FIN AND BLUE WHALES 293 



4*5 % of the adult females examined were non-pregnant, and all lactating females were pregnant. 

 This strongly suggests that the gestation period is less than 12 months (as there are not two foetal 

 length groups) and Jonsgard concluded that ' the majority of mature females bring forth young once 

 a year'. His material covers too short a period to allow for an estimate of the duration of gestation. 

 Nor is an estimate of the length of gestation possible from the data available to Omura and Sakiura 

 (1956). Earlier estimates of the duration of gestation as 10 months, made by Eschricht and Guldberg 

 are referred to by Jonsgard (1951), who concludes that pairing occurs from about January to the end 

 of May, with a maximum frequency probably in March. For reasons which Jonsgard discusses, 

 Grieg's (1889) material has not been used in the present paper. Although Jonsgard has concluded 

 that, owing to differential migration, the foetal measurements available are not truly representative 

 of the stock of whales, his mean lengths for the 4 months April to July are very similar to those given 

 by Omura and Sakiura (1956, fig. 23, curve A). 



The growth curve for these months appears to be linear as in other species (Text-fig. 12) and inter- 

 sects the abscissa at Lt^ in mid-March. By analogy with other species the period from conception to 

 LIq is probably about 43 days (i.e. intermediate between the humpback whale and the porpoise) 

 which gives an estimated average date of conception at the beginning of February. It is not possible 

 to come to any conclusions about the later part of pregnancy, but with a neonatal length of 2-46 m. 

 (Jonsgard, 1 951) or even 2-8 m. (Omura and Sakiura, 1956) the rate of growth must increase markedly 

 if pregnancy is to occupy no more than 12 months. 



The evidence presented so far strongly suggests that, in a baleen whale species foetal growth in 

 length may be represented by a curve, increasing very slowly at first, then linearly for about 4 months, 

 and finally increasing exponentially until parturition. Although the strong measure of agreement 

 between the different species, and particularly between the estimated and observed conception dates 

 for the humpback whale, is very suggestive, it must not be forgotten that mean values of small samples 

 may differ considerably from the true mean value in the population sampled, and the evidence for a 

 phase of exponential growth is not conclusive. 



FIN AND BLUE WHALES 

 The foregoing parts of this paper have been concerned with published data, and without the full 

 original records it was not possible to calculate the extent of variation within the monthly samples. 



For 956 southern hemisphere fin whales and 11 12 blue whales the original data on foetal lengths 

 are available and will now be used in a more detailed study of foetal growth in these species in order 

 to test the hypothesis put forward above. 



Of the fin whale records, five are from Saldanha Bay, South Africa, 268 are from South Georgia 

 and 683 are from the pelagic whaling grounds. The South Georgia sample extends by 2 months the 

 period of foetal growth covered by the pelagic data. Of the blue whale records, seven are from 

 Saldanha Bay, and ninety are from South Georgia. As they do not extend the period for which data 

 are available they are not used in calculating the monthly means. 



All these measurements were made either by biologists or by specially trained observers, and the 

 sources of error may therefore be considered to be minimal'. The foetal length records are summarized 

 in Tables 2 and 3 ; conclusions about the growth curves are presented in Tables 4 and 5 and Text-fig. 8. 



The variability of the length records is indicated by the mean plus or minus two standard errors. 

 There is a 95 % probability that the true mean of the population as a whole, in any one month, 

 assuming the sample is not biased, will lie within this range. For the fin whale the value of four 



1 For example, the uterus is searched for a small foetus if a functional corpus luteum is seen in the ovaries. 



