occurs (regularly, as a native taxon) in multiple nations
Regularity: Regularly occurring
Type of Residency: Year-round
Global Range: (5000-200,000 square km (about 2000-80,000 square miles)) The range includes southeastern California, extreme northeastern Baja California (Grismer 2002), northwestern Sonora, and southwestern Arizona (Funk 1981, Stebbins 2003).
In California, the species ranges southward from the Coachella Valley, including both sides of the Salton Sea and Imperial Valley, and westward into the Borrego Valley, Ocotillo Wells area, West Mesa, and Yuha Desert (Yuha Basin), and, on the east side of the Imperial Valley, to the vicinity of the Dos Palmas Bureau of Land Management (BLM) Area of Critical Environmental Concern (ACEC), but predominantly it occurs in East Mesa and in areas adjoining the Algodones Dunes (i.e., Imperial Sand Dunes, Glamis Sand Dunes) on the east side of the Imperial Valley. In southwestern Arizona, it occurs south of the Gila River and west of the Gila and Tinajas Altas Mountains in Yuma County (Rorabaugh et al. 1987). The range extends into Mexico from the international border in the Yuha Desert in California, south to Laguna Salada in Baja California, and from the international border in the Yuma Desert in Arizona, south and east through the Pinacate Region to the sandy plains around Puerto Penasco and Bahia de San Jorge, Sonora (Johnson and Spicer 1985, Gonzales-Romero and Alvarez-Cardenas 1989; these were cited, without full literature citations, by USFWS 2003).
The distribution of the flat-tailed horned lizard is not contiguous across its range; it is fragmented by large-scale agricultural and urban development, primarily in the Imperial Valley and the Coachella Valley. In addition, the Salton Sea, Colorado River, East Highline Canal, New Coachella Canal, and All American Canal are barriers to movement. Due to this habitat fragmentation and existing geographic barriers, the U.S. distribution appears to be currently divided on a broad scale into at least four geographically discrete populations: three in California and one in Arizona. The three populations 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. [from USFWS 2003]
The flat-tailed horned lizard has been recorded at elevations as high as 520 meters (1,706 feet) above sea level, but is more commonly found below 250 meters (820 feet) in areas with flat-to-modest slopes (Turner et al. 1980).
See Turner and Medica (1982) for the results of surveys done in southeastern California in 1978-1980; the area north of Highway 78 in the vicinity of Ocotillo Wells and Benson Dry Lake (= Ocotillo Dry Lake), eastern San Diego County, was identified as a particularly favorable area; other areas of relatively high abundance were in Imperial County--southern East Mesa, southeastern Yuha Desert, and the Superstition Mountain area. In Arizona, an area southeast of Yuma registered a relatively high abundance index (Rorabaugh et al. 1987).
Distribution: USA (SE California, SW Arizona), Mexico (NE Baja California Norte, NW Sonora)
Type locality: Great Desert of the Colorado, between Vallecita and Camp Yuma, about 200 miles east of San Diego.
Length: 12 cm
No other horned lizard has a dark middorsal stripe. Apparent hybrids between P. mcallii and P. platyrhinos, exhibiting a mix of morphological characteristics, have been observed in the vicinity of Ocotillo, California (Stebbins 1985), and southeast of Yuma, Arizona (K. Young, Utah State University, pers. comm., 2002, cited by USFWS 2002).
