436 DISCOVERY REPORTS 



These anomalous corpora are very similar histologically to the corpora aberrantia described above 

 (p. 380), but in size and gross morphology resemble normal corpora albicantia more closely. Like 

 the corpora aberrantia they probably represent former corpora lutea of ovulation which have under- 

 gone an aberrant type of regression. Not all corpora lutea of ovulation undergo this type of regression. 

 Nulliparous females, for instance, usually have normal 'young' corpora albicantia which cannot 

 certainly be distinguished from the single corpus albicans in primiparous lactating or resting 

 females. 



Thus, it seems likely that more than two-thirds of multiparous females, possibly all, experience a 

 post-partum ovulation. Indeed, the size distribution of corpora described above could not be expected 

 to show such a clear pattern if only two-thirds of females experience a post-partum ovulation. In 

 primiparous females, however, there is evidence that a substantial number do not have a post-partum 

 ovulation. 



Ovulation after abortion, stillbirth, or loss of calf 



Chittleborough (1958) has examined six female humpback whales which showed anatomical 

 evidence of a recent birth, but in which the mammary gland was involuting, suggesting that the calf 

 had been lost at, or just after, birth. In five of these ovarian activity had recommenced; in three cases 

 mature follicles were present; in another ovulation had just occurred and in the fifth a developing 

 corpus luteum indicated a recent ovulation. This is compatible with a normal post-partum ovulation 

 since these calves were presumably very young, and a post-partum ovulation seems to be a not un- 

 common feature of the sexual cycle of the female humpback whale. 



As regards the possibility of ovulation after the premature termination of pregnancy, another of 

 Chittleborough's observations is of interest. One of over seventy female humpback whales in late 

 pregnancy had a regressing corpus luteum and one mature follicle 4-8 cm. in diameter. This suggests 

 that the initiation of pro-oestrus changes is dependent on the regression of the corpus luteum of 

 pregnancy. It is unlikely that the fin whale is different in this respect. A further possibility is that 

 ovulation might closely follow abortion or foetal death in mid-pregnancy if this is also associated 

 with regression of the corpus luteum of pregnancy. It has been shown that there is a follicular cycle 

 during pregnancy in fin whales, with peak activity corresponding to a foetal length of 1-2 m. (p. 348). 

 The follicles enlarge at this time, but maturation and ovulation are suppressed owing to the presence 

 of an active corpus luteum. If for any reason, such as an abortion or foetal death, the corpus luteum 

 ceased to be functional at this stage of the cycle it might no longer suppress follicular maturation, and 

 ovulation might follow. 



Kimura (1957) reports on a case of fin whale triplets in which two (measuring 19 ft. 8 in. and 

 15 ft. o in.) were necrotic and one (5 ft. 2 in.) was normal. This would seem to be a case of foetal 

 death of twins, followed by a further ovulation which initiated a second pregnancy, but it is not possible 

 to say what was the interval between the death of the twins and the new conception. It may have been 

 quite short, but alternatively the further ovulation may have been delayed until the next ovulatory 

 period. The latter would seem to be the more probable sequence. 



We may conclude that the loss of a near-term foetus, a stillbirth, or the loss of a young calf, will 

 probably be followed by an oestrous cycle comparable with normal post-partum heat. The termination 

 of pregnancy by foetal death or abortion at an earlier stage will most likely be followed by ovulation 

 at the next ovulatory period. 



Post-lactation heat 



In the humpback whale the lactation period lasts io| months, weaning occurs on average at the end 

 of June and the majority of females are in oestrus between July and September, with pro-oestrus most 

 common in July. The female is not usually in anoestrus after lactation, and in some cases there is 



