AND PERSONALITY 



expected a bull hippopotamus, had we collected one, to 

 have a smaller thyroid gland than a giraffe of approxi- 

 mately the same weight. On the other hand, rapid extrac- 

 tion of body heat by water environment requires a large 

 thyroid gland to maintain the body temperature. But 

 nursing a i,2OO-pound mtoto is a work phenomenon, and 

 since the thyroid gland of animals enlarges during preg- 

 nancy and with the production of milk, it was not surprising 

 to find the weight of the thyroid gland of the nursing hippo- 

 potamus much greater than that of the bull giraffe. 



Of great significance is the fact that the standard-bred 

 milk cows, such as the Jersey, the Holstein, and the 

 Guernsey, show a larger thyroid gland than adrenal glands. 



The adrenal glands of the hippopotamus weighed 61.10 

 grams, whereas the adrenal glands of the giraffe weighed 

 78.12 grams. The smaller adrenal gland of the hippo- 

 potamus expresses the lesser activity of the hippopotamus 

 in comparison with that of the giraffe. 



The fact that the heart of the giraffe weighed 4,990 

 grams and the heart of the hippopotamus 4,536 grams may 

 be ascribed to the greater amount of energy required by the 

 giraffe to maintain his weight against gravity in comparison 

 with the hippopotamus, whose weight is partly sustained 

 by the buoyancy of his part-water medium. 



In the tropics, the size of the energy-controlling organs, 

 the brain, and the thyroid gland, in particular, is limited 

 by the ceiling of heatstroke. 



Because warm-bloodedness increases the speed of chem- 

 ical activity, the warm-blooded animals have a great 

 advantage over the cold-blooded animals. Since heat 

 elimination is at the surface, the larger the animal the less 

 is its facility for heat loss. 



Through natural selection, as the warm-blooded animals 

 became larger, the size of their brains and thyroid glands, 

 relative to their body weights, became smaller. Therefore, 

 it is not surprising to find, in the case of the giraffe and the 



75 



