254 PROCEEDINGS OF THE NATIONAL MUSEUM vol. iis 



Lava 



In contrast to numerous investigations of coloration in lavicolous 

 mammals, relatively few workers have shown simUar interest in 

 lava-dwelling amphibians and reptUes. Lewis (1949; 1951), Norris 

 (1958, p. 270), and Lawrence and Wilhoft (1958) have demonstrated, 

 however, the presence of abnormally dark reptiles on southwestern 

 lava flows. Apparently selection favoring procrypsis is operating in 

 these environments. Tertiary lava beds in the Zuni region were 

 described adequately by Hooper (1941) and Lindsey (1951). Hooper 

 found that mammals restricted to the malpais had not developed dark 

 races there. 



With the exception of Ufa stansburiana, Sceloporus graciosus, 

 Masticophis Jlagellum, and Crotalus atrox, the species characteristic 

 of the Plains Life Belt occasionally were taken on soil-covered lava. 

 This substrate, widespread near El Morro and CeboUeta Mesa, pre- 

 sents an appearance similar to the short-grass association except that 

 it is generally higher in elevation (see Lindsey, 1951, figs. 10, 11). 

 The Grants lava flow, on the other hand, is rough and relatively 

 unweathered, with less soil accumulation (see Hooper, 1941, pi. 2). 

 Crotaphytus collaris, Sceloporus undulatus, Urosaurus ornatus, Eumeces 

 obsoletus, Thamnophis elegans, Pituophis melanoleucus, and Crotalus 

 viridis were collected or observed on the rougher malpais. 



Only C. collaris, U. ornatus, and Scaphiopus hammondi show 

 evidence of unusually dark coloration. The latter two species are 

 predominantly dark gray with little trace of the usual patterned 

 dorsum. Because local populations of these species tend to resemble 

 substrate color to a remarkable extent, it appears unlikely that the 

 dark, lavicolous individuals represent unique variation. Lewis (1951) 

 found dark U. ornatus in southern New Mexico. Three adults of C. 

 collaris are patterned but are quite dark in general coloration. This 

 is the usual situation on New Mexico lava flows (Fitch, 1956a, p. 223). 



Numbers of lavicolous reptiles and amphibians were not large. 

 While the rubble of broken lava offers uncountable hiding places, the 

 presence of ponderosa pine and Douglas fir, postclimax to adjacent 

 nonlava vegetation (Lindsey, 1951, p. 220), may limit the ranges of 

 Plains species on such a substrate. Sections of sparsely vegetated 

 malpais dominated by apache plume (Fallugia paradoxa) held the most 

 reptiles, but there appeared to be easy access to and from the lava 

 margins, with the possibility of strong immigration pressure from 

 contiguous grassland supporting larger populations of several species. 

 Gene flow may thus reduce the effectiveness of local selective pres- 

 sures. Marginal habitats, relative accessibility, and geologic re- 

 cency of the Grants malpais undoubtedly account for the fact that no 

 indigenous dark races have developed there. 



