BREEDING AND GROWTH 429 



The time of the season of parturition may be worked out in the same way as the 

 pairing season. If we continue on the assumption that all the foetuses of a species grow 

 at the same rate the season of parturition will of course be identical in duration and 

 intensity with the pairing season. All that needs to be done is to reproduce the pairing 

 curves (Fig. 147) ten months later in the case of Blue whales and eleven and a half 

 months later in the case of Fin whales. The result is shown in Fig. 148. The details of 

 the shapes of these curves of course mean nothing, but they suggest in general that 

 Blue whales are mostly born in April and May and Fin whales in June and July. 



APR. ' MAY ■ JUNE ' JULY ' AUG. ' SEPT. """ OCT. ' NOV. 



Fig. 148. Curve of frequency of births. 

 Blue whales. Fin whales. 



As explained above it is being assumed that the foetuses grow at the same speed during 

 gestation. If, however, some foetuses grow faster than others the result would be 

 that the season of parturition might be more protracted than the pairing season. On 

 the other hand it is possible that a whale which was impregnated early might retain 

 the foetus slightly longer than one which was impregnated later, since birth apparently 

 takes place mostly when the parent migrates northwards to warmer waters, and the 

 attainment of the proper environment might have the effect of stimulating slightly 

 premature parturition in a whale in which the foetus was later than the average. 

 There is still another assumption involved in the construction of the foetal growth 

 curves, and that is that the earliness or lateness of a pregnant whale's visit to the 

 neighbourhood of South Georgia is not affected by the age of the foetus, i.e. the 

 time at which impregnation took place. But it seems quite possible that a female 

 which had not been impregnated before the later part of the pairing season might delay 

 her southern migration until impregnation took place, while those which had been 



