Round-tailed Horned Lizard – Tucson Herpetological Society

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Round-tailed Horned Lizard (Phrynosoma modestum)

[/vc_column_text][gap size=”12px” id=”” class=”” style=””][/vc_column][/vc_row][vc_row][vc_column width=”1/2″][vc_single_image image=”181″ img_size=”large” alignment=”center” style=”vc_box_rounded”][vc_column_text]Round-tailed Horned Lizard, E of Portal, Cochise Co., AZ. Photo by Jim Rorabaugh[/vc_column_text][/vc_column][vc_column width=”1/2″][vc_row_inner][vc_column_inner width=”1/2″][vc_single_image image=”1853″ img_size=”medium” alignment=”center” onclick=”img_link_large”][vc_column_text]Round-tailed Horned Lizard, Cochise Co., AZ. Photo by Jim Rorabaugh[/vc_column_text][gap size=”12px” id=”” class=”” style=””][/vc_column_inner][vc_column_inner width=”1/2″][vc_single_image image=”1854″ img_size=”medium” alignment=”center” style=”vc_box_rounded” onclick=”img_link_large”][vc_column_text]Round-tailed Horned Lizard. Photo by Jim Rorabaugh[/vc_column_text][gap size=”12px” id=”” class=”” style=””][/vc_column_inner][/vc_row_inner][/vc_column][/vc_row][vc_row][vc_column width=”1/6″][/vc_column][vc_column width=”2/3″][vc_column_text]

Description

The Round-tailed Horned Lizard (Phrynosoma modestum) is one of the smallest species of horned lizard (< 71 mm SVL) and the smallest in the 100-Mile Circle. Like other horned lizards, it has a wide, flat body. The tail is short; it narrows abruptly at the base, and is relatively slender and round for a horned lizard. Four short horns of equal or nearly equal length form a straight line at the back of the head. Dorsal scales are mostly granular, and there are no or only very few enlarged dorsal conical scales. The ventral scales are smooth, and there are no enlarged, fringed scales at the margin between the abdomen and dorsum (Sherbrooke 2003, Brennan and Holycross 2006, Hodges 2009).

The dorsal ground color is highly variable and can be white, pink, reddish, yellow, gray, brown, or bluish. Coloration usually closely matches the color of the substrate (Bundy and Ness 1958). Dark dorsal blotches are typically present on the neck and along the sides and the base of the tail, but they may be faint. The venter is uniform cream to white. The tail is banded, dark and light, but dark bands may be faint. The largest males (66 mm SVL) are smaller than the largest females (71 mm SVL, Whiting and Dixon 1996), and they have enlarged post-anal scales. Hatchlings resemble adults in color and pattern, but their cranial horns are proportionally smaller, and they measure 21-30 mm SVL (Sherbrooke 2003, Hodges 2009, Rorabaugh and Lemos-Espinal 2016).

Among the six horned lizard species that occur in the 100-Mile Circle, this is the only one to lack enlarged fringed scales at the margin of the abdomen and dorsum. It is also the only one to have four horns at the back of the head that are of equal or nearly length and are lined up in a straight row.

Range wide, this species occurs from southeastern Colorado, western Texas, New Mexico, and southeastern Arizona south through northeastern Sonora and portions of Chihuahua, Coahuila, Durango, Nuevo León, San Luis Potosí, Zacatecas, and Aguascalientes. In Arizona, the species occurs mainly in two apparently disjunct areas: 1) on the eastern bajada and lower slopes of the Pinaleño Mountains and the Gila River Valley and adjacent bajadas from near Bylas south and east probably into New Mexico, and 2) from about Interstate 10 south along the eastern bajada and foothills of the Dos Cabezas and Chiricahua Mountains. However, there is a 1950 collection from 6 miles north of Courtland and a 1956 collection from 2.2 miles east of Pearce in the southern Sulphur Springs Valley. It was not found in that valley by Rosen et al. (1996). UAZ 47341, collected in 1954 from “0.5 mi E Globe” in Gila County, is disjunct from other localities in the eastern Gila River Valley. The specimen and its locality need further investigation. In Arizona, the Round-tailed Horned Lizard is a species of Chihuahuan desertscrub and semi-desert grassland, primarily in areas of pebbly bajadas, but also lower montane slopes and foothills at elevations of about 915 to 1675 m.

