Field Observations on a Population of the Sand Lizard Uma exsul

Abstract

The fringe-toed lizard Uma exsul Schmidt and Bogert is the most primitive member of its genus, and has been reported by several authors from sandy areas of heavy plant growth in the dry Lagunas de Mayran and Viesca in southwestern Coahuila, Mexico. Such sandy areas fall clearly into Norris' (1958) category of sand hummocks or accretion The former term appears to be more applicable in this case, for there is a strong possibility, especially in light of the observations here reported, that the dunes in this region are the result of subsequent plant growth on dunes formed by other means than accretion around vegetation. Although U. exsul has been recorded from numerous such situations of small hills topped with shrubs, no record is known to us of the existence of this primitive form on live dunes. Four series of Uma exsul were collected by Martha L. Commins, Alan H. Savitzky, and Robert F. Zogg during August 1971, at 3 locations: dunes N Bilbao (a village 7.4 mi NW Viesca, along Coah. 15), elevation approximately 3,415 ft; dunes 4.1 mi NW Viesca, along Coah. 15, and 2.2 mi N Coah. 15, along a gravel road, elevation approximately 3,530 ft; and dunes 5.8 mi NW Viesca, along Coah. 15, elevation approximately 3,500 ft. The dunes are located in the Laguna de Viesca, in a part of the dry lake situated south of the Sie rr as, west of the Sierra de Parras and smaller ranges, and north of the Sierra deSierra de Santiago. The Sierra de las Burras separates the Laguna de Viesca from the more northern Laguna de Mayran, where the type locality of Uma exsul is located. The Laguna de Viesca extends westward to the vicinity of Torreon, and includes the area about Matamoros, where numerous specimens of Uma exsul have been collected by various workers. The Bilbao and Viesca localities extend the known range of the species southeast approximately 11 air mi. The dunes in the vicinity of Bilbao and Viesca were of the type considered by Norris (1958) as barchan crescent-shaped dunes which are formed by unidirectional wind and which are in a state of unidirectional motion as a result of such wind. The orientation of the Bilbao-Viesca dunes indicates a wind blowing from the east, forming dune crests which in places rise to a height of approximately 5 m. The dunes lying on the north edge of the village of Bilbao are the largest and least overgrown of those visited. Most of the Uma exsul were seen in the basins of the dunes and especially along the edges of the dunes, in the open or under scattered vegetation. Some were also seen on the dunes proper (between basins), and a few were observed off the main dune area on a small strip of less sandy flatlands by the south edge of the dunes. Specimens of Sceloporus magister bimaculatus, Cnemidophorus tigris variolosus, and Coleonyx brevis were also collected in this area, and Uta stansburiana steinegeri observed. This association of Uma exsul with the latter 3 species was also observed by Williams, Smith, and Chrapliwy (1960); and with Chemidophorus tigris variolosus by Zweifel (1959). When approached on the interior of the duhes, the Uma exsul would usually run and hide in tumbleweed found in the basins or in cracks sometimes formed by the accumulated under the tumbleweed. On the edges of the dunes, Uma would generally take refuge in vegetation. Sand swimming was also occasionally observed, as seen under laboratory conditions (Carpenter, 1967). Sometimes they would lie motionless and flat against the sand, blending