THE AGE OF FEMALE BLUE WHALES 237 



In Fig. 5 are shown the numbers of corpora lutea in the ovaries of whales of stated 

 lengths, and the distinction between physically immature and mature whales is made. 

 The result is very striking and informative. It will be seen that nearly every whale 

 having more than eleven corpora lutea is physically mature, while nearly every whale 

 having less than eleven is physically immature. If we may assume that physical maturity 

 is reached always at a definite age, we have here not only strong confirmation of the 

 theory of the persistence and accumulation of the corpora lutea but also strong evidence 

 that the number of corpora lutea is an absolute criterion of the age of a Blue whale. If 

 the relation between age and number of corpora were a loose one, one would not expect 

 to find such a sharp correlation between the numbers and such an important event in the 

 whale's life as the attainment of physical maturity. With one exception physical maturity 

 does not occur in female Blue whales under 86 ft. in length. At the same time there is 

 shown to be a great diversity in the lengths to which whales may grow before complete 

 ankylosis with the implied cessation of growth is attained. This is a good opportunity 

 to emphasize for Blue whales the unreliability of length measurements as a guide to age 

 (except in young whales) which Wheeler pointed out for Fin whales (p. 409). The 

 probability is that maturity comes at the same approximate age in all female Blue 

 whales, but at what great divergences of length has been seen in Fig. 6. Whether or not 

 this divergence is due to the varying experience of each whale in numbers of young 

 produced, the exertion of which might easily upset the growth rate, it is impossible to 

 say. It is interesting to note that a Blue whale can attain to considerable length (90 ft.) 

 and to physical maturity with but five corpora lutea to its credit, as one instance shows. 

 Such a situation can perhaps only exist if the whale became pregnant at an early 

 ovulation in each breeding season, thus reducing the number of ovulations to the 

 minimum for its age. This may be an exceptional whale, but its interest lies in the 

 probability that several pregnancies, each following the last as quickly as was practicable, 

 have not interfered with growth, since the whale grew to a length well above the average 

 before reaching physical maturity. 



Another form of graphical treatment (Fig. 6) contrasts in a different way the lengths 

 of, and the numbers of corpora lutea in, physically mature and immature whales. This 

 graph shows the average number of corpora lutea exhibited by each length class of whale, 

 lengths being taken at intervals of 1 ft. The averages show that up to 85 ft. there is a 

 fairly steady increase in numbers of corpora lutea with increasing length, and, taking the 

 means of adequate samples only, we can infer that length up to 85 ft. is a rough measure 

 of age. But at the length at which collectively growth slows down, i.e. 85 ft., there is 

 bound to be a sudden disproportionate increase in the numbers of corpora lutea. 

 Subdivision of the data as shown in the figure indicates how many more corpora lutea 

 are exhibited by the physically mature component of each length class over 85 ft. than 

 by the physically immature component. 



It is probably unsafe to try to make any further deductions from this material. To sum 

 up, the position is that from the small amount of material at hand a correlation between 

 the number of corpora lutea and the attainment of physical maturity has been established, 



