456 DISCOVERY REPORTS 



foetus was missed may have been included in the non-pregnant group, but this will not greatly affect 

 the results, and in any case the relative incidence of pregnancies is what we are here concerned with. 

 The percentages pregnant ± 2 s.E. are shown in Text-fig. 52 b, and the results for the last four age 

 groups have been combined to increase the size of the sample. 



There is again no statistically significant difference in the incidence of successful ovulations, but 

 considering the results as a whole it would appear that the youngest females show a slightly lower 

 fertility, that fertility is at a maximum at ages corresponding to 1 5-30 corpora (that is about 1 5-25 years, 

 see p. 466), after which the proportion of successful ovulations appears to decline. This is in agreement 

 with the generalization made by Hammond (1952, p. 656) that the time of submaximal fertility 

 corresponds to the time during which the skeleton is completing its full growth, because as we have 

 seen in the fin whale, ossification of the vertebral epiphyses is completed when 14-15 corpora have 

 accumulated in the ovaries. 



There are no data on which to base an accurate estimate of pre-natal mortality, which is another 

 factor in fecundity. This includes all females which conceive, but fail to complete pregnancy success- 

 fully, owing to absorption or abortion of the embryo or foetus. In sheep, losses from these causes 

 average 2-4%, but in cows they are higher, averaging 10% (Hammond, 1952, p. 703). It is likely that 

 in whales the average incidence of pre-natal mortality is very low, possibly as low as the average figure 

 quoted above for sheep. 



The climacteric which occurs in some mammals, and is perhaps most marked in man, marks the 

 end of active sexual life and is, therefore, the inverse of the developmental process of puberty. It is 

 of rare occurrence in natural populations. Taking into account all the evidence presented above, it is 

 clear that there is no real climacteric in fin whales, at least up to the ages represented in the sample. 

 It is significant that the oldest female (according to the number of corpora which had accumulated in 

 the ovaries) was pregnant. It does, however, seem that there is a slight tendency for fertility to be 

 reduced at ages in excess of about 30-40 years (that is when 35-50 corpora have accumulated, see 



P- 4 66 )- 



Variations in fertility with time 



Mackintosh (1942, p. 223) referring to blue and fin whales remarked that, 'the percentage of adult 

 females which are pregnant has been increasing in a remarkable degree year by year, as if the actual 

 rate of breeding were becoming faster '. He suggested that this was conceivably a reaction to whaling. 

 Since then additional material has become available. 



In order to make use of material collected by non-biologists, so as to keep the available samples as 

 large as possible, all females with an active corpus luteum in the ovaries have been assumed to be 

 pregnant whether or not an embryo or foetus is found. If this procedure were not adopted, then the 

 sample would have to be restricted to females of which the uterus has been carefully searched. This 

 would greatly reduce the size of the sample and would eliminate all samples provided by the whaling 

 companies. 



The term ' percentage pregnant ' means, therefore, the proportion of adult females in the catches 

 which had an active corpus luteum in the ovaries. The actual ' percentage pregnant ' will be lower than 

 this by about 8-10%, because as shown above a proportion of those females which have active corpora 

 lutea in the ovaries are not pregnant, but have recently ovulated. 



It should be emphasized that the term ' percentage pregnant ' refers to the catches only, and applies to 

 the total population in a relative way. This is due to the fact that a large proportion of lactating females 

 delay the southward migration into antarctic waters (p. 450), and because of this, and also because of the 

 prohibition on the taking of lactating females, this group is under-represented in the catches. This may 

 partly explain the lower values for the South Georgia catches, where more lactating females are taken. 



