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Black-tailed Rattlesnake (Crotalus molossus)

 

[/vc_column_text][gap size=”12px” id=”” class=”” style=””][/vc_column][/vc_row][vc_row][vc_column width=”1/2″][vc_single_image image=”2148″ img_size=”large” alignment=”center” style=”vc_box_rounded”][vc_column_text]Black-tailed Rattlesnake. Sonora. This snake is stretching its jaws after having its head pinned. 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=”2671″ img_size=”medium” alignment=”center” onclick=”img_link_large”][vc_column_text]Photo by Roger Repp[/vc_column_text][gap size=”12px” id=”” class=”” style=””][/vc_column_inner][vc_column_inner width=”1/2″][vc_single_image image=”2672″ img_size=”medium” alignment=”center” style=”vc_box_rounded” onclick=”img_link_large”][vc_column_text]Photo by Roger Repp[/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=”2673″ img_size=”medium” alignment=”center” style=”vc_box_rounded” onclick=”img_link_large”][vc_column_text]Black-tailed Rattlesnake. Photo by Young Cage[/vc_column_text][gap size=”12px” id=”” class=”” style=””][/vc_column_inner][vc_column_inner width=”1/2″][vc_single_image image=”2674″ img_size=”medium” alignment=”center” style=”vc_box_rounded” onclick=”img_link_large”][vc_column_text]Black-tailed Rattlesnake climbing in an oak ~ 1 meter off the ground. Chiricahua Mtns. 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=”2675″ img_size=”medium” alignment=”center” style=”vc_box_rounded” onclick=”img_link_large”][vc_column_text]Black-tailed Rattlesnake, Dragoon Mtns. Photo by Jim Rorabaugh[/vc_column_text][/vc_column_inner][vc_column_inner width=”1/2″][vc_single_image image=”2676″ img_size=”medium” alignment=”center” style=”vc_box_rounded” onclick=”img_link_large”][vc_column_text]Black-tailed Rattlesnake in a mesquite, Rancho El Aribabi, Sonora. Photo by Jim Rorabaugh[/vc_column_text][/vc_column_inner][/vc_row_inner][vc_row_inner][vc_column_inner width=”1/2″][vc_single_image image=”2678″ img_size=”medium” alignment=”center” style=”vc_box_rounded” onclick=”img_link_large”][vc_column_text]Black-tailed Rattlesnake, Photo by Young Cage[/vc_column_text][/vc_column_inner][vc_column_inner width=”1/2″][vc_single_image image=”2677″ img_size=”medium” alignment=”center” style=”vc_box_rounded” onclick=”img_link_large”][vc_column_text]Black-tailed Rattlesnake, Photo by Young Cage[/vc_column_text][/vc_column_inner][/vc_row_inner][vc_row_inner][vc_column_inner width=”1/2″][vc_single_image image=”2679″ img_size=”medium” alignment=”center” style=”vc_box_rounded” onclick=”img_link_large”][vc_column_text]Black-tailed Rattlesnake, Dragoon Mtns. Photo by Jim Rorabaugh[/vc_column_text][/vc_column_inner][vc_column_inner width=”1/2″][vc_single_image image=”2680″ img_size=”medium” alignment=”center” style=”vc_box_rounded” onclick=”img_link_large”][vc_column_text]Black-tailed Rattlesnake in a mesquite, Rancho El Aribabi, Sonora. Photo by Jim Rorabaugh[/vc_column_text][/vc_column_inner][/vc_row_inner][vc_row_inner][vc_column_inner width=”1/2″][vc_single_image image=”2681″ img_size=”medium” alignment=”center” style=”vc_box_rounded” onclick=”img_link_large”][vc_column_text]Black-tailed Rattlesnake with feathers stuck on its mouth, Cajon Bonito, Sonora. 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 Black-tailed Rattlesnake (Crotalus molossus) is a large (< 1524 mm total length, Platt and Rainwater 2009, but most are < 1000 mm TL in Arizona), heavy-bodied pit viper.  Atop the head is usually some darkening of the snout and normally a dark band or patch between the eyes, although some desert specimens lack that band. The dorsal pattern of the body is 20-41 light-edged, dark blotches or diamonds.  The anterior blotches and sometimes posterior blotches have some light interior scales, and many of the dark blotches coalesce with lateral blotches and become bands, especially towards the rear of the body.  The background dorsal color is greenish, gray or yellowish and each scale is unicolored.  The tail is typically a distinctive uniform black or dark gray in adults.  The infrequently observed neonates typically show some tail banding as do some adult snakes from the Sonoran Desert, which tend to be paler and grayer than snakes from the mountains.  Montane snakes have bold patterns and the background dorsal coloration is often yellow or yellowish green or yellowish brown.  The venter is cream to yellow sometimes with faint markings and a greenish tint posteriorly.

