Sonoran Shovel-nosed Snake – Tucson Herpetological Society

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Sonoran Shovel-nosed Snake (Chionactis palarostris)

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Description

The Sonoran Shovel-nosed Snake (Chionactis palarostris) is a colorfully tri-colored snake known in Arizona almost exclusively from Organ Pipe Cactus National Monument (NM). It is small (< 432 mm TL), and the tail length is 18.5-24.6% of TL in males and 16-20.7% in females. The dorsal pattern consists of a light-colored snout, a wide black hood on the head between and behind the eyes, the anterior margin of which is straight or nearly so, followed posteriorly by 13-22 (usually fewer than 21) black dorsal bands on the body, and 4-5 black bands on the tail that mostly encircle the body and tail. Between the black bands are red or red-orange saddles that do not cross the venter, and those saddles are separated from the black bands by cream or yellow background coloration (Rosen et al. 1996, Ernst and Ernst 2003).

The jaw is countersunk, an adaptation for burrowing, and the snout is relatively blunt. Dorsal scales are smooth and arranged in 15 rows at mid-body. There are 139-161 ventral scales and 42-50 divided subcaudals. The anal plate is divided. On the head, the internasals are in contact, and there is one loreal scale on each side. There are usually 7 supralabials and 7 infralabials. Males and females do not differ in TL, but males have, on average, longer tails proportionally (see above) as well as greater numbers of subcaudal scales. Males have 13-16 black body bands, whereas females have 18-22 (Ernst and Ernst 2003). Juveniles resemble adults. The smallest individual reported by Rosen et al. (1996) was about 150 mm SVL, which they surmised was about six months of age. Klauber (1937) described the Sonoran Shovel-nosed Snake (as Sonora palarostris) as a species distinct from the similar Sonora (=Chionactis) occipitalis. Two subspecies of C. palarostris are recognized; C.p. organica occurs in the 100-Mile Circle (Liner and Casa-Andreu 2008, Crother et al. 2012).

The Sonoran Shovel-nosed Snake could be mistaken for several other colorful, small banded snakes. The Western Shovel-nosed Snake (Chionactis annulata) has a black, crescent-shaped mark atop the head between and posterior to the eyes with a concave anterior edge, 17-40 (usually more than 21) black dorsal body bands, and 5-13 black bands on the tail. The Variable Sandsnake (Chilomeniscus stramineus) is comparatively stout, the rostral scale separates the internasals, and in the banded form found in Arizona, the yellow, orange or red-orange bands are in contact with the black bands. The snout of the Sonoran Coralsnake (Micruroides euryxanthus) is black, all bands encircle the body, and the tail lacks red bands. Banded forms of the Western Groundsnake (Sonora semiannulata) have red or orange bands that contact the black bands and a dark blotch on each scale in the lateral portions of the lighter bands. The Long-nosed Snake (Rhinocheilus lecontei) grows to a larger size (< 1520 mm TL), the dorsum is often speckled as well as banded, the rostral scale is enlarged, the anal plate is undivided, and most of the subcaudals are undivided.

In the 100-Mile Circle, the Sonoran Shovel-nosed Snake is strictly a species of the Arizona Upland subdivision of Sonoran desertscrub; in Arizona it is known only from the western bajada of the Ajo Mountains. Hensley (1950) was the first to report the species from Arizona and the United States. Also in the Circle, it was collected in 1966 from “1 mile SE of Sonoyta” (LACM 25897). In Sonora, it occurs in Sonoran desertscrub as far south as Guaymas. Its distribution extends marginally into foothills thornscrub about 35 km east of Hermosillo and northward to almost Magdalena. Three snakes labelled as this species were collected in Sonora from 22.5 miles east of San Luis Río Colorado (MCZ R-96813-15), presumably along Hwy 2; however, that locality is well west of the Gila and Tinajas Altas mountains in an area where only Chionactis annulata has been collected in nearby Arizona, suggesting the specimens are misidentified.

