144 



DISCOVERY REPORTS 



GROWTH AND AGE 



The smallest female Sperm whale examined was 9-5 m. long and was pregnant in its 

 second breeding season. Sexual maturity is therefore reached at some length shorter 

 than this, probably between 9-0 and 9-5 m. long. The smallest mature male examined 

 was 11-85 ^^- loi^g' ^^'^ its testis and epididymis contained very few spermatozoa: it is 

 judged to have only just reached sexual maturity. Two males were mature at i2-i m. 

 long, and three others 11-75, ^2-50 and 12-30 m. long respectively were doubtful. The 

 largest sexually immature male was 11-70 m. long and two of 11-50 m. were doubtfully 

 mature. All those over 13 m. in length were mature. Sexual maturity may, therefore, 

 be taken to be reached by the male between the lengths of 11-5 and 12-5 m. 



Data affording evidence on the rate of growth of the Sperm whale between birth and 

 sexual maturity are very scanty. Only a few immature whales, which were all males, 

 have been examined and their lengths are shown in Table XIII. If these figures are 

 plotted on Fig. 46 in a position about 18 months after birth they fall into place round a 

 projection of the ante-natal growth line. The average value for them, 10-81 m., is plotted 

 in the mean date position at ^. It is possible to plot these figures also in a position a 

 year earlier, when the average value would come at position B. But it is likely that the 

 first alternative is the right one, because not only does point A fall on to the projection 

 of the growth line, whereas point B does not, but if point B is accepted it is necessary to 

 assume an extremely rapid rate of growth during suckling. This does not seem to be 

 justified because not only does it require the length of the young to be doubled in 6 

 months, but it makes the growth curve take a sudden upward direction immediately 

 after birth, and this does not happen in the growth curves for those balaenopterid whales 

 in which it is known. Point A, therefore, appears to be the correct one. 



Table XIII. Sperm whale. Immature whales examined at Durban 



Sexual maturity is reached by the male at a length of 1 1-5 to 12-5 m., and from Fig. 46 

 we find that this corresponds to an age of about 22 months after birth. Growth in this 

 sex continues after sexual maturity is reached, and at the age of 3 years there is no sign 

 of physical maturity having been attained. 



The females become sexually mature at a much shorter length than the males, be- 

 tween 9-0 and 9-5 m. long. If they do so at the same age as the males their growth must 

 be much slower, as shown by the dotted line passing through point D in Fig. 46. On 



