Crotalus scutulatus is species of rattlesnake, a venomous pit viper in the family Viperidae.
The species is known commonly as the Mohave Rattlesnake. Other common English names include Mojave Rattlesnake Stebbins, R.C. (2003). A Field Guide to Western Reptiles and Amphibians. Boston: Houghton Mifflin & Co. pp. 416–417. Campbell JA, Lamar WW (2004). The Venomous Reptiles of the Western Hemisphere. Ithaca and London: Comstock Publishing Associates. 870 pp., 1,500 plates. . and, referring specifically to the nominate (northern) subspecies: Northern Mohave Rattlesnake and Mojave Green Rattlesnake, the latter name commonly shortened to the more colloquial "Mojave green". Campbell and Lamar (2004) supported the English name "Mohave (Mojave) rattlesnake" with some reluctance because so little of the snake's range lies within the Mojave Desert. The spelling of the English name with an "h" has been advocated by multiple authors in recent years for various reasons. The most recent iteration of standard English names for North American reptiles, endorsed by the major herpetological societies in the United States and Canada, concludes that spelling with either a "j" or an "h" is correct, based on "whether the word is used in a Spanish or English context." Thus, their standard English names list adopted the "h" spelling.
Crotalus scutulatus is a highly venomous pitviper (family Viperidae, subfamily Crotalinae) found in the deserts of the southwestern United States and deep into mainland Mexico. It is perhaps best known for producing two distinctly different venom types in different populations.
Two subspecies are currently recognized. This account describes the widely distributed nominate subspecies, the Northern Mohave Rattlesnake, Crotalus scutulatus scutulatus. The other subspecies, C. scutulatus salvini, occurs in a relatively small area deep in mainland Mexico.
Type specimen and type locality The type specimen (holotype) of Crotalus scutulatus is ANSP 7069, in the collection of the Philadelphia Academy of Natural Sciences (formerly one of two specimens of USNM 5027 at the Smithsonian Institution). The type locality is "Fort Buchanan, Arizona". The ruins of Fort Buchanan are in present day Santa Cruz County, Arizona.
Description Crotalus scutulatus grows to an average total length (tail included) of less than , with a maximum total length of 123.6 cm (4.1 ft) for males and 92.2 cm (3.0 ft) for females.
There is no single visual trait that reliably identifies C. scutulatus and the most reliable visual identifications result from careful consideration of multiple traits. C. scutulatus is broadly sympatric with C. atrox (the Western Diamond-backed Rattlesnake), which it closely resembles. The dorsal color of C. scutulatus varies from shades of green to browns, grays, and even yellowish. C. scutulatus has a row of large diamond-shaped dorsal blotches closely resembling C. atrox but lacking the abundant dark speckling, both coarse and fine, found throughout the dorsal surfaces of C. atrox. Additionally, C. scutulatus lacks the white margins along the caudal edges of the dorsal "diamonds" that are found in most C. atrox. The postocular light facial stripe usually bends toward the neck and does not intersect with the mouth in C. scutulatus, as it does in C. atrox. The tail is usually marked with alternating pale and dark rings in both species, with the dark rings often (but not always) being narrower than the pale ones in C. scutulatus and the colors are usually less distinct than the bright white and dark black caudal rings of most C. atrox. The proximal rattle segment contains live tissue and is usually bicolor – yellow and black, or entirely yellow, in C. scutulatus, but entirely black (sometimes with a brush of white) in C. atrox.
The crown scales (between the supraoculars) of C. scutulatus are relatively large compared to other rattlesnakes like C. atrox (but see C. molossus and C. ornatus), and the enlarged scales spill out behind the supraocular scales in a fan shape, usually with a well-defined margin. The minimum number of scales separating the supraoculars varies from two to four in C. scutulatus. Crown scales on C. atrox are usually smaller, more numerous, and they do not produce the well-defined fan where they integrate into the scales behind the crown. Comparison of the most significant distinguishing traits between C. scutulatus and other commonly-encountered sympatric rattlesnakes. Adapted from Cardwell et al. 2022TraitCrotalus scutulatusCrotalus atroxCrotalus molossus*Crotalus viridisGreenish colorSometimesNoSometimesOftenPattern specklingLittle or noneObvious, course and fineLittle or noneLittle or noneTail color & caudal ringsDark gray, black, or brown rings on pale gray or white backgroundHigh-contrast black rings on white backgroundTail uniformly black, gray, or dark brown, with occasional faint pale ringsNarrow dark and pale rings, same colors as dorsum, with little or no whitish colorProximal rattle segmentYellow or bicolor (yellow and black)Black with occasional brush of whiteBlackBlackPale postocular stripePasses above the mouthIntersects with mouthAbsentPasses above the mouthCrown scalesLarge and irregular, spilling out onto parietal areaSmall and granular, indistinct from parietal areaLarge and square anteriorly, others small and uniformSmall and granular, indistinct from parietal area* Same traits as C. ornatus
Geographic range
Crotalus scutulatus is found in arid habitats in the southwestern United States, from the Mohave Desert in California's Los Angeles and San Bernardino Counties, across most of western and southern Arizona (southwest of the Mogollon Rim), and from El Paso County south through the Big Bend region of western Texas. It also occurs as far north as Lincoln County in Nevada, the southwest corner of Washington County in Utah, and portions of extreme southern New Mexico. In Mexico, it is found in Sonora, Chihuahua, and south on the Mexican Plateau to the states of Mexico, Puebla, and Veracruz. It is found in deserts and other areas with xeric vegetation from near sea level to about elevation.
