BREEDING AND GROWTH 445 



The reliability of the statistical method of calculating this period might be questioned, 

 but a glance at Figs. 154 and 155 will show that it is during the periods of gestation 

 and nursing that the surprisingly rapid growth takes place and that the section of the 

 curve based on the statistical evidence shows a marked reduction in the rate of growth. 



Mention has already been made of a paper by Andrews (1914) on the Pacific Grey 

 whale, Rhachianectes glaiicus. Here there is more evidence of the rapid growth of 

 whalebone whales, for, from some notes on the period of gestation and rate of growth, 

 it appears that it more than doubles its length in its first year. An investigation of the 

 rate of growth of this species is also made by Risting, who shows that by the end of 

 this first year after birth this species is almost certainly adult. 



Direct evidence of the rapid growth of a young whale, probably a Fin whale, is also 

 mentioned in the Report of the Interdepartmental Committee on Research and Develop- 

 ment in the Dependencies of the Falkland Islands (p. 77). Reference is here made to a case 

 in which a recently born whale was observed early in May. It was presumably not more 

 than about 8 m. long and had a wound by means of which it was recognizable. It was 

 noticed by the whalers all through the summer, and by the autumn it had grown to 

 some 14 or 15 m. This indicates a rate of growth which corresponds fairly well with 

 that shown in Fig. 155. 



The great size of Blue and Fin whales is apt to result, perhaps naturally, in the im- 

 pression that they must require an exceptionally long time to grow to maturity and 

 must live to a great age in comparison with other animals. It has been shown, however, 

 that growth is surprisingly rapid during both gestation and adolescence, and that the 

 whale becomes adult within an unexpectedly short time. It will therefore be interesting 

 to make a comparison in this respect with some other mammals. 



The period of adolescence in most mammals varies from about two to five times the 

 length of the period of gestation, and up to a point it may be said that the larger the 

 animal the longer the periods of gestation and adolescence. In the rat, for instance, 

 the period of gestation is about three weeks, and it starts breeding about two months 

 after birth, while in cats and dogs the corresponding periods are about two and 

 ten months. The horse resembles the Blue and Fin whale in this respect, for 

 gestation lasts for some eleven months and breeding may take place two years after 

 birth. The longest period of gestation appears to be that of the elephant, in which 

 twenty months elapse between conception and birth. The age at which puberty 

 is reached is probably considerable, since full maturity is not reached before about 

 twenty-five years. 



Thus the periods of gestation and adolescence in whales are short in proportion to 

 their size when compared with the land mammals. This is not only the case in Blue 

 and Fin whales, but also, as we have seen, in Rhachianectes, in which both gestation and 

 adolescence last only about one year. It is probable in fact that in marine mammals 

 growth is in general relatively fast, for in the sea elephant also, which may be regarded 

 as one of the definitely large mammals, the period of gestation is eleven months and 

 breeding appears to begin about a year after birth. 



