406 ^VHALES 



rate is, therefore, exceptionally high for their size. From the fact that 

 Rorquals reach puberty relatively early in life (see the table on p. 384) 

 and that mammals in general become sexually mature at about one-eighth 

 to one-sixth their maximum age, we can therefore estimate their average 

 life span at from thirty-five to forty years. Moreover, whale marks, l^y 

 and large, bear out this estimate. Thus the oldest marks found in the 

 carcasses of Fin Whales had lodged there for twenty-five years, and the 

 animals must, therefore, have been at least twenty-seven years old, and a 

 Fin Whale cow from which a twenty-one-year-old mark was recovered 

 must have been at least twenty-three years old. The cow was pregnant, 

 and her ovaries showed one corpus luteum and twenty-two corpora 

 albicantia. The oldest Fin Whale, whose age was determined by corpora 

 albicantia, was thirty-five years old, the oldest pregnant cow being 

 thirty-three, whereas the ovaries of thirty- and thirty-one-year-old cows 

 showed definite signs of senility. The oldest known marked Humpback 

 was estimated to have been at least twenty years old, but Symons and 

 Weston (1957) mention a specimen that was twenty-nine years old. Now 

 these figures do not, of course, reflect the maximum age these animals can 

 reach, since none of the whales in question died of natural causes. How- 

 ever, from the days of Greenland whaling when harpoons used to be 

 marked with the year in which they were used, we know that Greenland 

 Whales and Biscayan Right Whales can live until forty years at least. 

 Sperm Whales are known to live until they are at least thirty-two, but no 

 marked Blue Whale has ever been found to grow older than twenty. 



Some Odontocetes can apparently reach the same age. Thus the Pilot 

 Whale (or Risso's Dolphin) 'Pelorous Jack' (see p. 184) is said to have 

 accompanied ships plying between Wellington and Nelson (New Zealand) 

 for thirty-two years, and a paper published in 1955 claims an average life 

 expectancy of twenty-five years for this species. Sleptsov estimates that the 

 very much smaller Common Dolphin lives for fifteen years. 



Because our knowledge of the life span of whales is still very vague, we 

 cannot say with any certainty whether they remain fertile throughout life, 

 or whether senility sets in at a given point, as it does, for instance, in 

 females of our own species. In 1955, an anonymous author asserted that 

 senility was a factor in the lives of Pilot Whales, but his assertion must 

 decidedly be put to further tests, since it runs counter to everything we 

 know about animals living in their natural habitat. True, our knowledge 

 of this aspect of wild life is somewhat scanty, but all the facts indicate that 

 most wild animals can propagate their kind throughout their lives, or else 

 have only the briefest periods of senility. For the time being, we had best 

 apply this general rule to whales, as well, though it seems possible that 

 their fertility, like that of other mammals, decreases with old age. 