Habitat Type: Terrestrial
Comments: Typical habitat consists of sandy desert flatlands with sparse vegetation and low plant species diversity; occasionally the species occurs on low hills, mud hills, alkali flats, or areas covered with small pebbles or desert pavement; it is most abundant where surface soils contain some loose or windblown sand but rarely occurs on dunes (see USFWS 1993, Beauchamp et al. 1998). Vegetation in favorable habitat may include creosotebush, bur-sage, indigo bush, saltbush, and ocotillo (Turner and Medica 1982); also salt-cedar (Grismer 2002). In Arizona, it is most abundant in areas with galleta grass, sandy soil, and many active black harvester ant nests (Rorabaugh et al. 1987). In southeastern California, abundance is positively correlated with density of perennial plants, and there is a strong positive association between lizard and ant densities (Turner and Medica 1982). This is a cryptic lizard that generally occurs on the ground; often it is immobile and difficult ot detect until it moves. Sometimes it perches on rocks or wood (Grismer 2002). Periods of inactivity may be spent burrowed in loose sand. When approached, it may attempt escape into a burrow or under a shrub (Wone and Beauchamp 1995). Hibernation burrows appear to be self-constructed (constructed by the lizards themselves versus using burrows constructed by other animals) and are within 10 cm of the surface (Muth and Fisher 1992). Mayhew (1965) found that the majority of lizards hibernated within 5 cm of the surface. The greatest depth recorded was 20 cm below the surface.
Habitat and Ecology
Non-Migrant: Yes. At least some populations of this species do not make significant seasonal migrations. Juvenile dispersal is not considered a migration.
Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).
Locally Migrant: No. No populations of this species make annual migrations of over 200 km.
Flat-tailed horned lizards can have relatively large home ranges (Foreman 1997). Muth and Fisher (1992) found the mean home range for lizards (N = 22) was 2.7 ha from a minimum of 19 locations in West Mesa. In the Yuma Desert of Arizona, Young and Young (2000) found mean home ranges for males differed between drought and wet years, while those of females did not. The mean home range for males was 2.5 ha during a dry year versus 10.3 ha during a wet year. Female mean home ranges were smaller at 1.3 ha and 1.9 ha in dry and wet years, respectively (Young and Young 2000). Young and Young (2000) noted a wide variation in movement patterns, with a few home ranges estimated at greater than 34.4 ha. [from USFWS 2003]
At Ocotillo Wells State Vehicular Recreation Area in California, Wone and Beauchamp (2003) found that the total area used ranged from 0.15 to 59.2 ha for males (mean 17.8 ha) and 0.06 to 36.0 ha for females (mean 9.0 ha).
Comments: Feeds almost exclusively on ants, especially harvester ants; sometimes also eats other insects (Turner and Medica 1982).
Number of Occurrences
Note: For many non-migratory species, occurrences are roughly equivalent to populations.
Estimated Number of Occurrences: 21 - 80
Comments: The number of distinct occurrences has not been established using consistent criteria; probably there are more than 20 and perhaps fewer than 80 (e.g., see Funk 1981). Jennings and Hayes (1994) mapped a few dozen extant populations in southern California, plus about 20 extirpated ones.
100,000 - 1,000,000 individuals
Comments: The BLM recently estimated the population size on the Yuha Basin Management Area (MA) (one of five management areas identified in a management strategy for the species) by using capture-mark-recapture (CMR) techniques incorporating detection probabilities (see Thompson et al. 1998, Williams et al. 2002). In the summer (June to August) of 2002, the population of flat-tailed horned lizards for the Yuha Basin MA (24,122 ha) was estimated at 18,494 adults (95 percent CI = 14,596 to 22,391) (Grant and Wright 2002) and 8,685 juveniles (95 percent CI = 6,860 to 10,510) (derived from Grant and Wright 2002). ''Adults'' included all lizards greater than 60 mm (Young and Young 2000), while ''juveniles'' included all lizards 60 mm or less in snout-to-vent length. Population estimates for the other four MAs using a CMR methodology will be conducted soon, for the first time (Gavin Wright, BLM biologist, pers. comm., 2002). [from USFWS 2003]. Based on this information, it seems likely that the total adult population size exceeds 100,000.
Relatively difficult to find due at least in part to low density; surveys in several areas of southeastern California yielded a maximum of only 0.4 lizards/hour (Turner and Medica 1982). Density typically is 0.3-3.8 per hectare (see USFWS, Federal Register, 15 July 1997). Beauchamp et al. (1998) recorded a density of up to 0.23 per ha at Ocotillo Wells State Vehicle Recreation Area, California.
