452 DISCOVERY REPORTS 



pregnancy. Some 18% of lactating females in the present material are found to be simultaneously 

 pregnant, but the variance of this percentage is large (s.E. = 4-34) which means that the true pro- 

 portion probably lies between 9 and 27%. It may be argued that if a post-partum ovulation is of 

 regular occurrence, then more females should be simultaneously lactating and pregnant. It is, how- 

 ever, quite likely that females are relatively infertile at the post-partum cycle, as compared with the 

 ovulation at the end of the 'resting' period which initiates pregnancy in the majority of females. 

 It is well known that a number of animals, including man, are less fertile during lactation. Similarly, 

 interoceptive factors may govern the initiation of an oestrous cycle after stillbirths and possibly after 

 abortions. 



Perhaps the most unexpected result of the present investigation is the evidence for an end-of- 

 lactation or post-lactation oestrous cycle in the fin whale. This is of regular occurrence in the humpback 

 whale, but in this species it occurs in low latitudes. In the fin whale the lactation period is shorter, 

 and a post-lactation ovulation occurs during the migration to the antarctic feeding grounds and is 

 associated with increasing photo-periods. Females taken in antarctic waters with an active corpus 

 luteum of ovulation in the ovaries, denoting a recent ovulation, show a. very low incidence of diatom 

 infection, suggesting that they have only recently moved into antarctic waters. 



It has been suggested that the southward movement of females with calves is in general restricted 

 to regions where the sea surface temperature is above o° C. Weaning of the calf occurs on average in 

 December, but probably owing to the unfavourable environmental conditions less than half of the 

 lactating females in that sector enter South Georgia waters, and probably less than one-fifth enter the 

 pelagic whaling grounds further south, over a month later. The majority of females, therefore, 

 experience the post-lactation oestrous cycle before attaining the latitude of the pelagic whaling grounds 

 (on average about 61-62 °S.). Almost all lactating females which are present in antarctic waters are 

 very near to the time of weaning, are weaning, or have recently weaned the calf. Many of the lactating 

 females passing through South Georgia waters probably experience a post-lactation oestrus while 

 migrating from the latitude of South Georgia to the pelagic whaling grounds further south. 



The few lactating females which penetrate to high latitudes late in the whaling season probably 

 enter oestrus at the end of lactation, but the earliest stages in the development of the corpus luteum 

 which have been accurately noted in antarctic waters, were estimated to be about one week post- 

 ovulation. As only ten such corpora lutea of ovulation have been noted since the recent investigations 

 began, it is not surprising that no very early stages in the development of the corpus luteum have been 

 recorded as yet. In the earlier records some corpora lutea are noted as 'very young', but it is not 

 known whether any of the very earliest stages after the rupture of the follicle and early development 

 of the corpus luteum have been observed. On rare occasions mating has been observed in antarctic 

 waters. 



The temporal distribution of these post-lactation ovulations has been investigated, and it is found 

 that very few if any occur before September, that mid-December is the average date, and that no 

 recent corpora lutea are found after March. 



It is significant that at the same time as this post-lactation oestrous cycle, pregnant females also 

 experience an ovarian cycle, following a southward movement to antarctic waters. In pregnant 

 females there is a cycle of follicular activity which is at a maximum (expressed as maximum follicle 

 size, and as numbers over 3 cm. in diameter), in December, or November/December. The follicles 

 increase in size, but maturation is suppressed owing to the presence of the corpus luteum of pregnancy. 

 The timing of this cycle is again in close agreement with the hypothesis that pro-oestrus is initiated 

 during a period of increasing day lengths related to the southward migration. 



The post-lactation oestrous cycle initiates a pregnancy only very infrequently (in an estimated 5 % of 



