468 DISCOVERY REPORTS 



sexual maturity and about 32 at physical maturity, corresponding to an incremental rate of approxi- 

 mately two a year. It was also found that the mean curve of growth in body length gave lengths at the 

 1 2th and 32nd laminations which were close to the previously estimated lengths at sexual maturity 

 and physical maturity. 



So far the best evidence for the biannual formation of ear-plug laminations, therefore, comes from 

 this check against the ovaries, using the figure of 1-4 for the annual increment of corpora. The nature 

 of the fin-whale sexual cycle, particularly of the female, which was discussed earlier in this paper 

 suggests additional strong circumstantial evidence in favour of a biannual cycle of lamina formation. 

 It also suggests a possible exteroceptive factor responsible for the regular formation of the laminations. 

 Owing to the migratory cycle, most fin whales are usually subjected each year to two periods of 

 increasing day lengths. In the female these are associated with oestrus (incompletely suppressed during 

 pregnancy). In the male it appears that the period of increasing day lengths associated with the spring 

 migration does not initiate a second period of sexual activity, because, it was suggested, of the long 

 period of spermatogenesis which precedes this migration and is followed by a refractory period. This 

 is paralleled by the suppression of oestrus in pregnant females. 



One of the most characteristic features of oestrus is the cornification of the vaginal and other 

 epithelia. It seems possible that the growth cycle of the ear-plug is in fact related to the migratory 

 cycle as was initially assumed, not because of arrested growth associated with the expenditure of 

 energy on migration, but because of a biannual hormonal cycle associated with and regulated by 

 varying day lengths. According to this hypothesis there is during each migration a rise in the amounts 

 of circulating oestrogens in both male and female, which results in the formation of a keratinized layer. 



This hypothesis explains lamina formation in adults satisfactorily, but little is known of the migra- 

 tions of immature fin whales and the early laminations are the most difficult to interpret. This inter- 

 pretation for adults is to some degree independent of the estimated figure for the annual increment 

 of corpora, so that the ear-plug age estimates are at least partly independent of the ovaries. 



Purves and Mountford (1959) give the ages, estimated from ear-plugs, of samples of fin whales, 

 and the ovaries of the females in these samples have been examined by the present author. Individual 

 ages have been estimated from the corpora numbers in the way described above (p. 466). In Text- 

 fig. 56 the ages estimated from ear-plugs are plotted against the ages estimated from the ovaries for 

 in individuals. The 45 ° line shown corresponds to an exact agreement between the two methods. 



It is expected that, owing to individual errors in the estimates based on corpora numbers (related 

 in the main to the variation in the age at puberty), the majority of the ovarian ages should lie within 

 ± 3 years of the regression line representing exact agreement. It is assumed that the ages estimated 

 from the ear-plug laminations are accurate, although some plugs are difficult to read and may give 

 inaccurate results. Also the interpretation of immature laminations is doubtful. Purves and Mount- 

 ford (1959) estimate the maximum error by this method to be ±2 years. In Text-fig. 57, the variation 

 of ovary ages about the theoretical mean regression line is shown. This curve was constructed by 

 drawing in other oblique lines parallel to the original regression line, displaced laterally at yearly 

 intervals. The total number of points lying within each pair of lines was obtained and plotted to show 

 the frequency distribution about the assumed mean. The results are near to expectation; 72% of the 

 estimated ovarian ages lie within ±3 years, and 82% within ±4 years of the regression line showing 

 an exact correlation, and the extreme range is ±12 years. Only seven values lie outside the range 

 ±6 years and it is probable that the five extreme values represent combined errors of the two methods. 

 The results are in close agreement with the distribution of corpora at physical maturity, shown in 

 Text-fig. 27. It should be noted that there is a group of points in Text-fig. 56, for which the ovarian 

 ages are too high. This is because 5 years is taken as the age at puberty, and some nine of these records 