Often occurs in association with the lizards CALLISAURUS DRACONOIDES and DIPSOSAURUS DORSALIS.
Predators include American kestrel, common raven, burrowing owl, loggerhead shrike, sidewinder, and kit fox (Duncan et al., 1994, Herpetol. Rev. 25:68); perhaps ravens obtain the lizards more often as carrion than as live prey.
Life History and Behavior
Comments: Inactive in cold temperatures or extreme heat. Burrows into sand to seek shelter from heat of day or cool of night. During the summer may be most active in the early morning and evening.
Adult flat-tailed horned lizards are reported to be obligatory hibernators (Mayhew 1965), although individuals have been noted on the surface during January and February (Eric Hollenbeck, Ocotillo Wells SVRA biologist, pers. comm., 2002). Hibernation may begin as early as October and end as late as March (Muth and Fisher 1992). While most adults apparently hibernate during winter months, some juveniles may remain active (Muth and Fisher 1992). In southern California, duration of winter dormancy for 8 telemetered lizards was 85 days (Wone and Beauchamp 2003).
Flat-tailed horned lizards are oviparous (egg-laying), early maturing, and may produce multiple clutches within a breeding season (Howard 1974). Flat-tailed horned lizards produce relatively small egg clutches (N = 31; mean clutch size = 4.7; range = 3 to 7; Howard 1974), compared to most other horned lizards (Pianka and Parker 1975). The first cohort hatches in July to August (Muth and Fisher 1992; Young and Young 2000), and in some years a second cohort may be produced (Howard 1974, Young and Young 2000). Hatchlings from the first cohort may reach sexual maturity after their first winter season, whereas hatchlings born later may require an additional growing season to mature (Howard 1974, Young and Young 2000). Flat-tailed horned lizards can live up to at least 6 years in the wild (FTHL-ICC 2002), and up to 9 years in captivity (Baur 1986). [from USFWS 2003]
Molecular Biology and Genetics
Statistics of barcoding coverage: Phrynosoma mcallii
Public Records: 0
Specimens with Barcodes: 2
Species With Barcodes: 1
National NatureServe Conservation Status
Rounded National Status Rank: N2 - Imperiled
NatureServe Conservation Status
Rounded Global Status Rank: G3 - Vulnerable
Reasons: Diminished, fragmented range in southeastern California, southwestern Arizona, and adjacent northwestern Mexico; threatened primarily by habitat loss/degradation resulting from land development and other human activities; conservation agreements have reduced threats on public lands.
Intrinsic Vulnerability: Moderately vulnerable
IUCN Red List Assessment
Red List Category
Red List Criteria
Global Short Term Trend: Relatively stable (=10% change)
Comments: Overall, extent of occurrence and area of occupancy have been relatively stable to slightly declining in recent years.
Global Long Term Trend: Decline of 30-50%
Comments: Hodges (1997) estimated that the flat-tailed horned lizard historically (prior to agricultural or urban development of either the Coachella or Imperial Valleys) occupied up to 979,037 ha in Arizona and California. Approximately 51 percent (503,173 ha) of this historical habitat remains in the United States, with about 56,770 ha in Arizona and 446,390 ha in California (Hodges 1997). The Salton Sea area could arguably be considered ephemeral historical habitat, present at some points and absent at others, as the area changed through time. Hodges (1977) included the Salton Sea as historical habitat. If the area the Salton Sea currently occupies is not considered historical habitat, then approximately 57 percent (557,072 ha) of historical habitat remains in the United States. [from USFWS 2003].