The Round-tailed Horned Lizard highly depends on concealment to avoid predators. When threatened, it tends to flattens itself and sit in place, not running away or showing aggressive responses (Bundy and Ness 1958). They can hunch their backs to closely resemble stones (Sherbrooke and Montanucci 1988, Cooper and Sherbrooke 2012). Flight initiation distances (distance from a predator when the lizard begins to run away) were shorter on rocky substrates than on sand, suggesting the lizards recognized they were more visible on sand (Cooper and Sherbrooke 2010). These lizards also prefer areas with small stones, rather than open, sandy areas (Cooper and Sherbrooke 2012).

Activity is mostly diurnal, in the morning and afternoon. Pianka and Parker (1975) reported that the mean body temperature of active lizards is 36.50 C. In New Mexico, when surface temperatures exceeded 400 C, the lizards restricted their activity to shady areas under or adjacent to shrubs (Shaffer and Whitford 1981). In southeastern New Mexico, densities of Round-tailed Horned Lizards were among the lowest of any lizard species in that Chihuahuan Desert community. However early summer densities were greater in a creosote bajada (up to about 10/ha) than in a mesquite/yucca desert grassland playa (less than 5/ha, Whitford and Creusere 1977). In an area of sand dunes in Chihuahua, Round-tailed Horned Lizards were most frequently encountered in bare areas, which is also where nests of their primary prey – ants – were most common (García-De la Peña et al. 2012).

Howard (1974) reported on reproduction in Round-tailed Horned Lizards from Arizona and Sonora. Males were reproductively competent from late April into mid-July. The smallest mature males and females were 42 and 43 mm SVL, respectively. Females with oviductal eggs were found from May through August, and there was evidence of two clutches per season. Mean clutch size was 11.2. Hatchlings were found from early July to 20 October, and most probably reached maturity the following year. Baur (1979) described mating behavior in captive Round-tailed Horned Lizards.

Goldberg (2012) examined reproduction in Round-tailed Horned Lizards from New Mexico. The smallest mature male and female were 37 and 44 mm SVL, respectively. Females were reproductively active from April into July, and clutch size ranged from five to 13 with a mean of 8.6. Two or more clutches can be produced each season. In a sample from New Mexico and Texas, Pianka and Parker (1975) reported a mean clutch size of 10.6, with clutches laid in May and June. Maximum clutch size is 19 (Hodges 2009).

Pianka and Parker (1975) found ants made up 86.1% (by frequency) and 66.1% (by volume) of the diet of Round-tailed Horned Lizards. On the Jornada Experimental Station, New Mexico, the primary prey was Honey Pot Ants (Myrmecocystus spp.), but they also fed upon other types of ants, other Hymenoptera, true bugs, beetles, and other insects (Shaffer and Whitford 1981). Based on analysis of stomach contents from 12 Round-tailed Horned Lizards collected at Samalayuca, Chihuahua, Lemos-Espinal et al. (2004) found that the diet was dominated by ants, particularly Camponotus sp., Octostruma sp., and Messor sp. A variety of other insects were found, as well as lizard skin and nematodes, but they contributed relatively little numerically and volumetrically to the diet. A variety of animals prey upon the Round-tailed Horned Lizard. Some species of horned lizards squirt blood at predators from the orbital sinuses around the eyes, but the Round-tailed Horned Lizard rarely exhibits this behavior (Sherbrooke 2003).

The Round-tailed Horned Lizard is a species of least concern on the IUCN Red List. In Arizona, with a valid hunting license, four can be taken per year or held in possession, alive or dead, except that collection is prohibited in protected areas, such as National Park Service units, without special permits. Collections from the southern Sulphur Springs Valley in the 1950s, as well as a 1950s collection near Globe (although the latter should be investigated for validity) without later records for those areas could indicate declines in Arizona. This is also a species that could be vulnerable to climate change. Lara-Resendez et al. (2015) predicted the effects of climate change on the Round-tailed Horned Lizard at three sites in Chihuahua. Because high temperatures restrict activity periods, they estimated the species will become extinct at 32% of sites by 2050 and 60% by 2080.

Round-tailed Horned Lizard (Phrynosoma modestum)

Description

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