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Species Description Taxonomy Midget Faded Rattlesnake Species Description Midget faded rattlesnakes are the smallest member of the western rattlesnake (Crotalus oreganus) species complex with a maximum total length only up to 75 cm, although most individuals measure 45- 55 cm. The mass of large adults can reach up to 300 grams, but are generally considerably less than this. There is sexual size dimorphism in both length (males average 44 cm snout-vent length and females Photo: Josh Parker 41 cm snout-vent length) and mass (males average 70 g and females 45 g). Midget faded rattlesnake coloration and patterning fades with age as the name suggests, but neonates and juveniles can have brighter coloration. Background coloration ranges from cream to yellowish brown to tan with dorsal blotches that are tan to light brown. Blotches tend to be shaped like a rectangle or oval, but can be highly irregular or fused and individuals can be identified by their unique pattern of blotch shapes. Facial markings include a small bar that extends from the eye to the corner of the mouth. Finally, as a rattlesnake, its tail ends in a series of keratinized segments that form the rattle. Midget faded rattlesnakes have a venom composition that is different from all other western rattlesnake subspecies and is one of the most lethal rattlesnake venoms known. This is due to the presence of a neurotoxic element known as concolor toxin. Interestingly, venom toxicity does not change between juveniles and adults, which is different than other western rattlesnake subspecies, which tend to shift to lower toxicity in adults. Taxonomy The midget faded rattlesnake was first formally described by Angus Woodbury in 1929 from a specimen in Utah as Crotalus concolor. The species name “concolor” means uniform color and this represents the tendency for older individuals to attain a faded, uniform color. In the 1930s the midget faded rattlesnake was considered a subspecies of the western rattlesnake and named Crotalus viridis concolor. This name stood until 2001 when genetic evidence suggested at least two species within the western rattlesnake complex and the scientific name was changed to Crotalus oreganus concolor. This is the name that is currently listed in the standard scientific names publication produced by the Society for the Study of Amphibian and Reptiles, although Douglas et al. (2002) proposed midget faded rattlesnakes to full species status as Crotalus concolor. The proposal for full species status is based on the fact there is no evidence for any current gene flow between midget faded rattlesnakes and any other western rattlesnake subspecies. Distribution Midget faded rattlesnakes are only found in southwest Wyoming, western Colorado and eastern Utah and their distribution is largely centered on the Green River formation of the Colorado Plateau in these three states. Their range is likely largely limited by geology, as they require exposed rocky outcrops (characteristic of the Green River formation) for denning habitat. Within the overall extent of their range, populations are not continuous, but rather patchy dependent on suitable denning and foraging habitat. However, little is known of their specific distribution within their potential range and we currently do not know whether their range is stable, declining or expanding. Habitat Midget faded rattlesnakes can be considered habitat specialists, especially in regards to denning habitat. They require rocky outcrops with southern exposure for denning habitat. These rocky outcrops not only serve as overwintering areas, but are also used for shedding aggregations and for parturition. Snakes use the same specific rocks every year for shedding and parturition (generally, the same rock is not used for both shedding and parturition). Furthermore, juveniles and post-partum females remain at the den site year-round. Adult males and adult females in reproductive condition do migrate from the denning areas during the summer active season and move to foraging/mating core areas. Less is known regarding the foraging areas as compared to den sites, but snakes tracked by radio telemetry have moved to sites characterized by intermediate densities of shrub vegetation, including sagebrush, as well as riparian areas. Canyon draws are often used as movement corridors between den and foraging areas. In most cases, migrating individuals return to the same den site each year. Movement and Home Range Movement patterns of midget faded rattlesnakes vary by age and reproductive status. Juveniles and gravid females are generally non-migratory and stay nearby the den site. For example, on average radio-tracked gravid females only moved a maximum of 115 meters from the den sites and had a home range of less than 5 hectares. Adult males and non-gravid adult females have