Johnson and Spicer (1985) estimated that in 1981 approximately 59 percent of the species range occurred in Mexico, with the majority of the range in Mexico occurring in the state of Sonora. However, the distribution of the species in Mexico is poorly understood because few surveys have been conducted to determine where the species occurs in Mexico (CEDO 2001). In 1981 in Sonora, about 14 percent of the habitat was estimated to be threatened by urban, agricultural or recreational use, and habitat degradation (Johnson and Spicer 1985). In Baja California, considerable habitat loss has occurred in the Mexicali Valley, where urban and agricultural development extends from Mexicali to the Colorado River (Johnson and Spicer 1985, Foreman 1997). [from USFWS 2003]
Degree of Threat: Medium
Comments: Based on information obtained since the withdrawal of the proposed listing rule in 1997 and information documented in the proposed rule, USFWS (2003) identified potential threats to the flat-tailed horned lizard, including the following: urban development, agricultural development, OHV activity, energy development, military activities, introduction of non-native plants, pesticide use, and habitat degradation due to Border Patrol and illegal drive-through traffic along the United States-Mexico border. These threats and their effects on flat-tailed horned lizards and their habitat are discussed in further detail in USFWS (2003).
After considering all the current available information, USFWS (2003) determined that the threats identified under Factor A (habitat loss/change) are not significant enough to conclude that the flat-tailed horned lizard is likely to become endangered throughout all or a significant portion of its range in the foreseeable future. However, the Coachella Valley has experienced a significant amount of habitat curtailment and there is the potential for significant habitat destruction in the immediate future, because of the predominant private ownership of habitat and the rate of development in the Coachella Valley.
Currently available information does not suggest that development of private lands on the west side of Salton Sea/Imperial Valley poses a threat in the foreseeable future. The only towns in this geographic area are Borrego Springs, Ocotillo, Ocotillo Wells, and Salton City. The largest of these towns is Borrego Springs with a population of approximately 3,000 people. It is likely that the size of these towns will not change significantly in the foreseeable future (USFWS 2003). Therefore, USFWS (2003) concluded that the threat of development of private lands in areas other than the Coachella Valley is not significant enough to endanger the species within the foreseeable future throughout a significant portion of its range. The available data do not suggest that habitat modification by OHV use threatens the flat-tailed horned lizard on the west side of the Salton Sea/Imperial Valley and east side of the Imperial Valley (USFWS 2003).
USFWS (2003) also concluded that the Arizona population is not likely to become endangered within the foreseeable future. The the low percentage of lands in private ownership makes for a low degree of threat from development. Further, OHV use has not been shown to be a threat to populations there, and this geographic area experiences a relatively low level of OHV activity.
Collection, predation, and disease are not believed to be significant threats (USFWS 2003).
Management Requirements: A flat-tailed horned lizard Population Viability Analysis (PVA) was conducted by a conservation team convened both to share research results involving this species and to evaluate the Management Strategy. The preliminary PVA provided no estimate of the minimum viable population size and did not determine whether populations contained within the MAs were viable, due to a lack of population demographic and stochastic (i.e., random events relevant to a population) information. However, the analysis illustrated the sensitivity of flat-tailed horned lizard population viability to certain factors, particularly changes in mortality and fecundity. Recommendations in the PVA report included controlling activities that result in mortality of flat-tailed horned lizards and degradation of their habitat. Large management areas were found to be desirable as a conservative approach to ensuring the long-term population persistence.
See Gardner et al. (2004, Herpetol. Rev.. 35:250-251) for a discussion of the use of barrier fences to prevent road mortality.
Global Protection: Several to many (4-40) occurrences appropriately protected and managed
Comments: In June of 1997, seven Federal and State agencies signed a Conservation Agreement (CA) to implement a Flat-tailed Horned Lizard Rangewide Management Strategy (Management Strategy). The purpose of the Management Strategy is to provide a framework for conserving sufficient habitat to maintain several viable populations of the species throughout the range in the United States. As part of the CA, agencies delineated specific areas under their jurisdiction as Management Areas (MAs). Approximately 181,100 ha of the remaining flat-tailed horned lizard habitat managed by signatories of the CA exists within five MAs, which occur in the Borrego Badlands, West Mesa, Yuha Desert, East Mesa, and the Yuma Desert. These managed areas are believed to represent approximately 35 percent of flat-tailed horned lizard habitat remaining in the United States.
