MULTIPAROUS FEMALES 45' 



advancement of the pairing season in multiparous females is clearly related to the fact that the north- 

 ward migration of mature females is earlier than that of immature females. Thus, there is an increasing 

 proportion of immature females in the catches towards the later part of the antarctic whaling season, 

 which suggests that immature females migrate into, and move out of, antarctic waters later in the 

 season than mature females. Only 13 % of multiparous conceptions are estimated to occur earlier than 

 the beginning of May and this figure is almost certainly too high, because the method of estimation 

 takes no account of variations in the rate of foetal growth. This has the effect of extending the 

 estimated period of conceptions. Thus, in calculating the frequency of conceptions, a faster growing 

 foetus would appear to have been conceived earlier than one which grew more slowly, whereas in fact 

 both might have been conceived at the same time. 



According to Text-fig. 42 (p. 423) the earliest period in which whales migrating northwards should 

 experience increasing day lengths is late April-May, and this is in very close agreement with the 

 estimated frequency of conceptions. The latest month during which fin whales can migrate northwards, 

 and yet be subjected to increasing day lengths, is seen to be September, although few if any animals 

 migrate northwards at this time. From July to November or December, animals remaining in one 

 latitude zone in the southern hemisphere will experience increasing day lengths (Text-fig. 42) ; this 

 extends over most of the period covered by the ' tail ' of multiparous conceptions. It is interesting that 

 in the apparently non-migratory female sperm whale pairing occurs in spring (Matthews, 1938; 

 Clarke, 1956), that is at a season of increasing photo-period. 



It is not, however, necessary to show that all conceptions follow a period of increasing day lengths, 

 because it has been shown that a post-partum ovulation is a regular feature of the sexual cycle. The 

 factors governing this post-partum ovulation are uncertain, but it is likely that interoceptive factors 

 relating to the end of pregnancy, the regression of the corpus luteum of pregnancy, and loss of the 

 placenta are responsible. In this event conceptions occurring before May could represent a successful 

 post-partum ovulation, for a calf conceived in May would on average be born 11 J months later in 

 April, so that the mother could experience a post-partum heat and mate again in April. In this 

 connexion it will be remembered that the average age of females which conceive in April is apparently 

 very much higher than in later months. 



It seems likely that the ' tail ' of the pairing curve of multiparous females is largely caused by such 

 post-partum conceptions, and it is perhaps significant that in the closely related blue whale the 

 frequency curve of conceptions obtained from foetal length records is not skewed in this way (Laws 

 and Purves, 1956, fig. 11). In the blue whale post-partum conceptions are not a regular feature of the 

 sexual cycle (p. 429). 



We may conclude that the pairing season of multiparous females is closely related to, and perhaps 

 regulated by the increasing day lengths associated with the northward migration from antarctic waters 

 to subtropical and tropical waters. For multiparous females in general, age does not influence the 

 time at which ovulation and conception occur, although the oldest females show a strong tendency 

 to mate early in the season. No convincing explanation of this anomaly suggests itself. It may be 

 relevant that in these older females linear growth is very slow, or has ceased, so that they may not 

 need to remain on the feeding grounds for as long as younger, faster-growing females and can migrate 

 north somewhat earlier. 



Detailed examination of the size distribution of corpora albicantia in females in the different phases 

 of the sexual cycle provides strong evidence that the majority of, if not all, adult females experience 

 a post-partum ovulation. It has been shown, however, that a substantial proportion of primiparous 

 females do not experience a post-partum ovulation. The evidence discussed above does not allow a 

 precise estimate of the proportion of females in which the post-partum ovulation initiates another 



