4 8o DISCOVERY REPORTS 



15. The pairing season of multiparous females is investigated as before from foetal length data. 

 The peak conception period is from April to July, with 8 June as the median date of pairing, and the 

 curve is skewed, with a long tail from July to December. Less than 13 % of all multiparous conceptions 

 are estimated to take place before May. The average age of females pairing in March and April is 

 much higher than for later months but, apart from this older group and the primiparous group, age 

 does not influence the conception date. 



16. Data on the incidence of females which are concurrently lactating and pregnant are given, and 

 suggest that some 18% of females conceive at a post-partum ovulation. Evidence is presented which 

 suggests that nearly all multiparous females experience a post-partum heat, and that a proportion of 

 primiparous females do not. This is inferred from the relative sizes of the corpora albicantia in 

 different classes of mature females, and from the presence in the ovaries of lactating females of 

 anomalous corpora which resemble corpora aberrantia in their histology. 



Loss of a near-term foetus, a stillbirth, or loss of a young calf, is probably followed by an oestrous 

 cycle comparable with normal post-partum heat. The termination of pregnancy at an earlier stage is 

 probably followed by ovulation at the ovulatory period which would normally occur next. 



17. A small proportion of females in late lactation are found to have recently ovulated, and some 

 14% of 'resting' females are also found to have ovulated recently and to have a corpus luteum of 

 ovulation in the ovaries. The incidence of diatom film is low in both lactating and recently-ovulated 

 females, suggesting that they are females which have only recently entered antarctic waters, and that 

 they have therefore ovulated either during or just after the southward migration. 



The incidence of recent ovulations in antarctic waters is not constant throughout the summer 

 months, but falls progressively from October/November to April. Evidence is presented which 

 strongly suggests that almost all fin whale females experience a post-lactation ovulation, and a theoretical 

 curve based on this hypothesis, which indicates the entry of post-lactation females into the antarctic 

 summer population, is in close agreement with one showing the monthly antarctic catch. It is 

 concluded that 50 % of non-pregnant mature females are south of the Antarctic Convergence by mid- 

 December, and that December is the average time of the post-lactation ovulation. This is in agree- 

 ment with the time of the suppressed oestrous cycle of pregnant females, and suggests that identical 

 exteroceptive factors are responsible for the regulation of this cycle in these two classes of females. 

 Very few pregnancies are initiated at this time, mainly owing to the absence of sexually active males. 



18. The cyclical activity of the mammary glands is briefly described. The criterion of lactation 

 which is adopted is the presence of apparently normal milk in the glands, but this is not completely 

 valid because it has been shown that probably about 25 % of such females in antarctic waters have 

 recently ceased to secrete milk and are weaning, or have just weaned, the calf. The average month of 

 weaning is estimated to be December, and the lactation period is about 7 months. The incidence of 

 diatom film, and the ages of foetuses in females simultaneously lactating and pregnant, suggest that 

 these females do not usually enter antarctic waters until they are just about to wean the calf. 



The relative abundance of lactating females in antarctic waters in different months is examined, and 

 it is concluded that they are not present in representative numbers in the early part of the whaling 

 season. It is suggested that the southward migration of females with suckling calves is dependent on, 

 and limited by, seasonal changes in sea temperature, and that the critical surface temperature is about 

 o° C. Thus, the influx of lactating females is earlier at South Georgia than on the pelagic whaling 

 grounds further south ; probably over 50% of females wean their calves before entering South Georgia 

 waters, and over 80% do so before they enter the pelagic whaling grounds. 



19. The relation of the sexual cycle to the migratory cycle is discussed. It is suggested that, as in 

 females at puberty, the biannual ovulatory periods are primarily related to the twice-yearly period of 



