calculations as to the condition 

 of the stock and the effect of whaling thereon. Mackintosh and Wheeler have shown 

 (p. 430) that the cycle of pairing, pregnancy, parturition, lactation, and recuperation 

 among Blue whales cannot, except in very exceptional circumstances, take less than two 

 years. Pregnancy is shown to take ten to eleven months, so that one year is virtually 

 occupied with gestation. The period of lactation was indicated to be of the order of six 

 to seven months (p. 436), followed by the period of recuperation. These conclusions were 

 arrived at from a variety of evidence, which included the measurements of the largest 

 unweaned and the smallest weaned whales. Considerable data have accumulated since 

 these conclusions were formulated, and it will be as well to examine the evidence from 

 South Georgia afresh and that from the Antarctic whaling grounds. 



south Georgia data. The estimate of the mean date of weaning was derived largely 

 from a study of the calves. The present intention in the light of more data is to inspect 

 the evidences for the duration of the lactatory function in adult females. The period of 

 lactation and that of nursing need not necessarily coincide, though it is improbable that 

 a whale would continue to secrete milk for many days after the calf had ceased to take it. 



We will now consider the relative occurrence of the three classes of female whales, 

 pregnant, lactating, and resting. Among the resting whales must be included those 

 which have recently ovulated without becoming pregnant (pp. 247-9). As they are not 

 pregnant they may be included among the non-productive class of resting whales. 



