FACTORS INFLUENCING RATE OF METABOLISM 39 



which are undergoing metamorphosis progressively in- 

 creases (Helff, 1923). Such changes in tadpoles are 

 associated with profound bodily modifications. The 

 stomach and intestine grow shorter through autotomy, 

 autolysis, and phagocytosis ; the food changes from vege- 

 tation to flesh. Of course other influences may be asso- 

 ciated with age in affecting the rate of metabolism — ^men 

 commonly have a higher rate than women; various 

 species of salamanders appear to show characteristic and 

 consistent differences (Helf, 1928). 



Temperature. — ^In general poikilotherms live faster 

 at higher temperatures and slower at lower tempera- 

 tures, while homoiotherms live continually at approxi- 

 mately the same rate, but expend more energy at lower 

 temperatures in order to furnish bodly heat. The meta- 

 bolic rate of a fish, amphibian or reptile is rather closely 

 associated with outside temperatures. The respiratory 

 exchange, which is a reliable indicator of the rate of 

 metabolism, in a turtle is approximately related directly 

 to the temperature of the surrounding medium (Hall, 

 1924). Van't Hoff's Law for the velocity of chemical 

 reactions in relation to temperature holds for the rate 

 of heart beat of salamander embryos (Laurens, 1914). 

 As the temperature of the water in which a fish lives 

 rises, the amount of nitrogen excreted steadily increases 

 (Knauthe, 1898). At temperatures below 6 to 9 deg. C. 

 adult freshwater fishes cease to eat altogether, but young 

 fishes continue to eat at lower temperatures than adults 

 (Haussman, 1897; Knauthe, 1898; Pearse, 1919; Hath- 

 away, 1927). However, there are species of fishes in 

 some localities which pass entire life cycles at or near 

 deg. C, and others which never know temperatures 

 below 45 deg. C. (Murray, 1914, 1914a). Particular 

 species may have peculiarities or have become adjusted 



