442 



DISCOVERY REPORTS 



in males, 23 m. in females), and (3) mature (24 m. in males, 26 m. in females). Again 

 the differences between the maxima are about equal to the ordinary differences in 

 length between the sexes. 



The tendency towards three dominant length groups is also to be seen in the curves 

 for the 1925-6 season, and in the graph showing the February-May 1925 figures 

 combined with those of season 1925-6. 



Although the numbers of whales on which these curves are based is small it is 

 difficult to believe that the appearance of these length groups is due to chance, and 

 the explanation seems to be as follows. If breeding took place regularly all the year 

 round one would expect the young whales to appear equally at all sizes at any given 

 time, but as breeding takes place mostly at a particular season there will be batches of 



29- 

 28- 

 27- 

 26- 

 25- 

 24- 

 23- 

 22- 

 21--- 

 20- 

 19- 

 18--- 

 17- 

 16--- 

 IS- 

 14- 

 13- 

 12- 

 11- 

 10- 

 9- 



.-«-WINTER->: 



; (north) 



females: sexual maturity at 23-7 M 

 .males: sexual maturity at 22-6 M. _ 



SECOND SEASON SOUTH: 21 M. (APPRO 



.FIRST SEASON SOUTH: 18 M. (APPROX.) 

 .WEANING AT 16 M. 



SUMMER 



(south) 



.-^WINTER->-:- 



(north) 



Fig. 154. Blue whales. Estimated mean cun-e of growth from conception to sexual maturity. 



young whales differing in length from one another by an amount equal to a year's 

 growth. It is therefore not unreasonable to suppose that the difference between our 

 two immature groups represents a year's growth. 



The most uniform results seem to be furnished by the male Blue whales. The facts 

 may be stated as follows. In the summer season at South Georgia male Blue whales 

 are most common at 18-19 m., 21 m. and 24 m. If the calf is weaned at 16 m. it 

 might quite reasonably be expected, judging by its rapid growth during nursing, to grow 

 to 18 m. before the end of its first summer (see Fig. 154). Then the inference from these 



