Twin-spotted Rattlesnake – Tucson Herpetological Society

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Twin-spotted Rattlesnake (Crotalus pricei)

[/vc_column_text][gap size=”12px” id=”” class=”” style=””][/vc_column][/vc_row][vc_row][vc_column width=”1/2″][vc_single_image image=”2155″ img_size=”large” alignment=”center” style=”vc_box_rounded”][vc_column_text]Photo by Tom Brennan[/vc_column_text][/vc_column][vc_column width=”1/2″][vc_row_inner][vc_column_inner width=”1/2″][vc_single_image image=”2769″ img_size=”medium” alignment=”center” onclick=”img_link_large”][vc_column_text]Photo by Cecil Schwalbe[/vc_column_text][gap size=”12px” id=”” class=”” style=””][/vc_column_inner][vc_column_inner width=”1/2″][vc_single_image image=”2770″ img_size=”medium” alignment=”center” style=”vc_box_rounded” onclick=”img_link_large”][vc_column_text]Twin-spotted Rattlesnake, Chiricahua Mtns, AZ. Photo by Jim Rorabaugh[/vc_column_text][gap size=”12px” id=”” class=”” style=””][/vc_column_inner][/vc_row_inner][vc_row_inner][vc_column_inner width=”1/2″][vc_single_image image=”2771″ img_size=”medium” alignment=”center” style=”vc_box_rounded” onclick=”img_link_large”][vc_column_text]Twin-spotted Rattlesnake, in-situ on the Rim Trail, Huachuca Mountains. 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=”2772″ img_size=”medium” alignment=”center” style=”vc_box_rounded” onclick=”img_link_large”][vc_column_text]Twin-spotted Rattlesnake, Chiricahua Mtns, AZ. Photo by Jim Rorabaugh[/vc_column_text][gap size=”12px” id=”” class=”” style=””][/vc_column_inner][/vc_row_inner][vc_row_inner][vc_column_inner width=”1/2″][vc_single_image image=”2773″ img_size=”medium” alignment=”center” style=”vc_box_rounded” onclick=”img_link_large”][vc_column_text]Twin-spotted Rattlesnake, Sierra Madre Occidental, Durango, Mexico. Photo by Jim Rorabaugh[/vc_column_text][/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 Twin-spotted Rattlesnake (Crotalus pricei) is relatively small (< 660 mm total length, Stebbins [2003]) and slender for a pitviper. In our area the dorsum is typically gray to pale brown with two longitudinal rows of 39-64 dark brown to black spots on the body, many of which may be fused across the back, forming irregular dark crossbands, particularly towards the rear of the body. The tail has only crossbands. Smaller dark spots are scattered down the sides, and a dark stripe runs from the eye back and down past the corner of the mouth. On rare occasions, Twin-spotted Rattlesnakes have been found in Arizona that lack spots, crossbands, and other dorsal patterning (Prival 2016). The basal segment of the rattle is orange in all but neonates. The throat is gray to salmon or salmon-pink. The rest of the venter is gray, and the posterior ventral scales contain dark mottling.

Dorsal scales are keeled and occur in 22-23 (usually 21) rows at mid-body. The species is the only rattlesnake to have a first supralabial that curves upward behind the postnasal scale to contact a small prefoveal scale that lies between the postnasal and the one or two loreals on each side. Males have tails that are 6.6-11.0% of total length, whereas females have tails that are 4.3-10.8% of total length. Males also average more subcaudal scales and grow to a slightly larger size than females. Two subspecies are recognized. Only Crotalus pricei pricei occurs north of Mexico.

The Twin-spotted Rattlesnake is unlikely to be confused with any other rattlesnake in Arizona or the 100-Mile Circle. It is the only rattlesnake with two longitudinal rows of dark spots on the dorsum. At the high elevations it inhabits, the only other rattlesnakes likely to be found are the Black-tailed, Ridge-nosed, and Rock Rattlesnakes. Although, within a few hundred miles of the border in the Sierra Madre Occidental and adjacent ranges are two non-venomous snakes, Storeria storeriodes and Conopsis nasus, with dorsal patterns and behaviors that appear to mimic the Twin-Spotted Rattlesnake (Lemos-Espinal and Smith 2007). Nightsnakes (Hypsiglena) can have two rows of dark spots on the dorsum, but lack rattles and are rare or absent at the high elevations where Twin-Spotted Rattlesnakes occur. The Terrestrial Gartersnake (Thamnophis elegans), which occurs at high elevation in the Pinaleño Mountains, as well as in the mountains of Mexico with Twin-Spotted Rattlesnakes, has dorsal spotting, but it also has mid-dorsal and lateral light stripes and lacks rattles.

