MULTIPAROUS FEMALES 437 



a recrudescence of ovarian activity before the end of lactation. Chittleborough (1958), found that in 

 this species 18% of females in late lactation had maturing follicles or had recently ovulated. 



In the fin whale the lactation period is shorter, and weaning probably occurs on average in December 

 (see below, p. 446); pairings are not frequent until May. There is, however, strong evidence to show 

 that, as in the humpback whale, there is an ovulatory period just after lactation, and that some females 

 may undergo oestrous changes and ovulate in late lactation. Some 20 % of female fin whales in late 

 lactation have maturing follicles in the ovaries (over 3 cm. in diameter (Text-fig. 4)), and some 5% 

 (4-6±4-2) of lactating females have recently ovulated, as evidenced by the presence of a corpus 

 luteum of ovulation in the ovaries (Table 19). This is in close agreement with Chittleborough's 

 findings in late-lactation humpback whales. 



The majority of lactating fin whales found in antarctic waters are in late lactation, and are very near 

 to the time of weaning. This is discussed in more detail in the next section (p. 445). The evidence as 

 to the size of the calf at weaning, the foetal age in pregnant lactating females, and the lower incidence 

 of diatom infection, all points to this conclusion. 



As Mackintosh and Wheeler (1929) showed, the period of lactation is succeeded by a ' resting ' period 

 of anoestrus which lasts until the following winter. However, when we examine the records for females 

 taken in the Antarctic which are neither lactating nor pregnant, and which should, therefore, be in the 

 Vesting' condition, we find that a proportion of them have recently ovulated. So as to minimize the 

 possibility of including pregnant females (perhaps with small foetuses, or aborted foetuses) in this 

 sample, the material on which the following discussion is based is restricted to whales which were 

 examined personally by the biologists. Where a corpus luteum was seen but no foetus or embryo was 

 found the uterus was searched. Although some females in early pregnancy may be included in this 

 sample, their numbers must be very small. 



The greatest numbers of recent ovulations are recorded from January (Table 22), a month in which 

 it is estimated that very few conceptions occur (Text-fig. 31). Apart from the care taken to include 

 only those females in which it is reasonably certain that no embryo was present there are two facts 

 which confirm the validity of the sample. The evidence of the mean size of the corpus luteum in these 

 whales, which is significantly smaller than the mean size of the corpus luteum of pregnant females 

 (p. 356), and the mean size of the largest follicle, which is significantly greater than the mean size of 

 the largest follicle of pregnant females (p. 348) confirm that most, if not all, of these recent ovulations 

 are correctly diagnosed. 



In the material up to 1958 there are 465 non-pregnant, non-lactating females which fulfil these 

 conditions and of these 59 or 127% had recently ovulated before death. This figure needs some 

 adjustment, because the discrepancy between the early period of the investigation at South Georgia 

 and the later seasons suggests that in the early seasons at South Georgia recent ovulations may have 

 been under-represented. Thus, in the four seasons 1924-28, some 55 non-pregnant, non-lactating 

 females are recorded, but no recent ovulations; in the three seasons 1928-31 eight out of 51 such 

 females had a corpus luteum of ovulation in the ovaries (157%). If these 55 records are eliminated 

 there are 410 non-pregnant, non-lactating females, of which 59 or 14^3-5% are deemed to have 

 had a corpus luteum of ovulation in the ovaries at the time of capture. It makes little difference 

 to the argument below whether or not we restrict the sample in this way. Another fact which should 

 be recalled is that, although the mean size of the largest follicle of ' resting ' females is similar to 

 that of lactating females (Text-fig. 4), the maximum follicle size of 'resting' females is 8 cm., only 

 slightly less than the largest follicles in recently ovulated females, whereas the maximum follicle size 

 in the sample of lactating females was about 5 cm. This suggests that some ' resting ' females are in an 

 immediately post-oestrus condition. Although there are a number of 'resting' females which have a 



