400 



WHALES 



Figure 220. The baleen of a Blue Whale, showing ridges and grooves. The variations in 

 thickness measured ivith special apparatus are reproduced on the graph. 



no measurable periodicity, this method is of no practical significance for 

 the moment. 



Still, one characteristic which shows gradual, periodic, changes is the 

 thickness of the baleen plates, in which ridges and grooves across the sur- 

 face can be made out with the naked eye (Fig. 220). In some baleen plates 

 the ridges are so regularly spaced that, as early as 1820, William Scoresby 

 suspected that the ridges might be akin to the annual rings found in the 

 horns of cattle, and in the scales and otoliths (ear-stones) of fish, in all of 

 which they are used for age determination. By simply opening a whale's 

 mouth we should therefore be able to tell its age. However, things are not 

 quite as simple as that, and it took Prof. Ruud and the Russian biologist 

 Tomilin, who worked independently, until 1940 to perfect an instrument 

 for measuring the thickness of baleen plates accurately, and the Japanese 

 biologist Hirata until 1959 to perfect a photographic method. 



We saw earlier that whalebone is formed inside the gum, from which it 

 is continually replenished to compensate for frictional effects. In other 

 words, the oldest whalebone is always on the top, and the whole baleen 

 plate consists of material formed over a limited number of years. In a 

 seven-year-old whale, for instance, probably nothing of the whalebone 

 formed during the first year of its life has remained, and the top of the 

 baleen plate consists of material formed during its second or third year. 

 On p. 265, we saw that the cornified tubules which constitute the inner- 

 most part of the baleen plate are of uniform diameter throughout, so that 

 the thickening of the baleen plate towards the gum rests exclusively on 

 increases of thickness of the cortical layer. (Baleen plates are naturally 



