The Midget Faded Rattlesnake
Crotalus concolor
by
Thomas C. Houston

Undergraduate | Department of Zoology | Oklahoma State University

Natural History Notes

The Midget Faded Rattlesnake is one of the smallest rattlesnakes in Colorado Plateau Region of the United States. These small rattlesnakes are believed to be a stunted form of the Great Basin Rattlesnake (Crotalus lutosus) similar in superficial appearance to the Hopi Rattlesnake (Crotalus viridis nuntius). Although nuntius and concolor are similar it is not believed to be a direct relationship (Douglas et al. 2002). Recent studies using mitochondrial DNA (mtDNA) and D-2 loop sequencing has suggested to elevate Midget Faded Rattlesnakes as well as others from the “western viridis complex” to full species status (Douglas et al. 2002) .

Midget Faded Rattlesnakes can grow to reach 63 cm in total body length and are a cream to yellow brown, or tan and generally have body blotches that are oval to rectangular. Their body blotches are usually only slightly darker than the background and become faded to sometimes absent in older animals. Young usually have prominent markings and can easily be confused with similar species.

Fig 1. Gravid female Crotalus concolor basking.

The Green River Formation in Southwestern Wyoming, Eastern Utah and Western Colorado makes up the entire range of this species. However, a new publication has suggested that the range extends down into a small part of Coconino County, Arizona (Brennan and Holycross 2004). These animals are protected to various degrees within the states they reside. They tend to prefer rocky outcrops in areas where sage is the abundant vegetation. Due to specific habitat preferences these animals do not travel great distances like other species, such as, lutosus and viridis (Ashton 2003).

These animals typically reproduce biennially, triennially or even longer. Gravid females have been found to only move short distances (>20m) from the den site after emergence until parturition in late August to mid September (Ashton and Patton 2001). The offspring are usually around 19-21.5 cm in total length and the number of offspring is usually directly related to female SVL (Ashton and Patton 2001) averaging around 5 per litter.

The venom of the Midget Faded Rattlesnake is composed of a much higher neurotoxin than one would assume. This species carries the presence of a phospholipase A2-based B-neurotoxin (concolor toxin) and several myotoxins (Mackessy et al. 2003) which makes concolor venoms highly toxic. In fact concolor is the most toxic out of the (Western Rattlesnake clade, Crotalus oreganus). It was hypothesized that concolor had an ontogenetic variation in venom composition due to is shifts in diet from juvenile to adulthood. However, it was discovered that although juveniles feed predominantly on lizards (Scleoporus ssp.) and adults on rodents (Peromyscus and Tamias) there isn't much in ontogenetic variation in venoms (Mackessy et al. 2003).

In Captivity

The Midget Faded Rattlesnake is quite rare in collections, in both the private and institutional sector, with only 3 animals registered under ISIS (International Species Information System) worldwide. Despite their rarity they are relatively simple to care for in captivity. The basic care is highly similar to lutosus, viridis, etc. One factor to consider is temperature, most concolor do not live in very hot areas so the extreme heat is detrimental to their health. The most difficult factor in concolor husbandry is the acquisition of legal animals. Because the three major states (Utah, Colorado, and Wyoming), that take up the vast majority of this species geographic range, have protected them from collection, it is highly difficult to get true legal specimens.

Animals taken from the wild in any of the three states without a permit and then transported across states lines are in direct felony violation of the Federal Lacy Act. In addition to that if the said specimen has a market value of greater than $350 and is illegally transported across state lines it is an additional felony violation. Each violation is punishable up to $250,000 fine and 5 years in prison. All in all, without legal documentation of captive propagation, concolor is as much a legal risk as owning a Gila Monster (Heloderma suspectum) without papers. On the other hand there is one locality that isn't yet protected by the state in which they reside, Arizona. However, like with abyssus most of their range within Arizona is within Nationally protected land (Glen Canyon, and the Vermillion Cliff Wilderness Area). Also, with specimens coming from Arizona it is highly difficult to determine if they are indeed true concolor, with out blood work and DNA samples, due to the superficial similarities between some abyssus, nuntius, viridis and even lutosus in the general vicinity.