The five MAs were designed to identify large areas of public land where flat-tailed horned lizards have been found, as well as to include most flat-tailed horned lizard habitat identified as key areas in previous studies (Turner et al. 1980, Turner and Medica 1982, Rorabaugh et al. 1987, Foreman 1997). The MAs were delineated to include areas as large as possible, while avoiding extensive, existing and predicted management conflicts (e.g., OHV open areas). The MAs are meant to be the core areas for maintaining self-sustaining populations of flat-tailed horned lizards in the U.S. (FTHL-ICC 2002).
This lizard commonly occurs in additional areas outside of the MAs. These areas include the Ocotillo Wells State Vehicle Recreation Area (Ocotillo Wells SVRA), Coachella Valley, the areas adjoining the Algodones Dunes, and east of the Algodones Dunes between Ogilby and the Mexican border (Norris 1949, Turner et al. 1980, Turner and Medica 1982). The Ocotillo Wells SVRA is currently a Research Area under the Management Strategy, and studies on the flat-tailed horned lizard have been encouraged and funded by the California Department of Parks and Recreation (CDPR) Division of Off-Highway Motor Vehicle Recreation
(Foreman 1997). The majority of the potential habitat is within and adjacent to the Algodones Dunes is within the Imperial Sand Dunes Recreation Area. Over 47,754 ha of the Imperial Sand Dunes Recreation Area is used as an OHV open area. The majority of the Algodones Dunes north of Highway 78 is a designated wilderness area. [from USFWS 2003]
Of the remaining habitat in the Coachella Valley, only about 2,150 ha of suitable flat-tailed horned lizard habitat is estimated to be protected as part of the Coachella Valley Fringe-Toed Lizard Preserve System (Coachella Valley Mountains Conservancy 2001). Approximately 75 percent of the horned lizard habitat in the Coachella Valley is either private or Tribal land and subject to development in the near future. [from USFWS 2003]
The majority (about 60 percent) of the range in Mexico lies within two federally protected areas: (1) The Upper Gulf of California and Colorado Delta Biosphere Reserve, and (2) the Pinacate and Gran Desierto de Altar Biosphere Reserve (CEDO 2001). The National Park of Pinacate is an area administered by the Mexican government with use restrictions similar to those in a national park in the United States. The Upper Gulf of California Biosphere Reserve includes flat-tailed horned lizard habitat in the vicinity of the Colorado River Delta in Sonora, Mexico. [from USFWS 2003]
Grismer (2002) stated that "it is conceivable that the remote, undeveloped areas of northeastern Baja California and northwestern Sonora may become strongholds for this species."
Needs: See draft "Flat-tailed Horned Lizard Rangewide Management Strategy," available in October 1996.
Flat-tail horned lizard
The flat-tail horned lizard (Phrynosoma mcallii) is a reptile of the Sonoran desert of the southwestern United States and northwestern Mexico. Its multiple adaptations for camouflage help to minimise its shadow. The species is threatened, with a restricted range under pressure from human activities such as agriculture and development, and is specially protected in the United States.
Description and range
The flat-tail horned lizard is named for Colonel George A. M'Call, who collected the first specimen in California in the 19th century. The species occupies a small range in the Sonoran Desert of southeastern California, southwestern Arizona, and extreme northern Mexico in the Baja California and Sonora states. Over time, horned lizard populations have adapted to climate, food, and predators, causing them to in some ways be distinct from one another.
- "A medium-sized flat-bodied lizard with a wide oval-shaped body and scattered enlarged pointed scales on the upper body and tail. The back skin is smooth with small spines. 8 horns extend from the back of the head. The two central horns are long, slender and sharp. Long and narrow spines on the lower jaw and two rows of fringe scales on the sides of the body, the bottom row scales smaller than the upper."
