A Second Reserve
for the Bolson Tortoise, Gopherus flavomarginatus,
at Rancho Sombreretillo, Chihuahua, Mexico

EDDIE TREVIÑO,¹ DAVID J. MORAFKA,¹ and GUSTAVO AGUIRRE ²

¹ Department of Biology, California State University, Dominguez Hills, Carson, CA 90747-0005, USA
² Instituto de Ecología, A. C., Km 2.5 Antigua Carretera a Coatepec, Ap. Postal 63, 91000 Xalapa, Veracruz, Mexico

        ABSTRACT:  In 1992 a team of U.S. and Mexican scientists, funded by the American Museum of Natural History Turtle Recovery Program, assessed the distinctiveness of a population of Mexican Bolson tortoises, Gopherus flavomarginatus, occurring on Rancho Sombreretillo in the state of Chihuahua.   Situated along the northern edge of the species’ current range, the ranch contains several demes of these tortoises.  Following the team’s investigative surveys, Rancho Sombreretillo was determined to be suitable and appropriate as a reserve for this endangered tortoise.
        Starch gel electrophoresis of frozen whole blood identified 18 isozymes (presumptive gene loci) through differential staining and mobility.  No differences in allozymes were found between the northern (Sombreretillo) population and the better-studied southern population from the Mapimí Reserve in Durango.  The close relationship of this species to the gopher tortoise, G. polyphemus, was confirmed, again by the absence of fixed allozyme differences.
        Surveys confirmed that yellow pigments in the Sombreretillo tortoises are largely confined to the marginals, especially in subadult to adult individuals.  Yellow pigment was more extensively distributed in juvenile carapaces than previously reported.
        Since the late 1980s habitat quality at Rancho Sombreretillo had deteriorated as a result of increased cattle grazing.  Cattle densities had increased, partly as a result of expanded pumping of underground water to local troughs and reservoirs.  However, tortoise populations remained robust, with ample evidence of nests, hatchlings, and juvenile age size classes.
        The process of establishing Rancho Sombreretillo as a Bolson tortoise reserve is reviewed in this paper.  From the options of (1) purchase, (2) lease, (3) easement, and/or (4) cooperative agreement, the latter was chosen because it was the most acceptable to the landowner, would generate local community support, and involved less investment.  Purchased land could also be more easily exploited by local poachers if subsequent patrolling proved inadequate.