402 WHALES 



Clark showed in 1938. lilting of finger-nails stimulates their growth 

 considerably. 



Now, in Mysticetes, and particularly in migrating Rorquals, annual 

 metabolic and frictional changes are only to be expected. In cold w^ater, 

 where food is plentiful, large fat reserves are stored up and the whalebone 

 is exposed to maximum friction, while in the tropics where food is scarcer, 

 the whalebone is used to a much lesser extent, and the animal growls thin. 

 Moreover, the annual return to the Antarctic taxes the strength of the 

 already underfed animals to straining point, with further metabolic 

 changes. Other contributory periodic causes are birth, lactation, weaning, 

 mating, etc. 



Prof. Ruud, who has been investigating baleen plates for niany years, 

 believes that, in the thickness of the plates, there occur different levels 

 which are associated with annual alternations of a 'storing metabolism' 

 in cold waters and a 'consumptive metabolism' in warm waters. Every 

 level (or period) would then represent a year in the whale's life, but, 

 because of friction, only six to seven years will be represented on the whale- 

 bone at one time. Prof Ruud also thinks that we can tell from the angle 

 between individual horn pipes (they converge to the whalebone 'top' 

 which is formed in the foetus) whether the original whalebone is still 

 present, or else how many years have been 'rubbed off'. However, more 

 recent investigations of Fin Whales on the part of Mrs C. van Utrecht- 

 Cock have cast some doubt on the correctness of Prof Ruud's interpreta- 

 tion of the so-called annual levels. She believes that the characteristic 

 recurrence of certain crests and troughs across the whalebone ridges is a 

 far better indication for dividing the baleen curves into periods, each 

 typical group of tops representing a certain event in every season. In order 

 to deduce even the most recent events in the history of the whalebone 

 (which can be checked by the ovaries, by the age of a calf, etc.), investiga- 

 tions have included that part of the baleen which is found inside the gum. 

 This has enabled Mrs van Utrecht to distinguish special tops which, prob- 

 ably, are correlated with ovulations. The more or less regular occurrence of 

 these ovulation tops in the curves of the baleen plates confirms the con- 

 conclusion of Laws that the Fin Whale shows an average ovulation rate 

 of 2-8 per two years. They point also to an average age of sexual maturity 

 of six years. Further researches on the baleen plates showed that Prof. 

 Ruud's age estimates are on the low side, and moreover that the age 

 determinations are only reliable up to an age of four years (i.e. only in 

 immature animals). For older animals it is better to rely on the corpora 

 albicantia or on a new method, first suggested by Purves, and subsequently 

 applied by him and by Mountford to mature and immature whales. 



This most recent and most promising method is based on investigations 



