AN EXPERIMENTAL STUDY OF THE REACTIONS OF 



THE HORNED LIZARD, PHRYNOSOMA MODESTUM 



GIR., A REPTILE OF THE SEMI-DESERT. 1 



A. O. WEESE, 



THE UNIVERSITY OF NEW MEXICO. 



PAGE. 



I. Introduction 98 



1. General Habits 98 



2. Color and Color Changes 100 



3. Behavior in Captivity 101 



II. Experimental Results 101 



1. Air Humidity Gradients 101 



2. Air Temperature Gradients 105 



3. Substratum Temperature Gradients 107 



III. Conclusions 109 



IV. Acknowledgments and Bibliography no 



I. INTRODUCTION. 



The species of Phrynosoma which is the subject of these ob- 

 servations is one of the smaller and less conspicuous members 

 of the genus, and is one of the commoner "horned toads" in New 

 Mexico, which lies at the center of its rather restricted range 

 (Ditmars, '08). This species, Phrynosoma modestnm Gir. (Girard 

 '53) has been confused to a certain extent in the literature (Her- 

 rick, et a/., '99) with another small member of the genus, Phry- 

 nosoma platyrhinos Gir. (Girard, '53) which extends into New 

 Mexico from the northwest, but a reference to the original de- 

 scriptions differentiates very clearly between the two groups. 



i . General Habits. 



Near Albuquerque, where the individuals observed were taken, 

 the species is widely distributed over the sparsely vegetated 

 "mesa" (Watson, '12) or clinoplane region varying in altitude 

 from 1,500 to 2,200 meters and having a rainfall of approximately 

 30 cm. annually. Bailey ('13) gives the distribution of this animal 

 as the Lower and Upper Sonoran Zones in New Mexico. In the 

 valley of the Rio Grande, just below the "mesa" region, and in 



1 Contribution from the Zoological Laboratory of the University of Illinois, 

 No. 86. 



98 



