REPRODUCTION 387 



In fact, the rate of growth is not so much related to the protein percentage 

 of the milk as to the total amount of protein ingested daily - a subject 

 on which we still know very little. On the other hand, Gregory and his 

 colleagues were able to determine that the vitamin A and B content of 

 Blue and Fin Whale milk, and also the proportion of potassium, magnesium 

 and chlorine in it, did not differ significantly from that of the milk of 

 terrestrial mammals. 



Calcium and phosphorus, however, do in fact occur in greater concentra- 

 tions in the milk of Blue and Fin Whales than in that of terrestrial 

 mammals, possibly because an inordinate amount of bone must be grown 

 within six to seven months, during which time Blue Whales, for instance, 

 grow from twenty-five feet to fifty feet, i.e. more than one and three- 

 quarter inches a day. In the same period their weight increases from two 

 to twenty-three tons, i.e. about two hundredweight a day. In fact, Blue 

 Whales grow so fast that they double their birth weight within seven days, 

 the corresponding figures for dogs, pigs, rhinoceroses, cows and horses 

 being nine, fourteen, thirty-four, forty-seven and sixty days respectively. 



We started this chapter by pointing out that those concerned with 

 whales are particularly interested in their reproductive processes, since, 

 once it is known how many calves a given cow can produce in a given time, 

 a check can be kept on the population level. 



To do so, we must first know at what age whales reach sexual maturity. 

 Now, while examinations of the sexual organs clearly show whether a 

 given whale is mature and thus ready to copulate, the determination of its 

 actual age is extremely difficult (see Chapter 14). The table on p. 384 must 

 therefore be treated with some reserve, particularly since more recent 

 investigations have shown time and again that Rorquals attain sexual 

 maturity later than was formerly thought. At present, it is believed that, 

 as the table shows, average propoises become sexually mature at about 

 fifteen months, that Blue and Fin Whales reach puberty at between fovu" 

 and a half and six years, and Bottlenose Dolphins only after five to six 

 years, with cows and bulls becoming sexually mature at about the same 

 age. The Pilot Whale seems to occvipy a very special position in the table 

 and we would do well to suspend judgement on it until more thorough 

 investigations have been made. Humpback cows generally start ovulating 

 when they are four and a half years old, bvit do not always conceive during 

 the season in which they reach maturity. Now, the age of sexual maturity 

 depends largely on size, and large animals generally mature later than 

 small ones. Seeing that water-buffaloes become inature at about two years, 

 American and European bisons and chamois and tapirs at about three, 

 camels and zebus at four and elephants at only eight or ten, we are forced 

 to conclude that large Cetaceans are singularly precocious for their size. 



