i88 DISCOVERY REPORTS 



about 60 per cent of males for all Sei whales and foetuses (Fig. i). Above a total length 

 of 15 m. the sex ratio of whales caught drops very suddenly and varies from 19-8 per cent 

 of males to zero. There are 136 whales above this length and of them only sixteen were 

 males, giving a proportion of 1 1 -8 per cent of males. The reason for this sudden falling 

 off in numbers of males is that the females normally reach a greater length than the 

 males, as is shown graphically in Fig. 2, in which the numbers of whales examined is 

 plotted against total length. Up to a total length of 15 m. the sample is probably 

 representative of the population, but above that length it consists almost entirely of 

 females. The whalers naturally select the largest whales, the females, for capture, and 

 consequently the figures as a whole are not representative of the total population ; but 

 if the sex ratio is worked out from data obtained from whales up to 15 m. in total length, 

 which may be expected to show the proportions of males and females more correctly, 

 it is seen that the sex ratio is about 60 per cent of males and not 29-5 per cent as would 

 appear if the total catch only is considered. These figures show the sex ratio among 

 whales of similar length, but not necessarily of similar age. The proportion of males 

 is probably less than 60 per cent among whales of similar age. The males stop growing 

 at a length of about 15 m. and consequently are removed from the length classes of 

 their contemporary females when the latter grow longer than 15 m. The length classes 

 near, but under, 15 m. must therefore contain the males of an age equal to that of the 

 females over 15 m. in length. Among whales of similar age, therefore, the primary sex 

 ratio found in the foetuses is probably steadily declining from the high proportion of 

 males to something nearer equality as the whales grow older. 



It is of interest to note that D'Arcy Thompson (1919) found that of the 1291 Sei 

 whales landed at Scottish whaling stations from 1908 to 1914, 55-3 per cent were males, 

 and this in the total catch, not making allowance for selective hunting. 



The sex ratio in the Sei whale thus appears to show very clearly an excess of males 

 from early stages of foetal life, and unless there is a differential foetal mortality in the 

 earliest stages, it must be due to the sex ratio at conception being unbalanced. In some 

 other mammals, such as the pig and man, an unbalanced sex ratio in favour of the male 

 is known, and Parkes (1926) in discussing this concluded "that the excess of males at 

 conception is probably brought about by virtue of the male-producing spermatozoa 

 being more efficient fertilising agents than the female-producing spermatozoa, the actual 

 diff"erence being probably in activity and vitality ". This difference between the X and Y 

 spermatozoa would appear to offer a satisfactory explanation of the unbalanced sex 

 ratio in favour of the male in the Sei whale. It is of interest to note that Laurie (1937) 

 finds a foetal sex ratio of 53 •6-54-4 P^^" ^^rit of males in the Blue Whale. 



EXTERNAL CHARACTERS 



BODY PROPORTIONS 



The standard series of measurements was taken from 220 whales, but from. 



many the series is not complete for each measurement. A few of the standard 



measurements were not taken at all, but for the sake of uniformity the standard 