In Arizona and the United States, Twin-spotted Rattlesnakes are only known from the Santa Rita, Huachuca, Chiricahua, and Pinaleño Mountains at elevations of 1860-3250 m (Prival 2016). Lowe et al. (1986) speculated that the area of greatest abundance occurs between 2440 and 2745 m in Arizona. Those authors also noted an unconfirmed sighting in the Dos Cabezas Mountains. BLM livestock grazing permittees in that area are very possessive of their leased public lands and actively discourage visitation, making herpetological work in the Dos Cabezas difficult; however, Dave Prival has visited that range to the highest elevations, did not find a Twin-spotted Rattlesnake, and believed the habitat was not particularly good for the species (Prival 2001). In Mexico, C. pricei pricei occurs in the Sierra Madre Occidental and adjacent ranges in Sonora and Chihuahua. In Sonora, the species is only known from the Sierras Los Ajos and El Tigre (Rorabaugh and Lemos-Espinal 2016). Crotalus pricei miquihuanas, the other subspecies, occurs in the Sierra Madre Oriental of Coahuila, Nuevo León, and Tamaulipas. Twin-spotted Rattlesnakes also occur in Durango, Jalisco, and Aguascalientes, which geographically places them with other Sierra Madrean populations (C. p. pricei), but snakes from Jalisco and Durango were more closely related to C. p. miquihuanas from the Sierra Madre Oriental than snakes from Chihuahua or Arizona (Bryson et al. 2011). Hence, the subspecific taxonomic placement of snakes from these three Mexican states is uncertain. The species may have been overlooked in the mountains of eastern Sinaloa. Elevational range in Mexico is 2135-3200 m (Armstrong and Murphy 1979, Lemos-Espinal and Smith 2007).

In Arizona, the Twin-spotted Rattlesnake occurs in four very isolated mountain tops. Presumably the species was more widespread in much cooler and wetter times, and became isolated in these high elevation forests when the climate warmed. But, not only is the Twin-spotted Rattlesnake a species of high elevation forests, it is almost always associated with rocks – either talus or rock outcrops. The intervening valleys between the four Arizona mountain ranges where the species occurs have no talus slopes and only few or scattered rock outcrops. So how did the Twin-spotted Rattlesnake traverse these valleys, even if conifers grew there and the climate was much cooler at some time in the past? Were there more rocks in the valleys at that time? Or perhaps the snake was less of a talus or outcrop specialist then?

In Arizona, Twin-spotted Rattlesnakes occupy pine-oak woodland, mixed conifer, and spruce-fir forest. They are most often found on talus slopes and rock outcrops; however, in the Chiricahua Mountains, male snakes spent considerably more time in wooded areas near a talus slope than on the talus itself in a dry monsoon year as compared to the previous wet summer. This activity away from the talus may have resulted from sunny, hot conditions on the talus that afforded better foraging conditions in the adjacent wooded area (Prival et al. 2002). This species is most active from March into November, but is most commonly encountered during the summer monsoon period from late June or early July into early September. They are occasionally found basking on quite cold days in December and January when snow is present on the ground, and are known to move short distances during the winter (Prival et al. 2003). Most activity is diurnal, but snakes are sometimes found coiled or active after dark. They are often heard before they are seen, and on talus slopes, will quickly disappear into the rocks when disturbed.

Prival et al. (2002, 2012) found that in the Chiricahua Mountains mating occurs in August and early September. Females apparently store sperm over winter, and litters of 1 to 9 (mean = 4) young are born in July or August, although births have been reported in late May, June, and September in captivity. In Prival’s study area, an average of 44.7% of adult females was gravid each year. There was no solid evidence that females attend to the young. The smallest reproductively active individuals reported by Goldberg (2000) were 330 mm (female) and 322 mm (male) SVL, although Klauber (1972) noted a pregnant female 301 mm TL. The smallest neonate reported in Arizona was 150 mm TL (Prival 2016).

Fecal samples from the Chiricahua and Pinaleño Mountains were examined by Prival et al. (2012). Lizards (all Sceloporus) were found in 77.5%, mammal hairs in 21.3%, bird feathers in 3.8%, and Crotalus pricei remains in 1.3% of the fecal samples. Yarrow’s Spiny Lizard (Sceloporus jarrovi) is a common inhabitant of talus slopes and rock outcrops where Twin-spotted Rattlesnakes occur, and is an important part of the diet. Juvenile Twin-spotted Rattlesnakes are not known to wave their tails (caudal luring) to attract prey.

Adults have fangs 2.0-3.3 mm in length. The venom is apparently quite toxic, but venom yield is low. Human envenomations resulted in more serious systematic symptoms than was expected from such a small snake (Minton and Weinstein 1984). Any bite from a Twin-spotted Rattlesnake should be considered life-threatening and immediate medical attention should be sought.

The Twin-spotted Rattlesnake is listed as a species of least concern on the 2015 IUCN’s Red List. However, collection in Arizona is prohibited without specific authorization from the Arizona Game and Fish Department. This species is subject to poaching by collectors. Prival et al. (2012) found a disproportionate number of small snakes at a site frequented by snake enthusiasts (and likely poachers) than at a site infrequently visited, suggesting collection pressure was influencing population structure. However, they found no evidence for a declining population at either site over a 13-year period. Prival and Schroff (2012) estimated 1000 to 4000 Twin-spotted Rattlesnakes reside in Arizona. As this is a snake of the highest elevations in the sky islands, it could be affected by climate change and associated increased wildfires. High elevation forests in all of the Arizona mountain ranges occupied by the Twin-spotted Rattlesnake have been decimated by fire and insect damage over the last three decades.

Twin-spotted Rattlesnake (Crotalus pricei)

Description

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