BREEDING, GROWTH AND AGE 225 



physically mature and immature Fin whales shows that it is not impossible for a whale of 

 this length even to have reached full physical maturity when growth ceases. There must 

 at least be plenty of Fin whales which have been sexually mature for a year or more 

 without exceeding this length. We can therefore say that this whale almost certainly 

 grew from birth to sexual maturity in not more than three years and may quite well have 

 done so in two. Had it been possible to secure the ovaries this point might have been 

 settled. 



The white scars, referred to on p. 214, are formed when whales are in warmer regions 

 during the winter migration, and since they heal when a whale returns to the Antarctic 

 a fresh batch should be added each winter if a whale undertakes regular annual migra- 

 tions. Wheeler (1934, p. 363) shows that it is often possible to distinguish in young Fin 

 whales separate sets of scars which appear to have been superimposed on one another. 

 His analysis of the data suggests that if in fact these sets of scars are annual accretions, 

 then some Fin whales should reach sexual maturity two years after birth and some three 

 years after birth. He points out, however, that this inference rests on unsatisfactory 

 evidence. W T e do not know very much of the circumstances in which the scars are 

 formed, it is not certain that all young Fin whales undertake regular annual migrations, 

 and we cannot be quite certain that what appear to be several age series of scars are 

 really annual additions. 



The position is that while the estimate of two years as the normal period from birth 

 to sexual maturity is not proved, it is unlikely to be more than three years and may very 

 likely be sometimes two and sometimes three in both Blue and Fin whales. Matthews 

 (1937, 1938c) considers that Humpback and Sei whales begin breeding about two years 

 after birth. 



THE DETERMINATION OF AGE 



It is not proposed to discuss here in much detail the estimation of the ages of whales 

 because, as mentioned on p. 216, the war has left a considerable amount of unfinished 

 work on the subject. Something may be said, however, of the general basis on which 

 such estimations can be made. 



There are five anatomical features which are correlated in one way or another with a 

 whale's age : (i) length ; (ii) sexual maturity, as indicated by the condition of the repro- 

 ductive organs ; (iii) physical maturity, as indicated by the condition of the vertebral 

 epiphyses ; (iv) the number of corpora lutea in the ovaries ; (v) the number of old scars on 

 the surface of the body. 



Length is obviously an indication of age up to a point, but has no relation to age after 

 physical maturity is reached. 



Sexual and physical maturity are two successive stages in the development of the 

 individual and form extremely useful landmarks with which to correlate other evidence 

 on age. All individuals of a given species do not of course necessarily reach either of 

 these stages at exactly the same age, but there is an inherent probability that they do so 

 at about the same age. According to Marshall (1922, p. 714) domestic animals reach 



