370 



Comparative Animal Physiology 



ture and increased their metabolism when cooled below zero, while others 

 kept for 3 months at 30 to 32° regulated less well, and when the air tempera- 

 ture dropped to 15° the increase in metabolism was insufficient to maintain 

 body temperature.**" Despite the physiological adaptation of individuals, it is 

 unlikely that tropical mammals could be adapted to Hve at arctic temperatures. 

 The genetic and physiological limits of temperature tolerance have not been 

 clearly defined in any species. 



A basic condition for homoiothermism is high basal metabolic rate. Heat 

 production can be measured by direct calorimetry or may be calculated from 

 CO2 production and O2 consumption. In man, with an R.Q. of 0.82, heat 

 is produced to the extent of 4.8 calories per liter of oxygen consumed. ^^ In 

 general, at a given body temperature, the metabolic rate is higher for small 

 animals of a kind than for large animals, although metabolic rate is not a 

 simple function of size (see pp. 225-228, Ch. 8). 



A small poikilotherm (e.g., frog or fish) has a metabohsm at 16° equal to 



nAnn/M5 



MtTAMUSn 

 MSAfM 



\ -U} 

 LOWf VT TP 



•n lAMM 



-30 -20 -10 O 10 



AIR TCnP[RATi;RF IN CEnri6RA0C 



Fig. 98. Metabolism of arctic and tropical animals in resting state as a function of air 

 temperature. Solid lines represent average measurements, broken lines are extrapolations. 

 From Scholander (unpublished). 



about 5 per cent of the heat production of a homoiotherm of similar weight 

 at 38°. A 20 gm. mouse at 37° produces 150 cal./kg., a 20 gm. canary produces 

 250 cal./kg., and a 20 gm. frog calculated to 37° produces about 30 cal./kg. 

 Measurements"' of heat production in a poikilotherm, in one good temperature 

 regulator, and in one poor regulator, of identical weight and at the same tem- 

 perature (2.5 kg. and 37°), are as follows: 



Rattlesnake 19.35 cal./kg./24 hours 



W(K)dchuck (Marmota) 72 " " 

 Rabbit 112 " " 



There has been much discussion as to whether weight, body surface area, 

 total nitrogen, or some other measure of size is the better correlated with 

 metabolism (Ch. 8). Certainly in homoiotherms the body surface is important 

 because for a given degree of insulation the total heat loss for a given amount 

 of heat production depends on the area over which a thermal gradient exists. 

 The basal metabolic rate is higher in individual mammals adapted to a cold 

 environment than in warm-adapted individuals (man, rat). 



