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Flat-Tailed Horned Lizard Demographic Monitoring


Background: Fig 1
The flat-tailed horned lizard (Phrynosoma mcallii) only occurs at low elevations within the Lower Colorado River Desertscrub community in the far southwestern corner of Arizona, southeastern California and Sonora, Mexico.  In Arizona, the species is mostly found in sandy flat or low-lying dune areas suitable for burrowing, and where creosote (Larrea tridentata), and white bursage (Ambrosia dumosa) are characteristic plants.   It is distinguished from other horned lizards (eight recognized species in the U.S.) by a dark mid-dorsal vertebral stripe, two rows of fringed scales on the sides of its body and a long flat tail.

This species is threatened by habitat loss from the expansion of agriculture, urban development and off-road vehicles. Recent increases in U.S. Border Patrol and other law enforcement activities, road expansions and construction of the border wall are also considered potential threats.  At present, Fig 2 the majority of flat-tailed horned lizard range in Arizona is restricted to the Yuma Desert area within the western portion of the Barry M. Goldwater Range, managed by the Marine Core Air Station in Yuma.

The purpose of this research project is to address concerns about the possible population decline in Arizona, and to support the management and conservation strategies developed by the Flat-tailed Horned Lizard Interagency Coordinating Committee, summarized in the Flat-tailed Horned Lizard Rangewide Management Strategy (2003). The primary goal of this management program is to "maintain self-sustaining populations".  In order to meet this goal, reduce possible threats and eliminate the potential need to list the species,  a strategy was devised that will provide a standardized assessment of the flat-tailed horned lizard population status within all management areas to determine if the population is stable, increasing or decreasing. Beginning in 2008, our objectives are: 1) delineate two long-term demography study plots and 2) collect baseline demographic information on local flat-tailed horned lizard populations within these plots.

The flat-tailed lizard study plots are located within the Yuma Desert Management Area south of Interstate 8, west of the Gila Mountains and southeast of Yuma, Arizona. One each is located within Bureau of Reclamation land and the Barry M. Goldwater Aerial Gunnery Range.

Monitoring surveys were conducted by teams of four to six observers within 9ha size plots.  Study areas were searched along designated transects by foot with emphasis on locating tracks or sign and investigating habitat features such as burrows, clumps of vegetation, sand mounds and ant colonies.  If tracks were detected, these were carefully followed until a lizard was found or the trail became too degraded to continue.  All flat-tailed horned lizards encountered were measured, weighed, sexed and marked.  Temporary external numerical marks were placed on the lizard's abdomen for rapid visual identification in the event of recapture.  A second marker using Passive Integrated Transponders (PIT tags) were inserted into the body cavity as permanent markers on all adult lizards.

Initial Results:
Fig 3During the 2008 field season, we documented 109 total individuals on both study plots combined. This included 48 adults and 61 juveniles.  Two adult mortalities were also recorded, with both the apparent victims of sidewinder rattlesnake (Crotalus cerastes) bites.  Several road-killed lizards were also observed along an access road, but outside the study area.

Collection of baseline information on flat-tailed horned lizard abundance, survival and recruitment during long-term monitoring will allow wildlife managers to analyze population trends and viability, and to assess the species status as a potential candidate for listing under the Endangered Species Act.   Results will benefit planners and managers for the local Yuma Management Area as well as contribute to the larger effort to monitor the flat-tailed horned lizard population status throughout its range.


For more information contact:
Dennis Abbate, Arizona Game and Fish Department
5000 W. Carefree Highway, Phoenix, AZ 85086
Phone: (520) 609-2167      E-mail:

Michael Ingraldi, Ph.D., Arizona Game and Fish Department
5000 W. Carefree Highway, Phoenix, AZ 85086
Phone: (928) 532-5625       E-mail:

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