Thornscrub Hook-nosed Snake – Tucson Herpetological Society

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Thornscrub Hook-nosed Snake (Gyalopion quadrangulare)

[/vc_column_text][gap size=”12px” id=”” class=”” style=””][/vc_column][/vc_row][vc_row][vc_column width=”1/2″][vc_single_image image=”2125″ img_size=”large” alignment=”center” style=”vc_box_rounded”][vc_column_text]Thornscrub Hook-nosed Snake, Santa Cruz 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=”2490″ img_size=”medium” alignment=”center” onclick=”img_link_large”][vc_column_text]Thornscrub Hook-nosed Snake, Sonora, MX. 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 Thornscrub Hook-nosed Snake (Gyalopion quadrangulare) is a subtropical species that just barely enters Arizona in southern Santa Cruz County. It is small (< 354 mm total length [TL]), relatively heavy-bodied, and the tail is short (12-15% of TL). The slightly concave rostral scale has a sharp-edged, upturned tip that is in contact with the prefrontal scales, separating the small internasals. The loreal scale is usually absent (a single loreal per side in some individuals from Guaymas south). The smooth dorsal scales are arranged in usually 17 rows at mid-body (16 in some Mexican specimens), the subcaudals are divided, and the anal plate is undivided (one specimen from Sonora had a partially divided anal plate). The dorsal pattern on the body and tail is 19-55 black, rectangular blotches on a background of cream, white, or grayish-white in the mid-dorsal region and orange on the sides. The dark blotches usually narrow on the sides and become infused with light spots. The venter is cream to greenish-yellow, and usually unpatterned in our area except for some dark spots on the subcaudals (Hardy and McDiarmid 1969, Ernst and Ernst 2003). Greer (1965) and Hardy and McDiarmid (1969) describe variation throughout the range of the species; some characters show clinal variation from north to south; however, no subspecies are currently recognized (Liner and Casa-Andreu 2008, Crother 2012).

The Thornscrub Hook-nosed Snake might be confused with small Long-nosed Snakes (Rhinocheilus lecontei, see the photo gallery for that species), Variable Sandsnakes (Chilomensicus stramineus), or banded forms of the Western Groundsnake (Sonora semiannulata). However, none of those snakes have a sharp-edged, upturned snout. Most of the subcaudals on the Long-nosed Snake are undivided, the Variable Sandsnake has a divided anal plate and usually 13 dorsal scale rows at mid-body, and the Western Groundsnake has a loreal scale on each side, the internasals are in contact, and dorsal scale rows at mid-body number 14-16.

This species has a very limited distribution in Arizona and the United States. In Arizona, records exist for 3.7 km north of Patagonia on Hwy 82, west to the vicinity of Peña Blanca Canyon, south to the city of Nogales, and north to 8 km north of Ruby Road on old Hwy 89 (replaced by Interstate 19). A specimen from near Salome in La Paz County (Trescott 1994) is believed to be in error (A. Holycross, pers. comm.). A single specimen with imprecise locality data exists for the Sonora side of the Circle: west of Cananea, probably by 8-16 km on Hwy 2. In the Circle, the Thornscrub Hook-nosed Snake occurs primarily in rolling terrain vegetated with mesquite-invaded semi-desert grassland, but also upslope into adjacent oak savanna at elevations of about 1035 to 1338 m. It probably occurs in foothills thornscrub in the southern portions of the Circle. The species is found from extreme south-central Arizona south through the foothills of east-central Sonora to Sinaloa and Nayarit.

In Arizona, these snakes are almost always taken at night on roads. However, two were found in late afternoon. In Sinaloa, one was found under a rock and another was floating on a stick after a morning rain (Hardy and McDiarmid 1969). Records in the Circle span the period from mid-June to 11 November, but most are found during the summer rainy season. In Sinaloa, this snake was never found when air temperatures were below 27⁰ C (Hardy and McDiarmid 1969). This is one of the most difficult snakes to find in the 100-Mile Circle. It was not found in Arizona until 1959 (Woodin 1962).

Virtually nothing is known of this snake’s reproductive behavior. Females with two and six eggs were found in Sinaloa; those clutches would have likely been laid in July (Greer 1966). Babb et al. (2005) examined stomachs of 50 specimens of Thornscrub Hook-nosed Snake from throughout the range of the species. Nine insects (crickets and grasshoppers were identified), nine spiders, and eight scorpions were found, plus three unidentifiable items. Stomachs of three specimens from Sonora all contained spiders in the genus Pachylomerides (Bogert and Oliver 1945). Woodin (1962) fed captives spiders and a small centipede. This snake has enlarged, grooved teeth in the rear of the upper jaw that may deliver toxins to prey items. Tanikawa et al. (2016) found a Thornscrub Hook-nosed Snake in the stomach of a Forrer’s Leopard Frog from southwestern Sonora or northwestern Sinaloa.

The Thornscrub Hook-nosed Snake is listed as a species of least concern on the 2014 IUCN Red List. With a valid Arizona hunting license, four can be collected per year or held in possession, alive or dead. There is no reason to believe the species is declining in the 100-Mile Circle, but the paucity of records precludes any analysis of population trend.

Thornscrub Hook-nosed Snake (Gyalopion quadrangulare)

Description

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