THE AGE OF FEMALE BLUE WHALES 251 



necessary to assume that the constitution of the population from which the catch is 

 taken is consistent from year to year and that every year adults undergo roughly the 

 same experiences at the breeding season. 



The percentage of pregnant whales found in the Kerguelen Area, for which we have 

 results for three seasons, may be taken as a rough indication that the same general state 

 of affairs exists each year. The pregnant whales calculated as percentages of the resting 

 and pregnant groups together were : 



i93 2 -3 6i-o% 



1934-5 52-5% 



1935-6 58-4% 



It is not considered that the variation in these numbers is great enough to indicate that 

 the experience of the breeding stock differs substantially from year to year. We may 

 therefore be justified in applying the annual increment in corpora lutea suggested above 

 to an estimate of the relation between age and corpora lutea numbers. It must be 

 remembered, however, that the number of ova produced by an individual whale in the 

 first breeding season may vary considerably (probably from one to seven). It is therefore 

 very important to note that the age of any individual may not legitimately be determined 

 from the number of its corpora lutea, but given a large number of whales with the same 

 number of corpora lutea it should be possible to decide with some certainty the average 

 age of the group (p. 262, fig. 13). 



corpora lutea frequencies for two seasons. The collections of ovaries made in 

 1934-5 and 1935-6 were not of the same size: the first provided 547 pairs, the second 

 464. This is in both cases after deduction of the doubtful specimens mentioned above 

 (p. 245) and damaged ovaries, some of which became detached from each other and from 

 their labels. The results from the several ships have been combined according to the 

 regions in which the ships were working at the time when the ovaries were collected, 

 and appear in the table below. The immature whales are included here, but not in the 

 corpora lutea frequency graphs nor in the percentage estimate of pregnancy. 



The frequencies for the three areas in which the collections were made are shown 

 graphically in Fig. 8. It will be seen that there is little consistency of outline between 

 one area and another, that is to say maxima and minima do not fall to the same numbers 

 in different areas. It may be that the age distribution of whales varies substantially from 

 one area to another and that numbers of corpora lutea, indicating a rough age group, 

 which are common in one area are lacking in another. Another and more likely explana- 

 tion may be that the sample from each area is not big enough to eliminate chance 

 fluctuations. 



More consistent results are obtained by combining the frequencies from all the areas 

 for each year. In order to make comparison between the two years easier, the frequencies 

 are expressed as percentages of the total sample for the year and are shown graphically 

 in Fig. 9. The frequency of each corpora lutea number is shown as a discrete point. The 

 variation is so great that it has been thought best to make up a curve representing the 



