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Lower Colorado River Multi-Species Conservation Program Balancing Resource Use and Conservation

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Flat-Tailed Horned Lizard

      (Phrynosoma mcalli)

Flat-Tailed Horned Lizard - Photo by Shelby HowardFlat-Tailed Horned Lizard - Photo by Shelby HowardA flat-tailed horned lizard, detected during surveys in May-June, 2011, moves across the desert pavement near the northwest boundary of the Yuha Basin Management Area - Helix Environmental Planning, Inc. – Erik LaCoste

General Description

The flat-tailed horned lizard (Phrynosoma mcalli) is a moderate-sized, flat-bodied lizard, with a white belly, a narrow dark stripe down the back from head to tail base, and a flattened tail. The back coloration is gray, tan, reddish-brown, or whitish with small spines. The lizard has eight horns that extend from the back of the head.  Adults weigh approximately 0.6 to 0.9 oz (17 to 25 g) with snout to vent lengths from 2.7 to 3.2 inches (70 to 80 mm). Males and females do not significantly differ in body size or growth rate.

Legal Status

The flat-tailed horned lizard is listed as a species of special concern in the State of Arizona and a species of concern in the State of California. The California Fish and Game (CDFG) Commission designated the this species as a candidate species, and collections are prohibited in California and Arizona.


One of 14 species of horned lizards, the flat-tailed horned lizard was first collected and described in 1852. The desert horned lizard (Phrynosoma platyrhinos) is the only other horned lizard known to live in the same area with the flat-tailed horned lizard. Hybrids between the two species have been reported in Ocotillo, California, and on the Barry M. Goldwater Range in Yuma, Arizona.


The flat-tailed horned lizard mates in April and May and eggs are laid in May and June. Flat-tailed horned lizards lay approximately 3 to 10 eggs per clutch and have up to two clutches per season.  They reach sexual maturity at age 1 year or less. Reproduction appears to be correlated with environmental conditions. Reproduction may be doubled in wet years as opposed to dry years. Following heavy fall precipitation, hatchlings reached adult size in less than a year, while under drought conditions, it generally takes 2 years to reach breeding condition.


Flat-tailed horned lizards feed on a variety of harvester ants and certain spiders that mimic ants. They may also eat beetles and other arthropods. Studies show that 75% of all insects in flat-tailed horned lizard scat were comprised of three species of harvester ants: Veromessor pergandei, Pogonomyrmex californicus, and Pogonomyrmex magnacantha. A fourth species of ant, Conomyrma insane, comprised 16% of insects in the scat. Feeding rates of up to 80 harvester ants per 15 minutes have been observed. The number of ant colonies in an area may be an important habitat requirement for the flat-tailed horned lizard. Several studies found the number of harvester ant colonies was correlated with high lizard abundance.


Threats to the flat-tailed horned lizard include habitat loss from urban and agricultural expansion, pesticide contamination, off-highway vehicle activities, geothermal development, roads, highways, railroads, power lines, military activities, wind turbines, invasive plant species, fires, land disposal, cattle grazing, military operations, border patrol activities, sand and gravel extraction and vehicular traffic.  Fencing was designed that was successful in keeping flat-tailed horned lizards off roads. Off-highway vehicle (OHV) usage is an increasingly popular activity that takes place in flat-tailed horned lizard habitat. OHV usage may pose direct threats (mortality by being run over) or indirect threats (destroying ant mounds, affecting vegetation, compacting soil) to flat-tailed horned lizard populations.

The primary predators of the flat-tailed horned lizard are the round-tailed ground squirrel and loggerhead shrike. Other predators of flat-tailed horned lizards include the grasshopper mice, American kestrel, common raven, burrowing owl, snakes, and feral cats and canids.

More Information

Additional information on this species, as well as source documentation, can be found in the species accounts located at this link (PDF). Technical Reports on this species can be found here.

Updated June 8, 2016

The flat-tailed horned lizard has the smallest range of any horned lizard in the United States. Assessing the historical habitat of the flat-tailed horned lizard is complicated by the fact that agriculture preceded knowledge of the species range in Imperial Valley, California, and Yuma Valley, Arizona.  There are four geographically discrete populations in the United States (three in California and one in Arizona). The three in California are located in the Coachella Valley, the west side of the Salton Sea/Imperial Valley, and the east side of the Imperial Valley. Populations in the Imperial Valley are divided into four major segments (Algodones Dunes, East Mesa, West Mesa/Anza Borrego, and Yuha) by Interstate 8 and the Coachella Canal. Populations in the Coachella Valley are divided into two segments by Interstate 10.