The flat-tail horned lizard has evolved elaborate camouflage measures to eliminate shadow. Their bodies are flattened, with the sides thinning to an edge; the animals habitually press their bodies to the ground; and their sides are fringed with white scales which effectively hide and disrupt any remaining areas of shadow there may be under the edge of the body.
Different populations of the species match their local backgrounds using a combination of colour-creating cells in their back scales. These cells include black melanophores and red chromatophores in an upper layer, scattered over a layer of white reflective iridophores, enabling the flat-tail horned lizard to match the local soil or rock. So for example the Algondones Dunes population of San Luis, Sonora is generally redder than the population on the whiter Thousand Palms dunes of California. In addition, the dark midline helps to disrupt the outline of the lizard, resembling the thin shadows of plant stems in its windswept sand habitat.
A threatened species
This lizard is threatened by development, agriculture, and other man-made intrusions into their small range. The majority of their remaining habitat in the US is administered by the Bureau of Land Management. The species (Phrynosoma mcallii) frequently coexists with sources of natural gas, oil, geothermal energy, and minerals which can found in its habitat. In 1982, the US Fish and Wildlife Service declared P. mcallii as a Candidate 2 Category for the list of threatened and endangered species due to concerns over potential threats to their habitat which could further diminish the population. P. mcallii has also been given special status in both California and Arizona, which prevents their collection.
Experiments have been conducted to investigate the "fluctuations in populations of flat-tailed horned lizards, Phrynosoma mcallii, in the Coachella Valley, California. This species has the smallest range of any horned lizard in the United States. In parts of its range, there are potentially conflicting activities, such as suburban development, agriculture, off-road recreation, and activities along the international border."
"Between 1978 and 1980 the Bureau of Land Management supported investigations of the status of P. mcallii in California. The purpose of this work was to determine the local distribution and relative abundance of P. mcallii, to correlate these parameters with various habitat attributes, and to gather information on the structure of the populations and mobility and food habits of individual lizards."
The study also looked at the flat-tailed horned lizard's distribution and abundance throughout Arizona. The species was found to be restricted to an area of desert, 650–700 km2 in size, in the southwestern corner of the state. The species was most abundant in places with the Western whiptail (Cnemidophorus tigris), nests of the black harvester ant (Messor pergandei), galleta grass (Hilaria rigida) and sandy soils.
- Rorabaugh, 1987.
- Sherbrooke, 2003.
- "Phrynosoma mcallii - Flat-tail Horned Lizard". Californiaherps.com. Retrieved 25 September 2011.
- Sherbrooke, 2003. pp.117–121
- Turner, Frederick B.; Medica, Philip A. (Dec 21, 1982). "The Distribution and Abundance of the Flat-Tailed Horned Lizard (Phrynosoma mcallii)". Copeia 4: 815–823. JSTOR 1444091.
- Barrows, Cameron W.; Allen, Michael F. (2009). "Conserving Species In Fragmented Habitats: Population Dynamics Of The Flat-Tailed Horned Lizard, Phrynosoma Mcallii". Southwestern Naturalist date=SEPTEMBER 2009 54: 307–316. doi:10.1894/wl-22.1.
- Rorabaugh, James C.; Carolyn L. Palermo; Steven C. Dunn (March 1987). Distribution and Relative Abundance of the Flat-Tailed Horned Lizard (Phrynosoma mcallii) in Arizona. Southwestern Association of Naturalists. p. 103. JSTOR 3672014.
- Sherbrooke, Wade C. (2003). Introduction to Horned Lizards of North America. London: University of California press.
Names and Taxonomy
Comments: Reeder and Montanucci (2001) examined phylogenetic relationships of horned lizards (Phrynosoma) based on mtDNA and morphology.