RESPONSES OF POIKILOTHERMS TO VARIATIONS 19 



The intrinsic factors are those that lie within the organ- 

 ism and act to produce a temperature different from that 

 of the environment. The extrinsic factors are those 

 imposed on the organism from without. These will be 

 discussed first. Heat may be gained or lost by (1) con- 

 duction and convection and by (2) radiation. 



Conduction. — Conduction means the loss of heat from 

 a body at a higher temperature to one at a lower tempera- 

 ture by passage from particle to particle. For example, 

 if one end of a copper rod is placed into a dish of hot 

 water, heat will pass into the rod and along it until the 

 other end becomes hot. This is molecular transference 

 of energy. The amount of heat gained or lost by a 

 poikilothermal animal through conduction will depend 

 upon several factors; the most important of these are 

 the amount of surface exposed, temperature gradients, 

 and atmospheric humidity. The amount of heat gained 

 or lost will vary directly with the area of the surface 

 exposed. Since the area of the surface of a body in- 

 creases to the second power when the mass is increased 

 to the third power, a larger animal will gain or lose heat 

 more slowly than a smaller one. The gain and loss of 

 heat also depend upon the moistness of the surface. 

 Most of the poikilotherms live in the water and are 

 little influenced by this factor, but amphibians and rep- 

 tiles which live in the air of course show marked differ- 

 ences in this respect. Amphibia nearly always have 

 moist skins whereas Reptilia have dry scaly skins. The 

 latter therefore lose heat more slowly. The nature of 

 the surface is likewise a factor in the gain and loss of 

 heat. A layer of fat at or near the surface will consid- 

 erably decrease the rate at which heat arrives at the sur- 

 face from the interior. The low thermal conductivity of 

 fat is thus an important factor in the retardation of heat 