The dorsal scales are keeled and in 27 (23-31) rows at mid-body.  The anal plate is divided but the subcaudals are undivided.  The pupils are vertically elliptical, a heat-sensing pit lies between the eye and nostril, and there are prominent rattles on the tail.  Males average fewer ventral scales, more subcaudals, and have longer tails than the females (Ernst and Ernst 2003).  Three subspecies are recognized.  The Mexican Black-tailed Rattlesnake (Crotalus molossus nigrescens) and the Oaxacan Black-tailed Rattlesnake (C. m. muertensis) only occur in Mexico.  The Northern Black-tailed Rattlesnake (C. m. molossus) occurs in our area and elsewhere in the United States.  Rattlesnakes on Isla San Esteban in the Gulf of California, Sonora, have in the past been considered a subspecies of C. molossus, but are now regarded as a separate species (C. estebanensis) by most authors (Grismer 2002, Liner and Casas-Andreu 2008, Rorabaugh and Lemos-Espinal 2016).  The Black-tailed Rattlesnake is unlikely to be misidentified in the 100-Mile Circle.  The black or dark gray tail that lacks banding, or in which tail banding is faint, is distinctive.

Throughout its range, the Black-tailed Rattlesnake occurs from western Arizona east to central Texas and south to Oaxaca.  This species is ubiquitous in montane habitats in the 100-Mile Circle, occurring from the lower slopes of arid desert mountains ranges to at least 2661 m in the Pinaleño Mountains (Persons et al. 2016) and 2592 m in the Chiricahuas (Armstrong and Murphy 1979).  Degenhardt et al. (1996) report it occurs to 3150 m in New Mexico, and Ernst and Ernst (2003) list 3750 m as the high elevation record for the species.  In the Circle, Black-tailed Rattlesnakes occur from Sonoran desertscrub upslope through semi-desert grassland, Chihuahuan desertscrub, oak woodland, pine-oak woodland, mixed conifer forest, and possibly spruce-fir forest.  The author observed one individual in Plains grassland near Sonoita, and Black-tailed Rattlesnakes can be relatively abundant in riparian corridors through rugged terrain.  This species is almost always found on or close to rocky areas in mountains, foothills, and upper bajadas.  However, there are occasional records from valleys that may represent low-density populations or individuals dispersing from nearby mountains.   Black-tailed Rattlesnakes are accomplished climbers.  The snake in a mesquite pictured above was about 2.5 m up in the tree.

Two studies provide a window on Black-tailed Rattlesnake ecology in Arizona.  Dan Beck (1995) radio-tracked this species in the Tucson Mountains, and Harry Greene and Dave Hardy (Greene 1999, Hardy and Greene 1999, Green et al. 2002) radio-tracked Black-tailed Rattlesnakes in the Chiricahua Mountains around Portal.  Based on Beck’s 1995 work, in the Sonoran Desert of the Tucson Mountains, the species is most active from 1 March to 15 October, but snakes are not uncommonly observed through the winter, and even some foraging occurs in the winter months.  During summer, most snakes are crepuscular or nocturnal (most activity is from 1600-2200), but in winter, spring, and fall they are often encountered mid-day.   Most snakes are found on rocky slopes, but during late summer and fall they also use arroyos and creosote flats.  Hibernacula are amidst rocks or in crevices on southwest-facing rocky slopes. Overwintering snakes are solitary.  Beck (1995) calculated a mean home range of 3.49 ha, annual mean movement of 14.99 km, and mean body temperature of active snakes was 29.6⁰ C.  Two or three large meals sustained a snake for a year.

