Gila Monsters in California Revisited

Lovich, Jeffrey E. U.S. Geological Survey Southwest Biological Science Center Flagstaff, Arizona, USA jeffrey_lovich@usgs.gov

Beaman, Kent R. Herpetology Division Natural History Museum of Los Angeles County Los Angeles, California, USA heloderma@roadrunner.com

The Gila Monster (Heloderma suspectum) is widely distributed in parts of the Mojave, Sonoran and Chihuahuan Deserts of the southwestern United States and northwestern Mexico. However, records and sightings in southeastern California are rare. Prior to 2007, only 26 credible records were documented from four California counties (Imperial, Inyo, Riverside, and San Bernardino). The earliest record appears to be ca. 1853. Since 2007, six new records have come to light, including a sighting in the southern Chocolate Mountains (1974), a record near Tecopa (1993), another one in the Providence Mountains (2009), two sightings in the Mesquite Mountains (2015 and 2016), and a sighting in the Kingston Mountains (2017). Habitat in which the species has been observed in California is characterized by rocky, deeply incised topography, and in most cases, associated with large and relatively high mountain ranges. Most sightings are within xeroriparian and riparian microhabitats (including the lower Colorado River) and range from near sea level to over 1,200 m. All records except one (Mojave River) occur east of about 116o longitude. Records documented with photographs or museum specimens generally show color patterns diagnostic of the geographically expected subspecies Heloderma s. cinctum. The distribution of the species in California suggests a possible invasion into the high mountain ranges of the northeastern Mojave during the last interglacial via the Colorado River corridor. We also explore the hypothesis that climate patterns shaped the current distribution of the Gila Monster in California. Precipitation in the California deserts is decidedly biphasic east of -116o longitude, with over 24 percent falling in the warm season. Warm season precipitation data from recording stations closest to Gila Monster localities are almost identical for those in western Arizona where the species is more common. Summer precipitation may be important in the foraging ecology of the species. Gila Monsters were probably already rare in California long before the arrival of Europeans due to changes in climate and landforms that delimited the marginal location of California in the range of this species. Most of the habitat for this species in California is protected or relatively free from human disturbance.