REACTIONS OF THE HORNED LIZARD. IOJ 



3. Substratum Temperature Gradient. 



The gradient cage was placed in a water bath so arranged that 

 hot water flowed into the latter at one end and cold water at the 

 other, the water being directed backward and forward beneath 

 the cage so as to produce a gradient in the temperature of the 

 substratum. This experiment was conducted with still air in 

 the cage, i. e., with air of approximately the same temperature as 

 the substratum and consequently varying in evaporating power, 

 and with air of a uniform temperature and evaporating power 

 flowing across the gradient. The results of the two types of 

 experiment checked very closely, indicating that the temperature 

 of the substratum is the dominant controlling factor under these 

 conditions. Both show an optimum substratum temperature of 

 35 to 40. In general the animals which had been kept at the 

 higher temperature, i. e., in cage No. 2, oriented themselves in the 

 gradient more quickly than those kept at the lower temperature 

 (Experiments 26, 2 7 a, b, Table III.). When the animals gathered 

 in the region of the optimum temperature an increase or decrease 

 of the temperature of the gradient caused a corresponding 

 movement of all the animals, which were repeatedly driven from 

 end to end of the cage in this manner. The lizards often bur- 

 rowed at or near the upper limit of the optimum region. They 

 also often burrowed when the temperature was reduced as low 



as 20. 



When the animals were placed in a cage the substratum of 

 which was gradually heated, rapid movements, interspersed with 

 digging reactions began at 35, and as the temperature approached 

 40, all animals attempted to burrow. However, under experi- 

 mental conditions, it was impossible to heat the substratum 

 without having the lower layers of the soil at least as hot as the 

 top, and the animals did not complete the burrowing reaction, 

 but again moved about the cage rapidly. Above 40, evidences 

 of extreme discomfort, such as wide-open mouth, spasmodic 

 movements, etc., were evidenced. 



When animals in a sand-bottom cage were subjected to a con- 

 tinuous blast of air of increasing temperature from above, so as 

 to heat the surface of the soil without greatly increasing the 

 temperature of the lower layers of the soil, they became very 



