AGE EVIDENCE FROM CORPORA LUTEA 413 



one old corpus luteum in the ovaries, immature mammary glands, and an unbroken 

 vaginal band (No. 3070). A similar whale was reported in 1925. 



Fin and Blue whales are thus polyoestrous animals in which ovulation is spon- 

 taneous, one ovum is discharged at a time, and all ovulations leave a permanent record 

 in the ovaries. The question now arises whether something cannot be done towards 

 the estimation of the actual age from the number of corpora lutea. 



It has been noticed this season, as in previous seasons, that certain numbers of corpora 

 lutea appear to be much more frequent than other numbers. An explanation of this 

 fact was attempted in 1929, and the theory of age, fully explained below, was briefly 

 outlined at the same time (p. 450). 



In polyoestrous animals with a definite sexual season, the number of ovulations in 

 any one season depends upon the occurrence of pregnancy within the limits imposed 

 by the season itself. In social animals like whales, that live in communities or schools 

 and undertake extensive migrations more or less simultaneously at certain times, one 

 would expect to find, at least for one or two seasons after sexual maturity, that the 

 experience of all had been somewhat similar, and that circumstances or conditions that 

 had affected one had affected others in the same way. This certainly applies to the course 

 of migration. There would be no "Fin years" or "Blue years" at South Georgia 

 (seasons when one species appears in great numbers while the other is practically 

 absent) if conditions aff'ecting migration did not affect the whole herd. 



Take then, for example, a group of whales in their first sexual season. At the close 

 of the season the minimum number of corpora lutea will be one (corpus luteum of 

 pregnancy) and the whale will be pregnant, the maximum will represent the number of 

 dioestrous cycles that are possible during the season. If, in the majority of these whales, 

 fertilization of the ovum for some reason does not occur for one or more oestrous 

 periods, there will be a majority of whales having the same number of corpora lutea 

 in the ovaries; that is, there will be traces of a certain number of ovulations and a 

 corpus luteum of pregnancy. In the following season these whales will be lactating, 

 while a fresh group is becoming mature. This fresh group may undergo the same 

 experience as the previous whales or it may not, but the number of corpora lutea cannot 

 exceed the maximum which is limited by the season. 



As gestation and lactation occupy most of two years, it is during their third season 

 that the first whales again experience a sexual season and the number of corpora lutea 

 again increases. The second group is now lactating, so the number of corpora lutea 

 remains stationary, and a third group is verging upon maturity. In the following year 

 the first group is again lactating, the second ovulating, the third lactating, and a fourth 

 is becoming mature. 



Thus, in representative samples from one area, the frequency of the numbers of 

 corpora lutea will show a succession of nodes or peaks marking the increase in numbers 

 every two years. 



The graph of the frequency of corpora lutea for season 1929-30 at South Georgia is 

 shown in Fig. 3. Peaks at one, seven, and eleven are clear, and there are indications 