Click on the map below to see the distribution range of the flat-tailed horned lizard using the interactive GIS map.

Click here to see the distribution range of the flat-tailed horned lizard

The flat-tailed horned lizard occurs along the Lower Colorado between Imperial Dam, just south of Yuma, AZ, and the Mexican border.


The flat-tailed horned lizard occurs in fine packed sand or pavement, overlain with loose, fine sand in areas that are sparse or lacking in vegetation. The species occurs in predominantly sandy flats associated with creosote bush, white bursage, burrobush, indigo bush, and big galleta. The lizards occur at elevations from below sea level to 820 feet (250 m). Vegetation may be an important requirement for egg laying sites. Typical habitat is sandy desert hardpan or gravel flats with scattered sparse vegetation of low species diversity. Most common in areas with a high density of harvester ants and fine windblown sand, but rarely occurs on dunes.


LCR MSCP Conservation Measures

The Habitat Conservation Plan (HCP) provides conservation measures specific to each species. Listed below are the species specific conservation measures for the flat-tailed horned lizard. Click on the arrows to expand the table.

FTHL1—Acquire and protect 230 acres of existing unprotected occupied flat-tailed horned lizard habitat (completed)

Consistent with the mitigation measures identified in the Flat-Tailed Horned Lizard Rangewide Management Strategy (Flat-tailed Horned Lizard Interagency Coordinating Committee 2003), the LCR MSCP will acquire and protect 230 acres of unprotected occupied flat-tailed horned lizard habitat. The acquired habitat will be transferred to an appropriate management agency for permanent protection of habitat for the species.

FTHL2—Implement conservation measures to avoid or minimize take of flat-tailed horned lizard

Reclamation will continue to implement measures to avoid or minimize take of flat-tailed horned lizard. These measures would include worker education programs and other procedures as described in the 1997 BO (U.S. Fish and Wildlife Service 1997) and are in accordance with the 2003 Flat-tailed Horned Lizard Interagency Coordinating Committee recommendations for the species.

AMM5—Avoid impacts of operation, maintenance, and replacement of hydroelectric generation and transmission facilities on covered species in the LCR MSCP planning area

To the extent practicable, before implementing activities associated with OM&R of hydroelectric generation and transmission facilities, measures will be identified and implemented that are necessary to avoid take of covered species where such activities could otherwise result in take. These measures could include conducting surveys to determine if covered species are present and, if so, deferring the implementation of activities to avoid disturbance during the breeding season; redesigning the activities to avoid the need to disturb covered species habitat use areas; staging of equipment outside of covered species habitats; delineating the limits of vegetation control activities to ensure that only the vegetation that needs to be removed to maintain infrastructure is removed; stockpiling and disposing of removed vegetation in a manner that minimizes the risk of fire; and implementing BMPs to control erosion when implementing ground disturbing activities.

The HCP requires the LCR MSCP to acquire 230 acres of existing unprotected occupied flat-tailed horned lizard habitat for permanent protection of the species’ habitat.

In November 2010 individual land owners within the Yuma Desert Area of Critical Environmental Concern (ACEC) were identified using county records and assessor tax information. Thirteen land owners were targeted for a total of 2,313 acres. Identified private land contained suitable habitat, no previous disturbance and was surrounded by suitable habitat. Two landowners within the Yuma Desert ACEC have granted access to conduct species surveys and federal appraisals on their properties totaling 240 acres. Additional right of entries for lands within either East or West Mesa ACEC’s may be required to meet the conservation measure.

Species surveys to determine occupied habitat occurred in Spring 2011. Federal appraisals will be initiated on lands with suitable habitat conditions for flat-tail horned lizards.

Once fair market value is established and a preliminary title is reviewed, offer letters from the LCR MSCP Program Manager will be mailed to land owners. The offer letter states the market value, possession of a clear title on the land and describes the federal acquisition process.

For additional information on this project, please refer to Work Task E30: Flat-tailed Horned Lizard (PDF). Find Technical Reports for this Work Task here.

Research and Monitoring Activities

The LCR MSCP conducts a variety of research and monitoring activities along the LCR encompassing both MSCP and non-MSCP species. For a complete list of all activities, please see the Research and Monitoring Activities web page.


This gallery includes photos of this species. If you require larger photos, please contact our webmaster Michelle Reilly at mreilly@usbr.gov.