364 WHALES 



sixteen feet and fourteen feet respectively. The newly-born Mysticete is 

 approximately 30 per cent its mother's length, and in Odontocetes the 

 corresponding figure may be up to 45 per cent. Similar percentages are 

 found also in other mammals Avhose young are born more or less complete. 



Accordingly, the Aveight at birth represents a fairly high percentage of 

 the mother's weight. For Rorquals, the figures are 5-6 per cent, and for 

 dolphins 10-15 per cent, i.e. they are of the same order as those for 

 Ungulates (8-10 per cent) and seals. 



In most orders of mammals, there is a definite relation between the 

 duration of the gestation period and the size of the young at birth, since 

 the rate of foetal growth is the same in all of them. Thus, horses carry their 

 young for eleven months, camels for twelve months, giraffes for fourteen— 

 fifteen months, rhinoceroses for eighteen-nineteen months, and elephants 

 for as much as twenty to twenty-t-\vo months. However, Huggett and his 

 collaborators (1951, 1959) have shown that every Cetacean species has 

 its own rate of foetal growth, so that, for instance, a 4,000-lb. Blue Whale 

 baby develops in exactly the same time (ten-eleven months) as a twelve- 

 pound baby of a porpoise. The reason for this characteristic must mainly 

 be sought in the food-situation. Investigations of terrestrial mammals 

 have shown that the mating season makes great physical demands on 

 the bull, while lactation makes similar demands on the cow. Similarly 

 heavy demands are made on the young when it is weaned and has to 

 fend for itself. Hence there is evidence that mating, lactation and 

 weaning often coincide with periods of plenty. 



In big Rorquals, mating takes place shortly after they have returned 

 well-fed to the tropics, and lactation begins ten— twelve months later when 

 the cow can once again draw on the reserves stored up during the southern 

 summer. The calves are weaned in the Antarctic where they can feed to 

 their heart's content. The period of gestation of these big animals is 

 therefore extremely short and they have an exceptionally high rate of 

 foetal growth. Similar factors are said to determine the periods of gestation 

 and the mating season of porpoises and dolphins as well, and further 

 investigations of the food supply of Sperm Whales will probably explain 

 why their period of pregnancy is so long. According to many sources from 

 various areas, Sperm Whales carry their young for sixteen months, and a 

 gestation period of sixteen months was also determined by Nishiwaki 

 and his colleagues for N. Pacific Killer Whales. 



The development of the Cetacean embryo can often be followed during 

 one whaling expedition, when countless pregnant females are caught 

 with foetuses at different stages of growth. This was particularly true in 

 earlier times, when the season lasted from November to April, but even 

 nowadays, when the season lasts from the beginning of January to the 