Fig 2. Variation to an extent is common within this species. This photograph illustrates the variation in Crotalus concolor.

Propagation

Mating behavior has been observed in the wild between 5-17^th of July and is the shortest duration of any species in the western clade (Ashton 2003). In captivity these animals breed quite easily. Cooling is suggested for stimulation of behavior and ovulation. Cooling should be done at a slow rate. Avoid feeding for two or three weeks prior to deducing their temperatures to avoid any left over undigested food items. Slowly lower the temperature by ten degrees (F) twice a week until a perfect temperature range of 48- 54 degrees is obtained. It is vital that you check on the animals every two weeks to ensure they are wintering successfully and to provide water.

To ensure proper stimulation the snakes should be cooled for a minimum period of 6-8 weeks. Keep in mind that in the wild hibernation in this particular species is one of the longest. Also, be aware that light is usually not associated with hibernation so try to avoid allowing white light to enter the containers during hibernation. This can be achieved by using hibernation enclosures that have solid walls on all sides and by using a red or black light while attending to the animals bi-weekly. Caution should be used at all times when working with these venomous animals but especially while working in low light situations.

The warming process should be slow and compliment the gradual cooling. After the animals have been warmed for two weeks, feeding should commence. The first meal should be smaller than normal to allow the digestive tract to adjust. Females that are scheduled to breed should be fed twice a week for six weeks to obtain a healthy body weight. Once all the animals have been feeding regularly for six weeks the females should be introduced to the males' enclosure. The pair should be supervised and observations should be conducted the entire cohabitation period to avoid any mishaps. Although, cannibalism is not documented in this species it is known within the family Crotalidae (Houston 2005). Copulation can last from minutes to several hours. After the female has copulated she should be removed from the males enclosure to avoid additional stressed from over stimulated males. This process can be repeated every other day for a week to ensure success.

In the wild parturition occurs in late August to late September (Ashton and Patton 2001). Gestation in this species lasts 395-425 days when properly cycled due to spermatozoa remaining viable in the female's oviduct until the following spring. The females should be placed on a 14:12 L:D light cycle and have access to ample thermal ranges. The female's enclosure should be inspected to insure that any offspring cannot escape. Offspring of this species are 50-100mm smaller than those of larger bodied taxa (Ashton and Patton 2001). However (Ashton and Patton) discovered that offspring size was not directly related to the females SVL (Snout to vent length) (2001).

The babies usually take small newborn pinky mice with little problems and should be fed on a weekly basis. Because of the juveniles wild diet consists of other reptiles, each animal should be housed individually to avoid ophiophagy and to determine successes with eating.

In conclusion, Midget Faded Rattlesnakes (Crotalus concolor) are a wonderful and unique species that requires little effort for successful captive propagation. Although, keeping and breeding concolor is a rewarding experience for all involved, these animals are at high risk from human encroachment and are a species that needs conservational attention. Therefore, with an increase in captive propagation, research, and public education this species can be around for many more generations to enjoy.

Literature Cited

Ashton, K. G. 2003. Movement and Mating Behavior of Adult Male Midget-Faded Rattlesnakes, Crotalus oreganus concolor, in Wyoming. Copeia 2003:190-194.

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.

Brennan, T. C., Holycross, A. T. 2004 Crotalus oreganus concolor. Geo. Dist. Herpetol. Rev. 35(2).

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. In Scheutt et al. [ eds. ], Biol. of the Vipers 2002: 11-50.

Houston, T.C. 2005. Ophiophagy In A Captive Sonoran Desert Sidewinder (Crotalus cerastes cercobombus). Herpetol. Rev. [In Review]

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.