416 DISCOVERY REPORTS 



that the mean of this second-pregnancy group is so close to the mean length of females in their first 

 lactation or resting period, suggests that the original assumption is correct and that this group is 

 correctly placed in Text-fig. 39. 



The variance in length in each group is rather large (standard deviations 2-3 ft.). The dotted lines 

 in Text-fig. 39 represent the mean plus or minus 6 in. and this represents the probable range of error 

 in the mean curve taking into account the values of two standard errors of the mean lengths at puberty, 

 some months later (groups (b) and (c) combined), and over a year later (groups (d) and (e) combined). 



The first ovulations 

 We have seen that prior to the first pregnancy there may be from one to four ovulations, with a mean 

 number of about 1-42. There are two important questions which we must now attempt to answer. 

 First, is ovulation spontaneous or is it induced by the stimulus of coition? Secondly, what is the 

 length of the oestrous cycle and the interval between successive ovulations ; that is to say is the fin 

 whale polyoestrous or monoestrous? 



Ovulation is probably spontaneous as in ungulates and most other mammals (Eckstein, 1949). 

 Mackintosh and Wheeler (1929, p. 381) describe the vaginal band which is present in about 21 % of 

 immature females. This structure is so placed that it is almost certain to be ruptured when mating 

 occurs and up to 1957 no intact vaginal bands had been observed in parous females. Wheeler (1930, 

 p. 413) mentions the case of a female fin whale with immature mammary glands, an unbroken vaginal 

 band and one corpus albicans in the ovaries ; a similar case was recorded by Mackintosh and Wheeler 

 (1929, p. 390), and there have been a few later unpublished records of females in a similar condition. 

 This is a rarely observed condition because, of a hundred females at puberty, about 70 become pregnant 

 at the first ovulation, and only about 21 % of the remainder would have a vaginal band ; this means that 

 less than about 6% of females which have ovulated only once can be expected to have an unbroken 

 vaginal band. The presence of corpora in the ovaries of females with unbroken vaginal bands would 

 appear to be conclusive evidence of spontaneous ovulation. However, in 1958 D. F. S. Raitt, a 

 biologist making observations and collections for the National Institute of Oceanography, reported 

 a pregnant fin whale which had an unbroken vaginal band. ' One case appeared to be an example 

 of a virgin ovulation. This was a 69 ft. fin... .The vaginal band was intact, though rather slack, 

 the mammary gland was 4-5 cm. deep and a 10 ft. n ins. male foetus was found. The ovaries were 

 obtained and one had an apparently functional corpus luteum (unpublished report). In spite of 

 this one exceptional record it is considered that the several other cases of virgin ovulations associated 

 with unbroken vaginal bands are evidence that ovulation is spontaneous. Some additional support 

 for this assumption is provided by the relatively high proportion of non-pregnant females in antarctic 

 waters which have recently ovulated (pp. 418 and 436), although mating behaviour has been very 

 rarely observed. 



It has been shown that some 68 % of newly mature female fin whales become pregnant at the first 

 ovulation. Furthermore, a large proportion of newly mature females, taken in the Antarctic several 

 months after the pairing season, have ovulated only once and regression of the corpus albicans 

 representing that ovulation is already advanced (mean diameter 4-4 cm.), indicating that it represents 

 a monoestrous cycle, and not the first of a series of polyoestrous cycles. The great majority of fin whale 

 females, if not all, are, therefore, effectively monoestrous in their first sexual season. Some 32% 

 of females in their first pregnancy had ovulated more than once before becoming pregnant. It is not 

 possible to say how long the oestrous cycle in the fin whale is, but it is probably about 16-23 days 

 as in ungulates (Eckstein, 1949) or longer; it is very unlikely to be shorter than this. Then, if the 

 female fin whale is polyoestrous in the first sexual season the curves of the monthly frequency of 



