68 VERTEBRATE RESPIRATION 



about the relative oxygen consumption of different poikilo- 

 thermic groups because it depends a great deal on the mode of 

 life of the individual species. This is also true among warm- 

 blooded vertebrates but when the metabolic rates of members 

 of the same class are related to their size some interesting features 

 emerge. 



Table 11 



Comparative metabolic rates of a reptile and mammals of 



similar weights (70 grams) at 37° C. (Data of Dawson 



and Bartholomew, 1956) 



In a given group the oxygen consumption/unit weight in- 

 creases inversely with the body weight. In homoiotherms this was 

 thought to be due to the relatively greater heat loss from smaller 

 animals. The latter have a relatively larger surface from which 

 heat may be lost and might therefore be expected to consume 

 more oxygen in maintaining a constant body temperature. This 

 has given rise to the so-called surface area law which suggests 

 that the metabolic rate is proportional to the surface area of the 

 animal. Over a limited range this appears to be true, e.g., 

 animals such as the hen (2 kg.), dog (15 kg.), man (65 kg.), and 

 pig (1 30 kg.) all have metabolic rates of about 9 ccs Og/sq. metre/ 

 hour. But such a relationship does not hold for the entire size 

 range of vertebrates. The formula best fitting all the data is 

 given by the relationship: 



Heat production or metabolic rate oc (body weight) ^''^^. 



It is generally believed that : 



Surface area oc (body weight) °-^^. 



