NEWLY MATURE FEMALES 4'7 



conceptions for the ' o corpus albicans ' group and the ' primiparous ' animals with 0-3 corpora albicantia 

 (diagnosed from mammary gland inspection) should differ. The latter group might be expected to 

 show more conceptions in the later months. In fact a comparison of these curves (Text-fig. 37) shows 

 that they are almost identical. 



Curves showing the frequency of conception in those females which had ovulated twice or more 

 before the current pregnancy have already been presented (Text-fig. 37). The ' 1 corpus albicans ' group 

 was shown to be in advance of the ' o corpus albicans ' group in this respect, whereas, if fin whale 

 females are polyoestrous it would be expected to be slightly later. The ' 2 corpora albicantia ' group 

 shows a similar advancement. 



The majority of the ' 1 corpus albicans ' group will probably be primiparous, but some of this group 

 may be individuals which are in the second pregnancy, having failed to complete the first pregnancy. 

 In this event the second ovulation, leading to the current pregnancy, would probably have occurred 

 earlier in the pairing season than the first ovulation (' o corpus albicans ' group) as is the tendency in 

 muciparous females. 



Others may be females which became pregnant a second time as a result of a post-partum ovulation 

 (see p. 429) following the successful completion of the first pregnancy. If pregnancy were to last 

 ill months in primiparous females, as in multiparous females, and this ovulation is about 3-4 weeks 

 post-partum then the group would tend to conceive about the end of July. This would partly counter 

 the effect of earlier mating of females in which the first pregnancy had been prematurely terminated. 

 The evidence presented above, although not conclusive, suggests that at puberty there is a single 

 ovulation and that if pregnancy does not intervene the female usually goes into anoestrus without 

 further ovulatory cycles. It is possible that a small proportion of females do experience up to three or 

 four polyoestrous cycles, but this is thought to be unlikely. 



Chittleborough (19556, p. 3 18) implies that this is also true of the humpback whale. He states that 

 some female humpback whales become pregnant at the first ovulation, but that the ovaries of a number 

 of nulliparous females 'contained one (or sometimes two) corpora albicantia' and these females 

 'would probably have become pregnant for the first time during their second ovulatory season'. 

 Presumably regression of the corpora albicantia was well advanced in these females and the follicles 

 were not undergoing pro-oestrus enlargement. The mean length of these females was about a foot 

 more than the mean length at puberty which also suggests, as in the fin whale (Text-fig. 39 C), that 

 puberty occurred some months earlier. 



Certain evidence, now to be presented, shows that puberty may be attained outside the usual pairing 

 season, and that in this case also the cycle at puberty is monoestrous. 



In the material there are 10 virgin females which have a corpus luteum of ovulation in the ovaries. 

 In seven of these animals (taken between November and February) this was the result of the first 

 ovulation at puberty; in one (20 January) it represented the second ovulation, and in two others 

 (11 November, 18 January) it was a product of the third ovulation. These are diagnosed as virgin 

 ovulations from the virgin appearance of the mammary gland, and the absence of any embryo in the 

 uterus. Only 2-4% of all pregnancies are estimated to begin in the 4 months November-February 



(Table 11). 



In one of these females (taken on 29 December) regression had begun but the corpus was still 7 cm. 

 in diameter. The others were taken between 15 November and 14 February, and the corpora lutea 

 ranged in size from 6 to 13 cm. The mean diameter was 8-8 cm., which is slightly higher than the mean 

 diameter of the corpus luteum of ovulation, but well within the probable range of variation of the 

 mean (8-28 ±0-82 cm., above, p. 356). It is well below the mean diameter of the corpus luteum of 

 pregnancy ( 1 1 -44 ± o- 1 5 cm.). This supports the diagnoses of recent ovulations. In the three nulliparous 



