GENERAL CONSIDERATIONS 335 



believing the females to have a polyoestrous cycle. The abundance of ripening graafian follicles is 

 suggestive ; the protracted breeding season allows time for several dioestrous cycles ; and the fact that 

 some whales, with more than 30 corpora albicantia, must be more than 30 years old if monoestrous 

 was thought to be important. 



If the female is polyoestrous and individual females become pregnant at 2-yearly intervals, then 

 up to, say, six ovulations may be possible in one season. Because female whales are gregarious it could 

 be expected that their experience would be similar and each year a certain number of ovulations 

 would tend to occur more commonly than others. When they plotted the frequencies of corpora in 

 their material they found peaks occurring at 4-5, 12, and 19 corpora and suggested that these repre- 

 sented the increase in numbers of corpora at intervals of 2 years. Wheeler (1930) developed and 

 modified this hypothesis. He examined the state of fusion to the centra of the vertebral epiphyses 

 and by this criterion classed individuals as physically mature or immature. With less than 15 corpora 

 females were almost invariably physically immature, and with more than this number, physically 

 mature. The frequency distribution of corpora in his larger material shows maxima at 1,7, 11, 18 and 

 21 corpora. Three of these peaks precede physical maturity. He pointed out that in the first group 

 the greatest number of corpora is at the beginning, which meant that the first ovulation was usually 

 followed by pregnancy, whereas in subsequent seasons unsuccessful ovulations were thought to 

 precede pregnancy. The importance of this work lies in the fact that it established a close relation 

 between the attainment of physical maturity and the rate of accumulation of corpora. The fact that 

 there is so little variation in the number of corpora accumulated at physical maturity implies that the 

 rate of accumulation is very regular and/or very small. Wheeler's work suggested that there were on 

 average three breeding seasons before the attainment of physical maturity so that about five ovulations 

 occur each breeding cycle, or on average 2-5 per year. 



In a later paper Wheeler (1934) applied this method of age-determination to 472 mature females 

 for which records were available and calculated average mortality rates. We know now that the peaks 

 in the frequency distribution of corpora in his material are not significant, but his figures demonstrate 

 that the decline in numbers of fin whales of increasing ages (as shown by corpora numbers) in his 

 sample, is exponential. Brinkmann (1948) published the results of investigations in 1939/40 based 

 on records of 918 female fin whales. His work confirmed the earlier estimates of size at sexual and 

 physical maturity, but he concluded that in the fin whale 13 corpora have been accumulated on the 

 attainment of physical maturity. This discrepancy between Brinkmann's material and that of Wheeler 

 (1930) and Peters (1939) who respectively found 15 and 14-15 corpora at physical maturity is readily 

 explained by differing criteria of physical maturity, for this is rather a subjective observation. More 

 recently, Japanese workers have obtained an even lower figure of 11-5 (Nishiwaki, 1950a, 1952). 

 Brinkmann discussed ' corpus luteum accumulation as a clue to age determination '. He found similari- 

 ties between the frequency distribution of corpora in his material and Wheeler's (1930), but did not 

 come to any definite conclusions. 



Laurie (1937) found a similar correlation in blue whales between the accumulation of 1 1-12 corpora 

 and the attainment of physical maturity. He found no regular occurrence of maxima in his frequency 

 distributions, but believed that comparison of certain features in the frequency curves for two suc- 

 cessive years indicated an increment of slightly more than one corpus each year. By consideration of 

 the fresh appearance of certain corpora he concluded that on average 1-13 were formed each breeding 

 season, but then states that this is the annual increment. If ovulatory periods recur at 2-yearly intervals, 

 then by this argument the annual increment should only be 0-57 corpora. 



Ruud (1940, 1945) developed a new method of estimating the age of whalebone whales, based on 

 the ridges present on the baleen. For 14 northern hemisphere fin whales he compared the number of 