At Organ Pipe Cactus NM and vicinity, it has been found on Hwy 85 from 0.25 mile north of the international boundary to 0.37 mile north of the Park boundary. In contrast, the Western Shovel-nosed Snake was not found on Hwy 85 in the Park except north of 23 miles north of the international boundary (Hensley 1950, Rosen et al. 1996). The latter authors only found Sonoran Shovel-nosed Snakes on Hwy 85, plus one in a funnel trap 1.5 mile west of Hwy 85. They reported that a Park Service employee found one on the north leg of Puerto Blanco Drive. Rosen et al. (1996) failed to find Sonoran Shovel-nosed Snakes while walking through apparently suitable habitat after dark, turning rocks, boards and other surface debris, and driving roads other than Hwy 85, despite the fact that many other snakes of other species were found during such searches. The Sonoran Shovel-nosed Snake was the third most encountered snake on Hwy 85 from Lukeville to Why during 1988-1991 (Rosen and Lowe 1994). Although based on a smaller sample size, in that area, its relative abundance was similar in 1949 (Hensley 1950).

Lowe (1964), Rosen et al. (1996), and Mahrdt et al. (2001) describe the habitat of the Sonoran Shovel-nosed Snake as higher on the bajada and in coarser, more gravelly and rockier soils than the Western Shovel-nosed Snake. Rosen et al. (1996) describe the Sonoran Shovel-nosed Snake habitat as high on the middle bajada to the uppermost bajada of the Ajo Mountains amidst hills and low mountains. These areas have very fine sandy loam soils overlain with desert pavement and sometimes larger rocks. Along Hwy 85, specific localities were often at wash crossings, especially in dry years. AGFD (2003) reports the elevational range in Arizona as 430-695 m. In Sonora it has been documented from about sea level to 830 m.

Records in Arizona run from 9 April to 23 September, but most are found from early April to mid July (Rosen et al. 1996). In Sonora, there are also collections from February, March, October, and December. Most activity occurs from 1900 to 2100 hours, but the individual on Puerto Blanco Drive was observed at 1000 (Rosen et al. 1996) and Klauber (1951) reported an individual from near Magdalena, Sonora, collected at 0700 on 11 April.

The reproductive behavior of this species is not well studied. However, at Organ Pipe Cactus NM, road-kill snakes were found with four and five eggs (Rosen et al. 1996) in May. Both Klauber (1951) and Hensley (1950) reported single individuals with 4 eggs each from Arizona. Goldberg and Rosen (1999) calculated mean clutch size as 4.77. Eggs are probably laid in late May to June and hatch about two months later. Based on 24 specimens from Arizona and Sonora, the smallest sexually mature male and female were 208 and 248 mm SVL, respectively (Goldberg and Rosen 1999). Sperm was found in the vasa deferentia of males from May into July, which defines the breeding season (Goldberg and Rosen 1999).

Klauber (1951) reported an individual collected from near Magdalena that had a spider in its stomach. At Organ Pipe Cactus NM, stomachs of Sonoran Shovel-nosed Snakes contained scorpions, a spider, one cricket, one unidentified insect and five unidentified arthropods. A captive readily ate centipedes (Rosen et al. 1996). Like its close relative, the Western Shovel-nosed Snake, this species possesses enlarged, grooved teeth in the rear of the upper jaw that may be used to deliver toxins to its prey, but the snake is harmless to humans. Rosen et al. (1996) captured a Kingsnake (presumably Lampropeltis californiae) on Hwy 85 that regurgitated a Sonoran Shovel-nosed Snake.

Collection of the Sonoran Shovel-nosed Snake is prohibited in Organ Pipe Cactus NM without specific permits. Outside the Park, in Arizona, four may be collected or held in possession, alive or dead, with a valid AGFD hunting license. This snake is considered a species of least concern on the 2015 IUCN Red List. However, 32 of 46 Sonoran Shovel-nosed Snakes found on Hwy 85 from 1988-1991 were dead (Rosen and Lowe 1994), illustrating the threat posed by highway traffic through the apparently limited distribution of this species in the United States. That said, the relative abundance of this snake, as enumerated by encounters on Hwy 85 relative to other snake species, was essentially unchanged from 1949 to 1991 (Hensley 1950, Rosen and Lowe 1994).

Sonoran Shovel-nosed Snake (Chionactis palarostris)

Description

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