THE METABOLIC ACTIVITY OF VERTEBRATES 



69 



However, the loss of heat in warm-blooded vertebrates can 

 occur from areas other than the external body surface, e.g., the 

 lungs, so that if it could be shown that : 



The surface area for heat loss oc (body weight) ^-^^j 



then it would be valid to accept the surface area law. 



The metabolic rate of small birds and mammals (fig. 19) 

 rises very steeply with decreasing size. It would be extremely 

 difficult for a mammal weighing less than 3-5 grams to obtain 

 sufficient food to maintain its activity. Furthermore, the strain 

 on the circulatory system in such animals becomes extremely 

 acute and it is notable that this increase in metabolic rate is 



c 

 o 



^£ 



§E 

 u en 





1 MASKED SHREW 



HARVEST MOUSE 



HOUSE MOUSE 

 ■ ' .1.1 



SHORT-TAILED SHREW 



RHOADS VOLE 

 |-^ 



DEER MOUSE "^WHITE MOUSE 

 I I I I I I I 



2 A 6 8 10 12 14 16 18 20 22 2A 26 

 Weight in grams 



Fig. 19. 



Graph showing the relationship between body weight and meta- 

 bolic rate in some small mammals (After Pearson, 1957). 



correlated with an increase in the percentage heart weight and 

 a pulse rate which, in small birds, may exceed 1000/minute. The 

 amount of work done by the heart in pumping sufficient blood 

 round the circulatory system in unit time becomes considerable. 

 One factor affecting the lower limit to size of homoiotherms 

 must undoubtedly be the difficulty of keeping up adequate 

 supplies of oxygen to the heart at these extremely high metabolic 

 rates. 



