AGE-DETERMINATION BY MEANS OF THE OVARIAN CORPORA 467 



lamination it seemed that the growth of the epidermis is arrested at regular intervals. It was assumed 

 that the period of arrested growth corresponds to the migratory periods (when the physiological 

 demands of active swimming coincide with complete absence of food), so that each lamination repre- 

 sents a growth period of approximately six months. Later Purves and Mountford (1959, p. 137) stated 

 that ' It is very doubtful whether environmental conditions have any direct influence on the time of 

 formation of the laminae of the ear plug in whales, although nutritional conditions may to some extent 

 determine their thickness. Since there is strong evidence [not given by them] that lamina formation 

 is an inherent moulting cycle it is very probable that it takes place whether the whale migrates or not. 

 If the rate of moulting can be established the ear plug would probably be a more accurate age indicator 

 than the fish scale.' 



Comparison of ages estimated in this way from the ear-plug with the corresponding baleen plates 

 for a small sample of North Atlantic fin whales (Laws and Purves, 1956), suggested that the hypothesis 



EAR -PLUG AGE (YRS.) 



Text-fig. 55. Comparison of the ages of individual fin whales estimated from baleen plates and ear-plugs 



(white, North Atlantic; black, Antarctic). 



of a biannual formation of laminations was correct. This agreement between baleen plates and ear- 

 plugs was partly confirmed with a small additional sample of antarctic fin whale material collected 

 in 1955-56. The ear-plug ages of this sample (estimated on the basis of two laminations per year) are 

 given by Purves and Mountford (1959) to whom the material was made available. Professor J. T. 

 Ruud and Mr Age Jonsgard (Statens Institutt for Hvalforskning, Oslo) kindly undertook the examina- 

 tion of the baleen plates. In Text-fig. 55 the ear-plug ages are plotted against the ages estimated from 

 the baleen plates. If the age-determinations by these two methods are in exact agreement then the 

 points should fall on the 45 line which is shown. It is apparent that there is fairly close though not 

 exact agreement between these two methods for the first 4-5 years, after which baleen plate ages 

 (owing to wear at the tip of the plates) are supposed to be minimum values. Chittleborough (1959) 

 finds similar agreement between baleen plates and ear-plugs of humpback whales. 



Purves (1958) and Purves and Mountford (1959) used the assumption that about 1-4 corpora are 

 accumulated annually in the ovaries of fin whales (Laws, 1956a) to confirm the assumed biannual 

 rate of lamina formation in sexually mature females. Some 14-15 corpora accumulate in the ovaries 

 between puberty and physical maturity, and this period is, therefore, estimated to be 10 years (but 

 see p. 388, where attention is drawn to an error in this work). About 12 laminations were found at 