Habitat Crotalus scutulatus is primarily an inhabitant of broad desert valleys or lower mountain slopes, C. scutulatus is often found in sparsely vegetated areas containing predominantly creosote (Larrea), sage (Ambrosia), mesquite (Prosopis), various cacti (Cactaceae), and Joshua trees (Yucca brevifolia), as well as juniper (Juniperus) woodland and grassland (Poaceae) habitats in some places. In general, C. scutulatus tends to avoid densely vegetated and extremely rocky areas, preferring relatively flat, open, and xeric habitats.
Conservation status Crotalus scutulatus is classified as Least Concern (LC) on the IUCN Red List of Threatened Species (v 3.1, 2001). Species are listed as such due to their wide distribution, presumed large population, or because they are unlikely to be declining fast enough to qualify for listing in a more threatened category. The population trend was stable when assessed in 2007.
Behavior
Crotalus scutulatus is most active from April to September. They are ambush predators, eating mostly small rodents and lizards. Courtship begins in late summer/fall, is interrupted by winter, and resumes in the spring. Females bear live young, from two to 17 (average about eight), from July through September.Klauber LM (1997). Rattlesnakes: Their Habitats, Life Histories, and Influence on Mankind. Second Edition. First printing in 1972. Berkeley: University of California Press. .
This species is not known to den communally and they have no need to seasonally migrate between winter dens and summer foraging areas, as do some other species living at higher elevations and higher latitudes. Instead, individuals occupy well defined home ranges year around, taking shelter during the winter and hot summer weather in burrows excavated by animals like rodents, tortoises, and kit foxes.
Like other rattlesnakes, this species is routinely preyed upon by larger predators like coyotes, bobcats, and raptors. As a result, it is shy, cryptic, and does not seek out confrontations with larger creatures, including humans. But like other rattlesnake species, it will strike and bite vigorously when disturbed, especially if surprised or when there is no nearby vegetation or burrow into which the snake can escape.
Recent genetic and morphological analyses Robust genetic analyses have revealed the population structure of Crotalus scutulatus throughout the species' range, correlating genetic evidence of isolation and subsequent secondary contact of subpopulations with corresponding geologic and climatic events. As a result, four genetically distinct clades among present-day C. scutulatus have been described.
The earliest split occurred at the northern margin of the Central Mexican Plateau about 4.1 million years ago (MYA), separating the species into northern and southern subpopulations. Then about 1.8 MYA, the subpopulation now identified as C. scutulatus salvini diverged genetically from the animals on the Central Mexican Plateau. Most recently, the northern subpopulation was divided at the Continental Divide (Cochise Filter) about 1.5 MYA, creating the Mojave-Sonoran clade to the west and the Chihuahuan clade to the southeast. Note that the boundaries between these clades correspond to elevational clines where climatic shifts during Pleistocene glacial advances and retreats likely isolated the subpopulations during cold periods but allowed secondary contact and resumption of gene flow during warmer periods, including the present.
These analyses indicate that the Central Mexican Plateau clade is more closely related (i.e., more recently shared a common ancestor) to the Huamantlan Rattlesnake (currently C. scutulatus salvini) than it is to the northern (Mojave-Sonoran and Chihuahuan) clades, suggesting that the designation of salvini as a subspecies of all other C. scutulatus is problematic.
More recently, qualitative, meristic, and morphometric traits from 347 specimens of C. scutulatus were analyzed, producing the conclusion that the species "is phenotypically cohesive without discrete subgroups, and that morphology follows a continuous cline in primary color pattern and meristic traits across the major axis of its expansive distribution," suggesting that "multiple episodes of isolation and secondary contact among metapopulations during the Pleistocene were sufficient to produce distinctive genetic populations, which have since experienced gene flow to produce clinal variation in phenotypes without discrete or diagnosable distinctions among these original populations." It was recommended that, for taxonomic purposes, Crotalus scutulatus "be retained as a single species, although it is possible that C. s. salvini, which is morphologically the most distinctive population, could represent a peripheral isolate in the initial stages of speciation."
Venom
History For decades, the bite of Crotalus scutulatus has been considered to be extraordinarily deadly, often described as "the" (or "one of the") deadliest or most dangerous rattlesnakes. For example: "the most lethal of the North American rattlesnake venoms"; "one of the most lethal venoms among the world's reptiles"; "an extremely dangerous snake"; "probably the most dangerous snake in the United States"; and "considered among the most venomous snakes on Earth".