NEWLY MATURE FEMALES 421 



The question is now, when the first ovulation of the ' 1 corpus albicans ' group of pregnant females 

 occurred. This could have been at the time when the average length and age at puberty was reached, 

 or possibly at an average length of about 68 ft. just before the second ovulation which led to the 

 current pregnancy. It seems most probable, taking into account all the evidence discussed in the 

 preceding pages, that the pregnant ' 1 corpus albicans ' group is composed of females which attain 

 puberty at a later date than the pregnant ' o corpus albicans ' group, during or just after the south- 

 ward migration, say in December or January. Pubertal females which were taken in the period 

 November-February and are discussed above (p. 417) belong to this group. After one ovulation they 

 probably go into anoestrus and conceive on average in July at the next ovulation after the northward 

 migration to the breeding grounds. 



Puberty and its relation to the migratory cycle 

 It has been shown that the gross structure of the mammary gland is diagnostic of nulliparous or 

 primiparous female fin whales. This enables certain groups of pubertal or newly mature females to be 

 defined and distinguished, and simplifies investigation of events at this stage of the life-cycle. It is 

 estimated that, on average, puberty is attained at an age of about 5 years (range 3-8 years), corres- 

 ponding to an average length of 65-25 ft. The standard deviation of the mean length at puberty is 

 2-07 ft. which means that 95% of females should attain puberty between about 61 and 69 ft., and 

 67% between 63 and 67 ft. Asdell (1946, p. 10) states that of the factors associated with puberty 

 length is much less variable than age, and this is true of the fin whale also. Laws (19566) has shown 

 that there is a very close relation between growth and sexual maturity in whales as in other mammals. 

 For 10 species of cetaceans the mean length at puberty expressed as a percentage of the mean length 

 of physically mature animals averages 85-1 % (a = 3-14, V = 3-69). It should be mentioned that the 

 true length variance at puberty (and in other age-groups) in the fin whale is probably rather less than 

 that given. Errors in length measurement are undoubtedly relatively large; the dragging of a 60-ton 

 whale up the slipway against gravity and friction might be expected to extend the whale by an amount 

 which should vary according to the post-mortem history. The true mean length at puberty is, there- 

 fore, even less variable than the figures given above suggest. The average rate of growth at puberty 

 is about 3 ft. a year, and in the majority of females puberty is preceded by a period of intensive feeding 

 in the Antarctic, when growth is probably rapid and follicular development may be stimulated by the 

 improved nutritive state. Maximum fertility is probably not reached until after the attainment of 

 physical maturity (p. 456). 



Ovulation is almost certainly spontaneous, and about 70% of females become pregnant at the first 

 ovulation. The curve of monthly frequencies of conceptions is very symmetrical for this group, and 

 the pairing season, which apparently extends from April to October or November, is later in the year 

 than the pairing period of multiparous females. 



A number of nulliparous females are found in the Antarctic between December and April which have 

 only one corpus albicans in the ovaries. Thus a single ovulation appears to be the rule in the first 

 ovulatory period even in the absence of conception. Furthermore, it is found that in general females 

 which become primiparous at the second ovulation conceive earlier in the year than those females which 

 become primiparous at the first ovulation. This also suggests a monoestrous cycle at puberty, for if 

 there were two or three cycles in succession females which conceive at the second or third ovulation 

 should in general pair rather later than females which become pregnant at the first ovulation. 



The evidence strongly suggests that there is a second, subsidiary, period when females which did 

 not attain puberty in the pairing season (April-November) ovulate during or just after the migration 

 to the antarctic feeding grounds. Pubertal females are found in antarctic waters which have recently 



