The Metabolic Activity of Vertebrates 



The amount of oxygen required by animals varies a great deal 

 but certain general patterns are recognisable. A major difficulty 

 in comparing the metabolic rate of different species is that they 

 have been determined under widely varying conditions of size, 

 temperature, activity, and acclimation (see p. 72), to enumerate 

 but a few of the factors involved. Animals vary in the effect of 

 these factors upon them, to say nothing of their reaction to 

 experimental conditions. Among vertebrates the presence of 

 groups (homoiotherms) which maintain a more or less constant 

 body temperature adds a further complicating factor. Neverthe- 

 less the metabolic rates obtained for a wide variety of animals 

 by different methods give a general picture which is surprisingly 

 constant. Unless it is stated otherwise, the figures used in the 

 following sections refer to the resting or *standard' metaboHsm, 

 i.e., the oxygen consumption measured for minimal activity. 



(a) BODY SIZE AND METABOLISM 



In general, birds and mammals have higher metabolic rates 

 than fish, amphibians and reptiles. This is partly because of the 

 higher body temperature of the homoiothermic groups but, even 

 when animals of the same size range are compared at the same 

 temperature, birds and mammals continue to show the highest 

 oxygen consumption. Thus the figures given in Table 1 1 for the 

 metabolic rates of reptiles and mammals of similar sizes at 

 37° C. show that mammals respire at about seven times the 

 rate of reptiles. Similar conclusions are reached when com- 

 parisons are made between mammals under hypothermia and 

 reptiles at the same temperature. It is difficult to generalise 



