Like the saguaro cactus (Carnegia gigantea) and the rattlesnake, the Gila monster is emblematic of the desert Southwest. The Gila monster is not only the largest lizard native to the United States, but also one of only two known species of venomous lizards in the Americas. This stout-bodied lizard can grow to 50 cm (20 in) and is covered with black and pink or orange markings and bead-like scales. The Gila monster’s range is centered in western and southern Arizona, continuing south to Sonora, Mexico.
Despite public fascination with the species, relatively little is known about the ecology and behavior of the Gila monster in the wild. For this reason, managers at Tonto National Monument, Arizona, contacted the U.S. Geological Survey (USGS) to investigate why Gila monsters were being seen in developed areas of the park, particularly crossing the main road. Managers were concerned about possible lizard-human conflicts and the risk of vehicle traffic killing Gila monsters. USGS scientists initiated a research effort in Tonto National Monument beginning in 2004 to provide information needed to make management decisions and improve scientific understanding of the species. Specifically, USGS scientists examined the movement patterns, range requirements, dietary habits, and use of developed areas by Gila monsters within the park. The 2004 research effort also extended a program begun in Tonto National Monument in 1994 to recognize individual Gila monsters based on unique dorsal patterns identified from photographs.