222 WHALES AND DOLPHTNS 



are fat and well-nourished, with stomachs usually filled with 

 Krill. 



The breeding season is a protracted one, but pairing is 

 commonest in these southern whales in the months of June 

 and July, during the southern winter when the animals are in 

 the warm water. The period of gestation is about a year, and 

 the calf is suckled for 6 or 7 months after birth. Growth is 

 very rapid, and sexual maturity is usually reached in 2 years, 

 when the animals are from 74 to 77 feet in length. Only one 

 calf is normally produced at a birth ; twins are infrequent, 

 but not unknown. Not less than 2 years elapse between 

 successive pregnancies — that is to say, pairing does not take 

 place during the suckling period. The almost equal proportion 

 of males and females in recorded catches indicates that the 

 Blue Whale is monogamous. The slow rate of propagation is 

 important economically ; the protracted breeding season and 

 rapid growth to maturity help greatly in the replacement of 

 the stock, but are counterbalanced by this other factor. 



Female animals are, on the average, a foot or two more in 

 length than males of the same age, but otherwise there is no 

 essential difference in general body form in the two sexes. 

 It is believed that the Blue Whale is physically mature and 

 fully grown when it is 12 to 14 years old, and although no 

 accurate knowledge is available of the age to which they live, 

 it is considered probable that a Blue Whale over 50 years old 

 has exceeded the norma) span of life for this species. 



External parasites are rare, and those found are believed to 

 be collected when the animals are in warm water. A species 

 of barnacle and another parasitic crustacean called Penella are 

 the most common, and the whales seem able to throw them off 

 when they migrate into colder water again. The film of 

 diatoms sometimes found on the skin of Blue Whales is an 

 exception to the more general warm-water infestation, for it is 

 those whales which have been long in cold water which get 

 covered with the microscopic unicellular organisms. This film 

 is sometimes so extensive that the normal colouring is overlaid 

 by that of the diatoms, and it was their yellow colour that 

 gave rise to the name " Sulphur-bottom ", and to the belief 

 that this was a separate species. 



A mysterious pitting of the skin occurs on the flanks and 



