304 WHALES 



relax a little more, and it is therefore not surprising that most reports of 

 sleeping whales come from lower latitudes. While no one has ever been 

 able to measure the actual metabolic rate of Cetaceans - to do so involves 

 using complicated instruments, and under water the difficulties are even 

 greater - we may get some idea of its extent if we are told that Bottlenose 

 Dolphins in New York Aquarium consumed about i8f lb. of herring a 

 day, e.g. 237-5 calories per pound of body weight, while an average 

 working man needs only 116-5 calories. Luckily, most Cetaceans eat food 

 with a high calorific value so that the actual weight of food they have to 

 consume is not so striking. 



We might have guessed that Cetaceans would have a high metabolic 

 rate from the size of their brain alone (see Chapter 9). Other organs, too, 

 and particularly their thyroid, suggest very much the same. The extremely 

 complicated processes which keep an animal alive and allow its body to 

 function are controlled, not only by the central nervous system, but also 

 by a number of endocrine glands which pour their secretions directly into 

 the blood stream, in contradistinction to other glands (e.g. the sweat 

 glands) whose products are excreted. The thyroid is one of these endocrine 

 glands, and plays a particularly important role in regulating the metabolic 

 rate. We shall therefore examine it in some detail. 



In Cetacean foetuses the thyroid has roughly the same shield-shape as 

 in man, but in adult specimens the gland forms two big lobes, on either 

 side of the trachea close behind the larynx, which are joined by a slender 

 bridge. The gland itself, about foru'teen inches long in big whales and dark 

 red, is externally divided into small lobes. Its weight varies between 

 2| and 9 lb., and in adult porpoises between half an ounce and an ounce. 

 These figures, and also the information that the thyroid of Rorquals, 

 dolphins and porpoises represents o -01 per cent, 0-02 per cent and 0-05 

 per cent of their body weight respectively, tell us very little in themselves, 

 since surface area and consequently loss of heat per pound of body weight 

 clearly decrease with increase in body size, thus causing a decrease in 

 metabolism and thyroid weight. However, Crile and Quiring (1940) 

 compared the weights of the thyroids of a Bekiga and a race-horse, two 

 animals with an identical weight of i ,048 lb., and found that while the 

 former weighed 3^ ozs. the latter weighed only i^ oz. This agrees with the 

 fact that terrestrial mammals the size of porpoises have a thyroid-to-body 

 ratio of about 0-015 per cent as against the porpoise's 0-05 per cent. 

 Comparing the thyroids of a large number of mammals, Crile and Quiring 

 discovered that particularly large thyroids were found only in aquatic 

 mammals and in terrestrial mammals which live in the Arctic, i.e. in 

 animals which lose an inordinate amoiuit of heat and whose metabolism 



