MELANOPHORES OF THE HORNED TOAD 279 



3. Adaptive color changes 



Horned toads show striking color changes which depend upon 

 the color of the environment in which they live. If they are 

 placed in a pen which is lined with black cinders, the ground- 

 color becomes so dark that the transverse bands are almost indis- 

 tinguishable. When such animals are transferred to a pen lined 

 with white sand, the ground-color becomes very pale in the course 

 of a few days. The contrast between these pale animals and 

 horned toads which have been left in the cinder-lined cage as 

 controls is illustrated in plate 1. This change in color becomes 

 noticeable within one day after the lizard has been placed in its 

 new environment, and reaches a maximum within one week. 



Upon the adaptive state of the chromatophores is superimposed 

 the daily rhythm of color change. Thus horned toads which are 

 adapted to a dark background become paler at night and at mid- 

 day, but never attain the extreme paleness of specimens adapted 

 to a light background. Similarly, horned toads which are 

 adapted to a light background become darker in the morning and 

 afternoon, especially on cool days, but never attain the dark color 

 of specimens which are adapted to a dark environment. 



4. Color changes due to nervous excitement 



Whenever horned toads are thrown into a state of nervous 

 excitement the melanophore pigment throughout the entire 

 surface of the body is contracted and the animal assumes the 

 maximal pale condition. This reaction is observed whenever one 

 of these lizards is subjected to any treatment which causes it to 

 struggle in an effort to free itself. Simply holding a horned toad 

 upon its back, attempting to pry open its mouth, or pinching its 

 tail may suffice to cause this reaction. A male horned toad, 

 attempting to copulate with a female, has been observed to 

 become very pale at a time when the pigment of the other ani- 

 mals in the pen was expanded fully. 



The contraction of the pigment which accompanies nervous 

 excitement occurs more rapidly than the two other types of reac- 

 tion which have been described. The complete change in color 



