BREEDING, GROWTH AND AGE 217 



The mean lengths at which Blue and Fin whales become adult were estimated by 

 Mackintosh and Wheeler (1929, p. 417) to be 22-6 and 237 m. for Blue males and 

 females and 19-5 and 20-0 m. for Fin males and females respectively, and Laurie (1937, 

 p. 231) considers that the data he obtained in a season's work in a factory ship confirm 

 these figures. 



Matthews (1937 and 1938c) estimates the corresponding lengths in Humpback and 

 Sei whales to be as follows : 



Humpback males 12-0 m. or 39 ft. 4 in. 



Humpback females 12-5 m. or 41 ft. o in. 



Sei males 13-5 m. or 44 ft. 3 in. 



Sei females 14-5 m. or 47 ft. 7 in. 



These figures are based on all the Humpback and Sei whales examined at South Georgia 

 and South Africa from 1925 to 193 1, and must therefore be taken as the nearest approxi- 

 mations which can be made at present. These two species, however, were scarce, and 

 during the whole period were far fewer than the Blue and Fin whales on which the 

 original estimates were based. 



Since we now have the additional material for Blue and Fin whales from 1927 to 1941 

 the original estimates for these two species can be re-examined. Table 8 shows all the 

 whales within certain lengths which can, from an examination of the records, be classed 

 as certainly or almost certainly sexually mature or immature, and they include the 

 longest immature and the shortest mature whales. The purpose of the table is to show 

 in what degree total length is itself a criterion of sexual maturity, and the extent to which 

 the lengths of mature and immature whales overlap. It is often difficult to draw the line 

 between whales which should or should not be included in the table. Females are more 

 easily distinguished as mature or immature than males, for in the latter there is little to 

 go by except the size of the testes. Hence a considerable number of males whose testes 

 were intermediate in size between the obviously mature or immature have been ex- 

 cluded. Whales are also excluded where there is any doubt of the accuracy of the total 

 length. The numbers of immature and mature whales are given for each tenth of a metre 

 of total length, and this perhaps comes near to splitting hairs, for we can scarcely expect 

 to fix exactly the mean length at sexual maturity to such a small unit. 



With this increased material the overlap is found to be a little more than appeared 

 when the mean length at maturity was originally estimated, but it is still not much more 

 than about 1-5 m. (4-9 ft.) if exceptional examples are omitted. 



Blue males. The table does not indicate any amendment to the estimated length of 

 22-6 m. or 74 ft. 2 in. This would mean that whales measured to the nearest whole foot 

 would be regarded as immature if less than 74 ft. 



Blue females. The figures suggest that 23-5 m. (77 ft. 1 in.) would be more nearly 

 correct than 237 m. (77 ft. 9 in.), but measurements about the critical length are rather 

 scarce. If the new length were accepted, whales measuring less than 77 ft. would be 

 regarded as immature. 



