224 DISCOVERY REPORTS 



has increased in recent years, conceivably as a reaction to whaling, it would probably 

 mean that fewer whales fail to become pregnant after two years from the last pregnancy. 

 There is some evidence that the occurrence of whales simultaneously pregnant and 

 lactating is also less rare than in former years. 



Laurie points out that if we assume that a large proportion of non-breeding females 

 are absent from the whaling grounds during part of the Antarctic summer, then we 

 should expect to find males correspondingly more numerous than females, and suggests 

 the possibility that these absent females are accompanied by a corresponding number of 

 males. I have, however, re-examined Laurie's data for 1932-3 (when lactating females 

 had not yet received protection), and find that in fact in the early part of the season 

 there was a considerable excess of males, while towards the end, when the majority of 

 lactating females made their appearance, there was an excess of females (see Table 25, 



P- 273)- 



GROWTH TO SEXUAL MATURITY 



In the absence of direct evidence it is also difficult to be certain of the rate of growth 

 from birth to sexual maturity. Mackintosh and Wheeler estimated that this period was 

 two years, and the basis of the calculation was mainly the incidence of unweaned calves 

 and the appearance of length groups in immature whales. Blue whales provided the best 

 evidence. Records of the smallest whales of this species to be found in the British 

 Museum's statistics of South African catches, when plotted according to the length and 

 month in which they occurred, suggested that they more than doubled their length in 

 the first seven months after birth, and since the baleen appeared to make a sharp increase 

 in growth at about 16 m. it was inferred that weaning takes place at about this length. 

 The appearance of two length groups in immature Blue whales measured at South 

 Georgia suggested that the total period from birth to sexual maturity was two years ; and 

 the curve of linear growth so obtained, when compared with the estimated growth during 

 the nursing period, indicated a slowing down of growth after weaning. This was ad- 

 mitted to be a rather speculative estimation, but such rapid growth agrees well enough 

 with Andrews's estimation (1914) of the growth of the Grey whale (in so far as analogy 

 with this species is significant) and with the observed growth of a young Fin whale 

 recorded by Hjort (see Harmer, 1920, p. 77). 



More recently, Rayner (1940), in his account of the results of whale marking by the 

 Discovery Committee, describes the important example of a Fin whale which was 

 marked in the Antarctic in February 1935. This whale he considers to have been not 

 more than 40-45 ft. long (i.e. about 13 m.), and possibly less. It was at any rate a calf 

 accompanied by its mother, and if our estimate of the rate of growth during lactation is 

 anywhere near correct it must have been born in the normal breeding season of 1934. 

 The whale was killed and the mark returned from South Africa on 1 July 1937, when it 

 should be just about three years old. Its length was then given as 68 ft. 9 in., or 20-95 m. 

 Table 8, p. 218, shows that this is well past the mean length at which sexual maturity is 

 reached, and Wheeler's comparison (1930, p. 411) of the numbers of corpora lutea in 



