226 



Comparative Animal Physiology 



to size (Fig. 45, and Table 38).-^*- -" The high oxygen uptake of the long- 

 tailed shrew, 13.7 cc./gm./hr., is startling; it may be noted that the food intake 

 too is enormous— something like its own weight in food every 24 hours! As 

 noted recently by Pearson the asymptotic increase in metabolic rate with 

 decreasing size places the lower limit of size for mammals at about 2.5 gm.; a 

 smaller mammal would be unable to obtain adequate food for its infinitely 

 rapid metabolism. The metabolic values for these small mammals exceed the 

 oxygen consumption of Protozoa; assuming a specific gravity of about 1, Para- 

 mecium respiration is 6.0 cc./gm./hr.,^*'^ and that of Tetrahymena geleii is 

 10-12 cc./gm./hr. at 25° C.27« 



The rate of oxygen consumption is correlated with body weight as an 

 exponential function and tends to follow the increase in surface area rather 

 than weight per se. For instance, in fish (pike and tench), oxygen uptake 



512- 



N.n- 



8- 



2 6- 



o 4- 



o 2- 



Matked Shr««r 



Harvest Mouse 



Houf* Mouse 



_Shorf-t»iled shrew 



Rhoads' 

 Capper's Vole . P ine Mo use ♦* 



■ 



Deer Mouse' White 



Mouse 



J I L 



J I I I I I I I I I I I L__l I I I I I L 



10 12 



WEJGHT 



IN 



16 18 



ORAMS 



20 



22 



24 



26 



Fig. 45. Oxygen consumption of small mammals plotted as a function of body weight, 

 indicating the surface area rule within this group. From Pearson."^ 



seems to increase in direct proportion to the surface area, i.e., cc.02=kW^-^^, 

 where W is the body weight, and fe is a constant for the organism.^^* Most 

 determinations yield an exponent in excess of that indicated by the "two- 

 thirds" rule. On a series of rodents Morrison'-'^^ found good agreement with an 

 exponent of 0.73 (adopted by the Conference on Energy Metabolism, 1935) 

 relating weight to basal metabolism, and a slightly higher value of 0.798 was 

 obtained for the kelp crab, Pugettia producta, by Weymouth and his co- 

 workers'^"^ (Fig. 46). Results on a number of different species of Crustacea 

 indicate the same general correlation between body surface and oxygen con- 

 sumption. Failing to establish direct correlation with surface area and body 

 weight in mammals, Brody'^*' relates basal metabolism to "metabolically effec- 

 tive body weight." 



The general relationship of decreasing metabolic rate with increase in body 

 size holds for small marine organisms, although those weighing less than 1 

 gram frequently do not obey the surface law.^**'* Zeuthen's thorough-going 

 study on the marine microfauna indicates that comparison of organisms on a 

 basis of respiration per nitrogen content reveals considerable agreement among 



