ACCUMULATION OF CORPORA UP TO ATTAINMENT OF PHYSICAL MATURITY 393 

 Probably neither hypothesis alone is entirely correct and the actual mechanism is likely to be very 

 complicated. In the absence of a sufficiency of marked whales of known age it is unlikely that a 

 satisfactory explanation can be found, but the available evidence suggests that there is a very regular 

 annual increment of corpora in individuals and that the variation in the number of corpora at the 

 threshold of physical maturity is mainly influenced by the spread in the ages at which sexual and 

 physical maturity are attained, and partly by the variation in the annual production of corpora in 

 individuals. 



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 RELATIVE NUMBER OF CORPORA 



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Text-fig. 27. Frequency distribution of corpora at two stages of the life-cycle compared. 



The fact that, despite the possible sources of error in the collecting of the data, there is so good a 

 correlation between corpora accumulation and physical maturity points to a rather regular annual rate 

 of accumulation of corpora. This is difficult to account for if the female fin whale is polyoestrous 

 with an average of three ovulations (representing a range of say 1-6 ovulations) per breeding season. 

 This individual variation in the rate of ovulation would alone account for the variance in the number 

 of corpora at physical maturity, without taking account of the age spread at sexual and physical 

 maturity or the possibility of human error. Chittleborough (1955 b, p. 56) presents data which strongly 

 suggest that ' the mean number of ovulations per female humpback during the ovulatory period was 

 only slightly above one'. But if, as seems likely, some two laminations of the ear-plug represent one 

 year (Laws and Purves, 1956; Nishiwaki, 1957; Purves and Mountford, 1959) then the female 

 humpback accumulates about 2-4 corpora on average during each 2-year breeding cycle (Symons and 

 Weston, 1958). That is to say, there is more than one ovulatory period. It has been shown above 

 that newly mature female fin whales, which might be expected to be less successful breeders than 

 multiparous females, have on average only 1-42 ovulations (range 1-4) before becoming pregnant. 



In the following sections the reproductive cycle of the fin whale is re-examined in order to find an 

 explanation of the regular rate of ovulation and it is concluded that the female fin whale is probably 

 not polyoestrous as has been assumed by all previous workers. A preliminary announcement of this 

 finding was made in 1956 (Laws, 1956a). 



9-2 



