454 



DISCOVERY REPORTS 



Variation of fertility with age 

 It has been shown that the female fin whale experiences a seasonally monoestrous sexual cycle. If this 

 is true then variations in the rate of production of ova will be mainly related to the incidence of 

 multiple ovulations, or to the percentage of females ovulating at each ovulatory period, rather than 

 to variations in the number of dioestrous cycles in each ovulatory period. 



Multiple ovulations 

 The varying incidence of multiple ovulations with age in the present material is shown in Table 26. 

 For different numbers of corpora albicantia are shown the numbers of females with more than one 

 corpus luteum in the ovaries. The incidence of multiple ovulations appears to increase with increasing 

 numbers of corpora albicantia (and therefore with increasing age), but there is a significant difference 

 only between the first two groups. It seems probable that there is a real increase in the occurrence of 

 multiple ovulations, because Kimura (1957, fig. 10) shows, for a much larger sample, that the frequency 

 of dizygotic twins similarly increases with age from about o-i % (at 1-5 corpora) to 6-25% (at 30-35 

 corpora). The average incidence of multiple ovulations in the present material is 2-i6±o-82%. So 

 far we have not considered the variations in actual numbers of ovulations in cases of multiple ovulation. 

 In different cases of multiple ovulations the number of corpora lutea produced varies from 2 to 13 

 (in the present material). There are 22 females with 2 corpora lutea, 3 with 3 corpora lutea, 1 with 

 4 corpora lutea and 1 with 13 corpora lutea. 



Table 26. 



No. of 



corpora 



albicantia 



0-4 



5-9 

 10-14 



15-19 



20-24 

 25-29 

 30-34 

 35-39 

 40-44 



45-49 

 5°-54 

 Total 



Variation in incidence of multiple ovulations with age 



(number of corpora albicantia) 



Total 



ovaries with 



corpora lutea 



490 

 281 

 178 

 128 

 76 



49 

 29 



15 

 3 

 2 



Multiple 

 ovulations 



1 

 7 

 5 

 6 



5 



1 



Percentage 



multiple 



ovulations 



0-20 

 249 

 2-81 

 4-69 

 6-58 



6-oo 



2 S.E. 



0-40 

 i-86 

 2-47 



374 



57° 



672 



1252 



27 



2-16 



0-82 



Table 27. Details of multiple ovulation in which 13 ova were shed and 5 foetuses developed 



Ovary 1 



Ovary 11 



