NEWLY MATURE FEMALES 425 



According to this hypothesis animals approaching the age of puberty, which migrate northwards 

 at such a time or at such an age that the postulated ' threshold of stimulation ' is not quite attained 

 (see p. 422) should not attain puberty until they are next subjected to increasing day lengths. For the 

 great majority, if not all, of these animals this will be during the next southward migration to the 

 feeding grounds. Animals migrating southwards in the 6 months from September to February (which 

 includes almost all individuals) will be subjected to increasing day lengths, and animals making this 

 migration in November will experience the greatest change in day length. It is again significant that 

 the incidence of recent ovulations in nulliparous females in antarctic waters appears to be highest in 

 November and December, falling to zero in March and April (Text-fig. 40). It has also been shown 

 above that animals taken with corpora lutea of ovulation in the ovaries have only recently completed 

 the southward migration. 



So far only the influence of light has been considered, but the effects of seasonal feeding and tempera- 

 ture changes are probably also important. Thus, a period of heavy feeding precedes the northward 

 migration and undoubtedly influences the time of puberty. These factors will be discussed again in 

 relation to the adult reproductive cycle. There may also be other less obvious influences at present 

 unsuspected and owing to the lack of precise information about the timing and duration of the 

 migrations of fin whales it is not possible to treat this subject in more detail. It is possible, for instance, 

 that the duration of northward and southward migrations may differ, and the speed of migration no 

 doubt varies for different classes of whales. The curves showing the spread in time of conception may 

 also be less precise than has been assumed, owing to individual variations in foetal growth. 



This hypothesis has been developed to show that the observed facts relating to the reproductive 

 cycle at puberty are in close agreement with the two periods of increasing day lengths related to 

 migrations between the breeding and feeding areas. This does not mean that other factors may not 

 influence the timing of the cycle. Thus, in some mammals which have two breeding seasons the first, 

 in the spring, follows a period of increasing day length, and the second follows a period of decreasing 

 day length in the autumn (Eckstein and Zuckerman, 1956). Nevertheless, the thesis that in the fin 

 whale the response to increasing light is the primary exteroceptive factor in the regulation of an 

 inherent sexual cycle appears to be reasonable. 



MULTIPAROUS FEMALES 

 The pairing season and the calving season 

 The curve showing the monthly frequency of conceptions of pregnant females in the 'o corpus 

 albicans ' and ' 1 corpus albicans ' groups combined is very close to that for females known to be in 

 the first pregnancy by reason of the state of the mammary gland (p. 409, Text-fig. 37). For the purpose 

 of determining the pairing season of multiparous females it is, therefore, assumed that all females with 

 three or more corpora in the ovaries are multiparous. A small proportion of primiparous females will 

 be included in this group since 9 % of primiparous females have been found to have three or more 

 corpora lutea and corpora albicantia in the ovaries. This will have a negligible effect on the resulting 

 curve showing the seasonal frequency of pairing. 



There are in the material 694 pregnant females which have three or more corpora in the ovaries and 

 for which there are foetal length records. The seasonal frequency of pairing was estimated for this 

 group by relating foetal lengths to the mean curve of foetal growth as was done for all females (p. 403, 

 Text-fig. 31) and for selected groups of newly mature females (p. 412, Text-fig. 37). In this way 

 foetuses have been divided into groups conceived in the periods 12 June- 11 July, 12 July- 11 August, 

 etc. The estimated frequencies of conceptions in the different monthly periods are set out in Table 17 



13-2 



