EXTERNAL CHARACTERS OF BLUE WHALES 275 



total of all Blue whales, of which there are records., consists of 48 per cent females and 

 52 per cent males. Any attempt to explain these differences must, however, be very 

 speculative. The main fact is that in general there is very little difference in the abun- 

 dance of the two sexes. 



It is well known that among the Rorquals the female reaches a greater size than 

 the male, and it will be interesting to examine briefly the extent of the difference 

 which exists. The largest recorded specimen of each sex hardly provides a fair com- 

 parison in itself. Perhaps the best criterion of the difference in size is to be found in 

 a comparison of the lengths at which each sex becomes adult. It will be shown later 

 that up to a point the rate of growth of the two sexes is probably equal, but that the 

 female appears to begin growing faster than the male some time after it is weaned, 

 and that when sexual maturity is reached there is a definite difference between the 

 two. The mean length at which sexual maturity is reached in the female Blue whale 

 can be fairly accurately estimated at 23-7 m. and the corresponding length in the male 

 at 22-6 m. Thus there is approximately i m. difference between the two or the length 

 of the male is 95-4 per cent of that of the female. The largest female measured was 

 28-5 m. and the largest male only 26-45 m. Thus although the largest specimen met with 

 of either sex is perhaps a matter of chance, this seems to show that there must be an 

 increased divergence in size after maturity is reached. The difference between these 

 two specimens was 2-05 m., and the length of the male was 92-5 per cent of the female. 



Besides the difference in size there are one or two differences between the sexes in 

 respect of the bodily proportions, but these will be considered in the general analysis 

 of the measurement records. 



EXTERNAL PROPORTIONS 



It has already been explained that an essential part of the work at whaling stations 

 is to establish as thoroughly as possible the external characters of the southern whales 

 and the limits of the ordinary individual variations which may occur. There has been 

 no evidence to show that the southern whales differ specifically from the corresponding 

 northern forms, or that there are racial distinctions among the southern species them- 

 selves, but the equatorial regions appear to constitute something of a natural barrier 

 between the whales of the two hemispheres (so far at least as the genus Balaetioptera 

 is concerned) and the circumstances are therefore not unfavourable to the development 

 of separate sub-species. Similarly, it is not proved that the whales of the Dependencies 

 are the same whales which are found at a different time of year in South African 

 waters, so that here again some distinction may exist. Consequently a basis is required 

 for the comparison of the whales of different localities, and the first step will be to 

 quote the average condition (in respect of the external characters) of a large number 

 of whales in each region. 



The bodily proportions, recorded by the system of measurements described on 

 p. 265, are of course included in this connection among the external characters. The 

 entire series of measurements has hardly ever been carried out on any one whale, for 



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