FIN AND BLUE WHALES 299 



This is undoubtedly an over-simplified picture. Conceptions probably extend over a longer period 

 than 6 months and are not normally distributed, and there is no doubt that the individual variation 

 in growth rates may be quite large. Kimura (1957) has shown that the differences in the body lengths 

 of a pair of twin fin whales increases with time and suggests that at birth it will average about 2 ft. 

 (0-62 m.). The average neonatal length of the blue whale is estimated to be 7-0 m. (Mackintosh and 

 Wheeler, 1929), but one foetus examined at South Georgia measured 7-46 m. (PI. XIII, fig. 2). Such 

 variation should have no effect on the calculated average growth curves, but would explain the 

 presence of some 7% of fin whale foetuses in the March sample which are above 5 m. m length. 



Zemskiy (1950 a) claimed that female embryos are larger than male embryos. He gives the mean 

 length of sixty-six females as 247-6 cm. and of sixty-seven males as 221-1 cm. Reference to Table 4 

 (in the present paper) where the values of two standard errors are shown to be 12 to 19 cm. for much 

 larger samples suggests that this supposed sex difference in growth rate is not statistically significant. 

 Kimura (1957, p. 113) studied the lengths of pairs of twins of diflferent sex and concluded that there 

 is no sex difference in the rate of foetal growth. 



It appears then, that foetal growth in length in the fin whale can best be described by a grovvth 

 curve of the shape given in Table 4, and drawn in Text-fig. 8. It is possible that groxvth in the second 

 half of pregnancy is not exponential, but if pregnancy is to occupy less that 12 months (required by 

 the high percentage of adult females which are pregnant), then at least two more periods of differing 

 linear growth rates are required making three in all, or the growth curve must be similar to but more 

 complicated than an exponential curve. While this is not impossible there is no evidence for it and 

 exponential growth appears to provide a simpler explanation. The samples taken in the Antarctic in 

 the c; months from October to February may be considered to be representative of the progress ot 

 foetal growth, but differential migration out of the area affects the validity of the samples from March 



°Tour of the five foetal length records from August and September (from Saldanha Bay, South Africa) 

 are below the calculated average lengths for these months (Text-fig. 9). I" view of the small number 

 of records, the largest of which is undoubtedly aberrant (and will be discussed later) this is statistically 

 insignificant, but there are reasons for supposing that there is a differential migration southwards 

 from the breeding grounds (complementary to the northward migration of near-term females from the 

 Antarctic feeding grounds in March) so that samples from Saldanha Bay (33° S.) are likely to be 

 biased towards small foetuses. In general, females with larger foetuses might be expected to have 

 moved further south. It is relevant that Chittleborough (1954. iQSS) found only two early embryos 

 among several hundred recently pregnant females examined on the west coast of Australia which 

 suggests migration out of the area shortly after conception. Jonsgard (1951) reached similar con- 

 clusions about the minke whale on the west coast of Norway. 



It is also to be expected that very early embryos (i.e. less than one month post-conception) wil be 

 absent in Antarctic samples. The effect of this on the mean foetal length would be most marked 

 in the earlier months prior to November, for which in any case very few length records are 



^"^Therl'are three foetuses in Text-fig. 9 which are conspicuously out of phase with the rest. These 

 three foetuses are displaced by 4, Sf and 6 months from the average growth curve and it is though 

 that they represent foetuses conceived following supplementary ovulations in November to December 

 (Laws 1956 1958). It is also likely that some of the smallest embryos from December onwards are 

 he pr;ducts of matings at this time. This question will be considered in more detail in a ater paper, 

 but is mentioned here for the sake of completeness and to draw attention to the wide spread of months 

 in which conception may occur in this species. 



