39 2 DISCOVERY REPORTS 



ness of the correlation is that, of 53 individuals with 14 or 15 corpora, 26 are physically immature 

 and 27 are mature. 



The mean number of corpora at physical maturity is now concluded to be 14-3, and is in close 

 agreement with the findings of Wheeler (1930) and Peters (1939), which were based on smaller samples. 



Comparisons between the number of corpora in young age groups 



and at physical maturity 

 In Text-fig. 27 the frequency distributions of corpora numbers at baleen group V (mean number 5-6) 

 and at the threshold of physical maturity (mean number 14-0) are illustrated. For comparison both 

 curves have been converted to percentage frequencies for there is a great discrepancy in the size of 

 the samples. The curves are based on 405 females in baleen group V and only 54 at the threshold of 

 physical maturity. They have been arranged so that the modal number of corpora (5 in the case of 

 baleen group V, and 14-3, the intersect, in the case of the group at physical maturity) corresponds to 

 o and the corpora numbers have been converted to values relative to the mode. It is then apparent 

 that the frequency distributions are very similar in shape, the agreement between the right-hand 

 parts of the curves being particularly close. The discrepancy between the left-hand slopes is largely 

 due to the truncation of the group V frequency curve as mentioned above (p. 386). The standard 

 deviations for the curves are 3-30 for group V and 3-94 at physical maturity, and they are in reasonably 

 close agreement. If the two extremes in the frequency at physical maturity (at 1 and 28 corpora) are 

 eliminated the standard deviation becomes 3.00. 



We may conclude that the range of variation in corpora numbers at baleen group V is similar to 

 that obtaining at the threshold of physical maturity. It has been suggested that baleen group V 

 includes several year classes and cannot, therefore, represent the range of variation at one specific 

 age, which must be less than this sample shows. Similarly, the group at physical maturity is unlikely 

 to represent a single year class, or even a single age class relative to sexual maturity, but it is not possible 

 to say over how many year classes it is spread and therefore impossible to estimate the probable 

 increase in variation by comparing these two samples. 



It would seem to be clear, however, from the small variance at the threshold of physical maturity 

 that there is little variation in the average number accumulated annually by individual whales. In the 

 simplest case linear growth is a decelerating process which begins before sexual maturity and ceases a 

 number of years later. The corpora accumulate annually and the range of variation in the number 

 present at the threshold of physical maturity depends first, on the individual variation in the age at 

 sexual maturity, secondly on the time taken to reach physical maturity and thirdly on the cumulative 

 variation in the annual production of corpora. In this case there can be very little individual variation 

 in the average annual rate of corpora production, because almost all the variation can be accounted for 

 by the age spread at puberty (see below, p. 407). This argument assumes that the accumulation of 

 corpora and ossification of the epiphyses are independent physiological processes. It should be pointed 

 out, however, that oestrogen production (by follicles, corpus luteum and placenta) is associated with 

 each ovulation and pregnancy. One of the biological actions of oestrogens is to delay the ossification of 

 epiphyses in mammals, so that the number of such cycles might well have a direct effect, independent 

 of chronological age, on the attainment of physical maturity. According to this hypothesis females 

 ovulating less frequently than the average would attain physical maturity at an earlier age than those 

 with a higher rate of ovulation, and the frequency distribution of corpora at the attainment of physical 

 maturity might be very similar to that obtained in the present study. If there is such a direct correla- 

 tion between rate of ovulation and age at maturity then the amount of variation at physical maturity 

 will nevertheless reflect the extent of annual variations between individuals. 



