THE REPRODUCTIVE CYCLE 407 



from area V. This close agreement between material from different areas of the Antarctic is 

 particularly interesting in view of the much lower size at sexual maturity of northern hemisphere fin 

 whales (Jonsgard, 1952; Pike, 1953) and the reported difference between the lengths at maturity of 

 this species in the North Pacific and in the East China Sea (Miyazaki, 1958); also in antarctic areas I 

 and II (Purves and Mountford, 1959). 



The question of the mean length of the female fin whale at sexual maturity has been re-examined 

 using the material available to Mackintosh (1942) and in addition several hundred observations made 

 up to 1954 on behalf of the ' Discovery ' Committee and the National Institute of Oceanography. This 

 new material increases the number of records of immature and mature females between 60 and 71 ft. 

 from 402 to 1 145. The data are presented in Table 12. In Text-fig. 33 these length frequencies, 

 smoothed by threes, are shown and it will be seen that the intersection of the curves for sexually 

 immature and mature females is at 65-25 ft. There are 80 sexually mature females below the inter- 

 section point and 86 sexually immature females above. Taking the lengths of these 166 individuals 



100 



>- 100 



-- IMMATURE 

 — MATURE 



LENGTH IN FEET 



Text-fig. 33. Smoothed length frequencies of 

 sexually immature and sexually mature females. 



2 3 4 5 6 7 8 



AGE FROM EAR -PLUG (YEARS) 



Text-fig. 34. Percentage of sexually mature 

 females in successive age groups. 



as a normal frequency distribution with a mean value corresponding to the average length at puberty, 

 the mean is 65-41 ft. (standard deviation 2-07, standard error 0-16). This is in close agreement with the 

 value of the intersection point of the curves. The mean length at puberty is therefore taken to be 

 65-25 ft. The larger sample has extended the range of overlap by a few feet from 61-69 to 60-71 ft., 

 but the mean length at sexual maturity remains identical with the earlier estimate of Mackintosh. It 

 should be remembered that this mean length is derived from material representing several areas. 



The diagnosis of sexually mature males is not so easy, nor so precise. It depends either on histo- 

 logical criteria or on consideration of the size of the testis. The estimate given by Mackintosh (1942) 

 is based on 770 sexually immature and mature males up to 69 ft. in length, and his figure of 63 ft. is 

 accepted here, because the new material did not necessitate revision of his estimate for the female. 



The age at sexual maturity 

 Earlier estimates of the age of female fin whales at sexual maturity have been discussed in the introduc- 

 tion to this paper (pp. 334-38). These were based on the occurrence of modal groups in analyses of 

 length frequencies, and on methods of age-determination, such as baleen plate, crystalline lens, etc. 



