An Evaluation of the Possibility of Hybridization between the Rattlesnakes Crotalus atrox and C. scutulatus in the Southwestern United States

Abstract

The possibility that Crotalus atrox and C. scutulatus might hybridize in southeastern Arizona-southwestern New Mexico or in the Big Bend region of Texas was analyzed morphometrically and electrophoretically. Intraspecific variation and interspecific overlap in key morphological characters has led to, the incorrect assumption that these species hybridize. Even though they are syntopic primarily in an unstable desert-grassland ecotone where habitat differences are not evident, C. atrox and C. scutulatus are morphologically and biochemically distinct. Two large diamond-backed rattlesnakes, Crotalus atrox Baird and Girard and C. scutulatus (Kennicott), often have been confused in areas of sympatry in the southwestern United States. These species are so similar morphologically that data from specimens of C. scutulatus have been included in descriptions and character summaries prepared for C. atrox (Klauber 1930). This happened so frequently that Klauber (1930) thought it advisable to present character statistics which reemphasized the differences between the two species. The repeated occurrence of specimens possessing unusual characters led to the speculation that C. atrox and C. scutulatus might hybridize, and Conant (1975) was prompted to note that C. scutulatus in western Texas is often aberrant and that one or more of the characters that normally serve to distinguish it approximate those of C. atrox. Captive-born interspecific hybrids between several species of Crotalus have been reported (Klauber 1956), but hybridization of Crotalus under natural conditions is rare. Klauber (1956) mentioned only three cases, none of which involved C. atrox or C. scutulatus. These two species occur sympatrically throughout much of the Southwest (Klau-