NEWLY MATURE FEMALES 413 



are first to be impregnated, while older females arrive on the breeding grounds later, after one or more 

 unsuccessful ovulations. In fact it is now known that the pubertal females migrate and conceive later 

 than the mature females. The tendency for the advancement of the pairing season with increasing age 

 is further discussed below (p. 427). 



Growth in length just after puberty 



Females in the ' o corpus albicans ' group form a distinct, narrow, and relatively uniform class of 

 animals. For this group of females it is possible to study the rate of growth over almost a year by 

 comparing the maternal growth with the growth of the foetus. The average curve of foetal growth in 

 length of the fin whale has been established with reasonable accuracy (p. 403), and may be used as an 

 absolute time-scale over the period of gestation. 



Text-fig. 38. 



Ol 234 5678 9 IO II 12 



JASONDJFMAMJJ 



GESTATION PERIOD (MONTHS) 



Probable growth in length during first pregnancy of females which conceived 

 at the first ovulation. 



The mean length at conception is taken to be 65-25 ft. (see above, p. 406), but this may be a slightly 

 high (or low) value because females which become pregnant at the first ovulation may be shorter (or 

 longer) than those which ovulate unsuccessfully for the first time. 



The mean lengths of 86 primiparous females in the 'o corpus albicans' group are set out in 

 Table 14, according to the month of pregnancy, and in Text-fig. 38 the smoothed maternal lengths 

 are plotted against the months of pregnancy, taking July as the mean period of conception. A growth 

 curve has been fitted by inspection to these points taking its origin at a length of 65-25 ft. at conception. 

 There is no statistical justification for fitting an approximately sigmoid curve to the points, but with 

 the exception of the second point, which represents a small sample, the lightly smoothed mean lengths 

 do not show large fluctuations and it is reasonable to suppose that the average curve which has been 

 drawn does approximate to the true growth curve (Text-fig. 38). It is significant that the supposed 

 maternal increase in growth-rate begins about 6-7 months after conception ; it is at this time that the 

 exponential phase of foetal growth begins (p. 401, Text-fig. 30) and this increased growth is related 

 to the short annual period of intensive feeding. A mass curve of this kind is likely to reduce 

 apparent changes in the growth rate. It is likely, therefore, though not proven, that fin whales 

 experience a period of increased growth in length, similar to the increase in weight known to take 

 place on the antarctic feeding grounds (Ash, 1956). Probably all fin whales show a similar rapid 

 seasonal increase in length on the feeding grounds, which yearly becomes smaller and smaller until 

 physical maturity is attained. These data suggest that just after puberty the average rate of growth is 

 between 2 and 4 ft. a year, probably just over 3 ft. 