much larger movement distances and home ranges and feature disjunct summer and winter ranges. The average distance moved from the denning sites is 779 meters for males and 681 meters for females. These distances are smaller than most known movement distances for other northern temperate rattlesnakes, and likely are due to the small size of this snake. However, adult midget faded rattlesnakes have home range areas that are comparable or larger than most other rattlesnakes. Total home ranges ranged from 200-300 hectares and core areas averaged 57 hectares for males and 28 hectares for females. This suggests that although midget faded rattlesnakes do not typically move long linear distances, they may move more actively during the summer activity season. Movement patterns are not uniform among individuals. Three general patterns have been observed: (1) direct straight-line movement to and from the den, (2) straight-line movement to a core area in which multi- directional movements occur and (3) multi-directional movements without an initial straight-line movement. However, individuals tend to move to the same habitat features each year. Midget faded rattlesnakes are the only rattlesnakes known to have at least some individuals remain at the denning area year-round. Thus, at least for juvenile and gravid snakes, the level of population density and food availability appear to be sufficient at the den sites. Migrating snakes are likely involved in mate searching during much of the active season as well as foraging for small mammal prey. Observations of migrating individuals suggest that individuals participate in both foraging and mate searching activities throughout the active season. Movements away from the den typically take place between June and September. Diet Midget faded rattlesnakes are limited in their dietary breadth due to their small size. Previous research has demonstrated a distinct ontogenetic shift in dietary preferences. Juveniles feed almost exclusively on lizards with adults shifting to a diet of 80% small mammals and 20% lizards. Adults also will occasionally feed on small birds. Known prey species include sagebrush lizards (Sceloporus graciosus), plateau fence lizards (Sceloporus tristichus), side-blotched lizards (Uta stansburiana) tiger whiptails (Aspidoscelis tigris), plateau striped whiptails (Aspidoscelis velox), deer mouse (Peromyscus maniculatus), bushy-tailed woodrat (Neotoma cinerea), western harvest mouse (Reithrodontomys megalotis) and the least chipmunk (Tamias minimus). Among adults, deer mice appear to be the most common food item. Reproduction Midget faded rattlesnakes are a relatively long lived species with low reproductive output. They are ovoviviparous, which means that eggs are retained in the female and young are born live. Age to first reproduction is probably around 5 years (at least for females), although there are some instances of younger gravid females. Average longevity is unknown, although individuals can live at least 15-20 years. On average, 25% of adult females are gravid and females probably reproduce once every three years. Sex ratio appears biased toward females, ranging from 1.25-2 females for every male. Mating occurs during July and August after migration from the denning area. The following year gravid females give birth to four to five young in August. Neonates measure 19 cm and average mass is 8 grams. The clutch size for midget faded rattlesnakes is lower than most other rattlesnake species and is correlated with snout-vent length in females. Therefore, small body size likely limits the reproductive potential of midget faded rattlesnakes and probably increases their susceptibility to population disturbance. Literature Cited: Ashton, K.G. and T.M. Patton. 2001. Movement and reproductive biology of female midget faded rattlesnakes, Crotalus viridis concolor, in Wyoming. Copeia 2001: 229-234. Douglas, M.E., M.R. Douglas, G.W.Schuett, L.W. Porras, and A.T. Holycross. 2002. Phylogeography of the western rattlesnake (Crotalus viridis) complex, with emphasis on the Colorado Plateau. Pages 11-50 in G.W. Schuett, M. Hoggren, M.E. Douglas, and H.W. Greene (editors). Biology of the Vipers.Eagle Mountain Publishing, Company. Mackessy, S.P., K. Williams, and K.G. Ashton. 2003. Ontogenetic variation in venom composition and diet of Crotalus oreganus concolor: A case of venom paedomorphosis? Copeia 2003: 769-782. Parker, J.M. 2003. The ecology and behavior of the midget faded rattlesnake in Wyoming. PhD Dissertation, University of Wyoming. Laramie, Wyoming, USA. Parker, J.M., and S.H. Anderson. 2007. Ecology and behavior of the midget faded rattlesnake (Crotalus oreganus concolor) in Wyoming. Journal of Herpetology 41: 41-51. Woodbury, A.M. 1929. A new rattlesnake from Utah. Bulletin of the University of Utah 20:1. .
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