In the Chihuahuan desertscrub and oak savanna of the Chiricahuas, Black-tailed Rattlesnakes usually overwinter in solitary within rock outcrops high on canyon slopes.  They begin activity in late March or early April.  Prey consists mainly of woodrats, rabbits, and rock squirrels, and individuals hunt by following scent trails and then positioning themselves in ambush coils near runways or nest entrances.  Feeding activity extends into late November or early December, after which the snakes become dormant.  Mating occurs from late July to early September, sperm is stored overwinter, and females give birth to 2-6 young in July of the following year.  Females and young stay at the birth site from one day prior to five days after the neonate snakes shed their skin for the first time, or about 8-10 days after birth.  Females do not produce young every year (Greene 1999, Hardy and Green 1999, Green et al. 2002).  Males have been observed to engage in battles over mates (Green 1999, Hamilton and Wreidon 2004), although such behavior is infrequently documented (Persons et al. 2016).

Although not the norm in either the Tucson Mountains or the Chiricahuas, Hamilton and Nowak (2009) reported communal winter denning in Yavapai County where 2-8 Black-tailed Rattlesnakes congregated for the winter and shared their hibernacula with Arizona Black Rattlesnakes (Crotalus cerberus) and Sonoran Lyresnakes (Trimorphodon lambda).  Ernst and Ernst (2003) report that in addition to the prey items noted by Greene and his colleagues above, Black-tailed Rattlesnakes also take various other rodents, birds, lizards (including a Gila Monster), snakes, anurans, and insects.  The smallest reproductively active males and females examined by Goldberg (1999) were 576 and 653 mm SVL, respectively.  Throughout the species’ range, litter size ranges from 3-16.  Neonates are 229-316 total length (Armstrong and Murphy 1979, Lowe et al. 1986, Ernst and Ernst 2003, Persons et al. 2016).  Savary (1999) reported what appeared to be a female defending her young.  Enderson (1999) witnessed a predation attempt by a Sonoran Whipsnake (Coluber bilineatus) on a neonate Black-tailed Rattlesnake in the Whetstone Mountains.

Relative to other rattlesnakes, this species is generally considered to be somewhat docile, rarely rattling when approached or threatened.  The highly hemorrhagic venom is of moderate toxicity, but venom yield from a large snake can be substantial.  Any bite from one of these snakes should be considered life-threatening, and immediate medical attention should be sought.

The Black-tailed Rattlesnake is listed as a species of least concern on the 2014 IUCN Red List.  With a valid Arizona hunting license, four can be captured or held in possession, alive or dead.  There is no reason to believe the species is in decline in the 100-Mile Circle or elsewhere, however in the mountains of southeastern Arizona and northeastern Sonora, the woodland habitats of this species are increasingly at risk from climate change and wildfire.  Precisely how loss of woodlands might affect this species is unknown.

Suggested Reading:

Anderson, C.G. and E. Greenbaum 2012. Phylogeography of northern populations of the Black-tailed Rattlesnake (Crotalus molossus Baird And Girard, 1853), with the revalidation of C. ornatus Hallowell, 1854. Herpetological Monographs 26 (1):19-57.

Armstrong, B.L., and J.B. Murphy. 1979. The Natural History of Mexican Rattlesnakes. University of Kansas Museum of Natural History Special Publication 5:1-88.

Beck, D.D. 1995. Ecology and energetics of three sympatric rattlesnake species in the Sonoran Desert. Journal of Herpetology 29:211-223.

Brennan, T.C., and A.T. Holycross. 2006. Amphibians and Reptiles in Arizona. Arizona Game and Fish Department, Phoenix, AZ.

Degenhardt, W.G., C.W. Painter, and A.H. Price. 1996.  Amphibians and Reptiles of New Mexico. University of New Mexico Press, Albuquerque.

