390 WHALES 



interval between t^vo successive births to be less than two years, and the 

 fact that the calculated interval is as much as 2 -16 years must mean that 

 some cows do not conceive even during the winter following weaning. 

 It seems reasonable to assume that most of these are cows which, in the 

 year before, conceived soon after giving birth, and thus overtaxed their 

 strength. 



Sperm Whales have their next oestrous phase only seven months after 

 weaning their calves, so that they produce offspring once every three 

 years. Narwhals, too, seem to have a similarly long interval between 

 successive births, while Blue Whales and Humpbacks (just like Fin 

 Whales) are known to miss a season occasionally, and thus to have an 

 interval of more than two years between successive births. Other Ceta- 

 ceans also miss a season from time to time, with Common Dolphins 

 'resting' after every three births. 



With the help of all these data we should be able, by and large, to 

 calculate how many calves a given whale or dolphin cow can produce 

 during her lifetime, if we also knew her average life expectancy, and her 

 average fertile life. We shall return to these questions in the next chapter 

 in greater detail, but we may say, straight away, that while we have well- 

 founded arguments showing that senility plays hardly any role in the 

 life of Cetaceans, we know very little indeed about their average life- 

 span. The relevant figures in the table (p. 384) are therefore based on 

 inferences from other mammals. Crude approximations though these 

 figures are, they nevertheless show that Cetaceans cannot produce a 

 great many offspring, the figures varying from six to fifteen, and we would 

 do well to assume that Rorqual cows bear a maximum of twelve calves. 

 (These figures agree, by and large, with what we know of undomesticated 

 bovines, i.e. bisons.) 



