FACTORS INFLUENCING RATE OF METABOLISM 45 



pituitrin which exert a profound influence on metabolic 

 activity and growth. There is at present no evidence 

 that poikilotherms differ essentially in their digestive 

 and assimilative processes from homoiotherms. They 

 are susceptible to injury by lack of vitamins, monotonous 

 diets, and other dietary deficiencies in the same way that 

 homoiotherms are (Pearse, Lepkovsky, and Hintze, 

 1926). They are affected in much the same way by thy- 

 roid extracts and those of other ductless glands (Helff, 

 1923). 



Starvation. — Among vertebrates there are many 

 species of fishes, amphibians, reptiles and mammals 

 which regularly pass long periods without food during 

 aestivation or hibernation, and there are many which 

 can endure long fasts. Both these types of quiescence 

 are associated with changes in metabolic rate. An eel 

 has been kept without food for 657 days. During this . 

 time the body cells decreased 10 per cent in size, but their 

 number remained approximately unchanged (D'Ancona, 

 1928). Two species of centrarchid fishes were kept in 

 filtered water at about 20 deg. C. for two months without 

 food (Pearse, 1925). Turtles (CJirysemys) lived for 

 nearly a year under the same conditions (Pearse, Lep- 

 kovsky, and Hintze, 1926). Some birds when starved, 

 lived for a month. Fasting dogs have lived for 96 to 117 

 days, and men have survived for over seventy days. 

 Unfortunately, no careful records have been kept of the 

 temperatures at which fasting poikilotherms have been 

 kept and hence no direct comparison with homoiotherms 

 is possible. The former probably can usually endure 

 longer periods without food because their metabolic rate 

 is lower at low temperatures. 



Fasting animals, of course, lose weight. In lobsters 

 the organic body constituents fell to one-half their origi- 