Enderson, E.F. 1999. Predation of Crotalus molossus molossus (Black-tailed Rattlesnake) by a Masticophis bilineatus (Sonoran Whipsnake), Whetstone Mountains, Arizona. Sonoran Herpetologist 12(7):72-73. 

Ernst, C.H., and E.M. Ernst. 2003. Snakes of the United States and Canada. Smithsonian Books, Washington, D.C.

Ernst, C.H., and E.M. Ernst. 2012. Venomous Reptiles of the United States, Canada, and Northern Mexico. Volumes 1 & 2. Johns Hopkins University Press, Baltimore, MD.

Goldberg, S.R. 1999. Reproduction in the blacktail rattlesnake, Crotalus molossus (Serpentes: Viperidae). Texas Journal of Science 51:323-328.

Greene, H.W. 1999. Radiotelemetry and the annual cycle of Black-tailed Rattlesnakes (Crotalus molossus). Pages 134-135 in H.W. Greene, Snakes, the Evolution of Mystery in Nature. University of California Press, Berkeley.

Greene, H.W., P.G. May, D.L. Hardy, and J.M. Sciturro. 2002. Parental behavior in vipers. Pages 179-206 in G.W. Schuett, M. Höggren, and H.W. Greene, Biology of the Vipers, Eagle Mountain Publishing.

Grismer, L. 2002. Amphibians and Reptiles of Baja California Including its Pacific Islands and the Islands in the Sea of Cortes. University of California Press, Berkeley.

Hamilton, B.T., and E.M. Nowak. 2009. Relationships between insolation and rattlesnake hibernacula. Western North American Naturalist 69(3):319–328.

Hamilton, P.S., and J. Wreidon. 2004. Crotalus molossus molossus (Black-tailed Rattlesnake). Male-male fighting. Herpetological Review. 35:63.

Hardy, D.L., and H.W. Greene. 1999. Borderland blacktails: radiotelemetry, natural history, and living with venomous snakes. Pages 117-121 in G.J. Gottfried, L.G. Eskew, C.G. Curtin, and C.B. Edminster (compilers), Towards Integrated Research, Land Management, and Ecosystem Protection in the Malpai Borderlands: Conference Summary. RMRS-P-10, Fort Collins, CO, U.S. Forest Service, Rocky Mountain Research Station.

Liner, E.A., & G. Casas-Andreu. 2008. Nombres estándar en Español en Inglés y nombres científicos de los anfibios y reptiles de México/Standard Spanish, English and scientific names of the amphibians and reptiles of Mexico. Second edition. Society for the Study of Amphibians and Reptiles, Herpetological Circular 38.

Lowe, C.H., C.R. Schwalbe, and T.B. Johnson. 1986. The Venomous Reptiles of Arizona. Arizona Game and Fish Department, Phoenix.

Persons, T.B., M.J. Feldner, and R.A. Repp. 2016. Black-tailed Rattlesnake, Crotalus molossus (Baird and Girard 1853). Pages 461-505 in G.W. Schuett, M.J. Feldner, C.F. Smith, and R.S. Reiserer (eds). 2016. Rattlesnakes of Arizona, Volume 1. ECO Herpetological Publishing, Rodeo, NM.

Platt, S.G., and T.R. Rainwater. 2009. A new maximum size record for Crotalus molossus (Baird & Girard, 1853). Journal of Kansas Herpetology (29):11.

Rorabaugh, J.C., and J.A. Lemos Espinal. 2016. A Field Guide to the Amphibians and Reptiles of Sonora, Mexico. ECO Herpetological Publishing and Distribution, Rodeo, NM.

Savary, W. 1999. Crotalus molossus molossus (Northern Black-tailed Rattlesnake). Brood defense. Herpetological Review 30:45.

Author: Jim Rorabaugh

For additional information on this species, please see the following volumes and pages in the Sonoran Herpetologist: 1998 Jul:80; 1999 Jul:72; 2001 Jun:62-63.

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