BREEDING, GROWTH AND AGE 219 



Fin males. The original estimate of 19-5 m. (64 ft. o in.) was clearly a little too long. 

 19-2 m. (63 ft. o in.) is probably about right. Those measuring less than 63 ft. would 

 then count as immature. 



Fin females. The original estimate of 20-0 m. (65 ft. 7 in.) seems nearly right, but is 

 probably too long by about o-io m., and the revised estimate is 19-9 m. (65 ft. 3 in.). 

 Those measuring less than 65 ft. would then count as immature. 



These revised figures should not be regarded as final. A certain amount of biological 

 work has been undertaken in recent years in the factory ships of more than one nation, 

 and it may be that a larger body of data will be forthcoming which may provide for more 

 precise adjustments to the mean length at sexual maturity. In the meantime it is doubtful 

 whether the original estimates should be disturbed, for they have been used for various 

 calculations of the percentage of immature whales in catch statistics and remain valid for 

 purposes of comparison, e.g. for noting any changes in the ratio of immature whales in 

 successive years. We have to consider the effect on such calculations of revising the mean 

 length at sexual maturity. The corrections suggested above vary only from 4 to 12 in. 

 which seems very small, but a correction of 12 in. can make a distinct difference (of the 

 order of 4 or 5 %) to the estimated ratio of immature whales in statistics of catches. On 

 the other hand, it should be emphasized that such estimates can never be very accurate, 

 for the whaler's measurements are made only to the nearest whole foot, and 12 in. is thus 

 in any case the minimum correction that could affect such calculations. 



THE REPRODUCTIVE ORGANS 



Systematic examination of the reproductive organs forms the principal basis for the 

 study of the breeding of whales. Mackintosh and Wheeler (1929) give a general account 

 of the anatomy and physiology of the genitalia of Blue and Fin whales so far as they 

 throw light on breeding problems, and Matthews (1937, 1938c) shows that in this re- 

 spect Humpback and Sei whales differ very little from Blue and Fin whales. For a more 

 detailed account of the morphology of the reproductive organs reference should be made 

 to Ommanney (1932). 



Examination of the external genitalia is not very useful except as a quick means of 

 ascertaining whether a whale is sexually mature or immature, and even then is uncertain 

 in borderline cases. Of the internal genitalia inspection of the uterus and mammary 

 glands is also useful in the diagnosis of maturity and for determining the stage in the 

 reproductive cycle, i.e. whether pregnant, lactating or resting. The size of the testis is 

 usually, but by no means always, sufficient to show whether a male is immature or 

 mature, but histological examination is the only unfailing method. Histological examina- 

 tion has also provided clear evidence of a male sexual season. Material from an adequate 

 number of adult whales is difficult to obtain during the breeding season, and at present 

 it can only be said that the testes become most active in the early winter months, roughly 

 from April to June in the southern hemisphere. The ovaries are by far the most instructive 

 of the reproductive organs. Their size and weight differentiate mature from immature 

 females, and the ripening follicles indicate the approach to maturity or to ovulation ; but 



