4 o8 DISCOVERY REPORTS 



Nishiwaki (1957) re-examined this question by comparing the number of laminations in the ear- 

 plug with the number of corpora in the ovaries of 34 antarctic fin whales. He found that 10 laminations 

 corresponded to one corpus albicans and, assuming that laminae are laid down at the rate of two per 

 year, concluded that sexual maturity is attained at 4-5 years. This estimate agrees with his previously 

 published estimate based on other methods (Nishiwaki, 1952). It appears from his fig. 3 that he used 

 six records of immature females (with no corpora) in fitting his regression line, and this would result 

 in a figure for the age at sexual maturity which is slightly low. 



Purves and Mountford (1959) correlated sexual maturity with lamination number for a large sample 

 of antarctic fin whales and found that in the female approximately 12 laminations were laid down at 

 sexual maturity. They based this estimate on the youngest pregnant females in their sample, and on 

 their growth curve showing the number of laminae corresponding to the average length at sexual 

 maturity which they assume to be 66 ft. (Actually 65-25 ft. is the best estimate.) They conclude that 

 the age of the female fin whale at sexual maturity is 5-6 years. 



Not all females become pregnant at the first ovulation and there is usually a gap of several months 

 between first and second ovulations (see below, p. 416). If all sexually mature females (including those 

 at puberty which have ovulated without becoming pregnant) are examined, a more precise estimate of 

 the age at puberty should be possible. 



The ovaries relating to the ear-plugs examined by Purves and Mountford (1959) have now been 

 examined and a brief preliminary account of the findings is helpful at this point. A more detailed 

 account based on a much larger sample of ear-plugs and ovaries now being examined by the author 

 will be published at a later date and may lead to slight modification of the results now put forward. 

 Females are taken to be sexually immature if there is no corpus luteum or corpus albicans in the 

 ovaries ; they are sexually mature if the ovaries contain one or more corpora. 



Table 1 3 . Frequency of sexually immature and mature female fin whales in different age groups 



Number Percentage 



Mature 



3 

 6 



7 

 7 

 8 



9 — 5 



The results of this study are set out in Table 13 and Text-fig. 34. The estimated ages of individual 

 females are taken from Purves and Mountford (1959, table A) and the material has been confined to 

 area I as their sample from area II is rather small. 



In Text-fig. 34 the curve shows the percentage frequency of sexually mature females in different 

 age-groups up to 9 years. The age corresponding to 50% of mature females is about 5 years, which is 

 taken to be the average age at sexual maturity. This is in fairly good agreement with the conclusions 

 of Purves and Mountford (1959); the age above which the majority of females were pregnant was 

 estimated to be about 6 years, but the antarctic catch is taken in January, February and March, and 

 these females would, on average have ovulated during the breeding season (July), that is some 

 7 months previously, giving an estimate for the age at sexual maturity of about 5-4 years. 



Although the average age at maturity is here taken to be 5 years, Text-fig. 34 shows that some 3 -year- 



