Rubber Boa (Charina bottae)
Tetrapoda: Amniota: Reptilia: Squamata: Serpentes: Boidae
Rubber Boa (Charina bottae) showing the vertical pupils and rounded
head.
Photograph by Joe Holbrook
Identification: The genus Charina is unique morphologically and defined by a short, blunt tail composed of fused terminal vertebrae. It is due to this blunt tail that rubber boas are also known as the "two-headed snake." The term "rubber" in the common name is derived from the smooth, shiny skin, which folds when the body is sharply bent resembling a rubber hose. Rubber boas are a uniform olive green, yellow brown, or dark brown dorsally fading into a light yellow color ventrally. Young individuals will appear more tan or pinkish than brown, both dorsally and ventrally. The head of the rubber boa is covered with large symmetrical scales (internasal, prefrontal, supraocular, frontal and parietal), but lack enlarged chin scales like those found on many other snakes in Idaho. When in hand, you can see vertical pupils within a small eye, a hint to their nocturnal habit. Rubber boas are fairly sexually dimorphic. Females are larger, but lack or have small anal spurs, a remnant of hind limbs. Males have visible anal spurs that are hooked and used to stroke the female during mating.
Distribution: The rubber boa is one of two boa species found in the United States, both of which are found on the west coast. The rubber boa is restricted to the western states, including northern California although small populations have been found at higher elevations in southern California, Nevada, Utah, Oregon, Idaho, Washington, northwestern Wyoming, far western Montana, and southern British Columbia. Rubber boas are found throughout Idaho, but are noticably lacking from higher elevations in the middle of the state (Fig. 1).
Figure 1: Idaho range and distribution of the rubber boa
Map from the Digital Atlas of Idaho
Habitat: Rubber boas can be found in many habitat types, but are generally not present at higher elevations. Although they are rarely encountered due to their nocturnal and elusive nature, rubber boas are actually fairly common in grasslands, chaparral, woodlands and forest. Within these habitats, they are found underneath and in rotting logs, under rocks and under the bark of snags. Rubber boas are equally competent as swimmers, burrowers, and climbers, which allows them to thrive in so many different habitats.
Behavior: Rubber boas are largely nocturnal snakes that are active from mid-march until late November. Unlike other snakes, rubber boas will actually be active at surface temperatures approaching the mid-50's, although temperatures this low slow the metabolism. The diet of rubber boas largely consists of young mice, pocket gophers, and shrews, however, some have been reported to eat lizards, salamanders, birds, and even other snakes. It has been reported that when a rubber boa finds a nest of young rodents, it will enter the nest and consume the young, sometimes constricting one or two while swallowing another. While eating, the snake has also been witnessed pushing the female rodent away and striking at it with its blunt tail.
These snakes have a fairly simple defense mechanism during which they will role into a ball with their head in the middle and strike at the attacker with its blunt, head-like tail. If this does not work, rubber boas, like many other snakes, will release a pungent musk from their anal glands.
Reproduction: Reproduction normally occurs in late April through early May. These are not definite dates, however, due to variation in weather and other abiotic factors. In September, young rubber boas hatch from clutches ranging in size from two to eight eggs. In some areas, young have even been reported to hatch as late as November. In the wild, young snakes that survive to adulthood will can live 40-50 years.
Scientific Study: Several studies of reptile thermal physiology have found that head temperatures are maintained within a narrower set of limits than body temperatures. The underlying mechanisms for these temperature differences are largely unknown, however, higher temperatures in the head may hel p to optimize nervous system function. Because snakes are elongate, they provide an excellent opportunity to study regional differences in body temperature. Snakes that are nocturnal, including the rubber boa, are an interesting case because they have no acces to solar radiation at night. Michael Dorcas and Chuck Peterson at Idaho State University examined the head and body temperatures of free-ranging rubber boas in southeastern Idaho.
Methods--To be able to compare free-ranging rubber boa temperatures to a standard, 19 rubber boas were kept in a lab setting with an available thermal gradient. Thermal preference was determined by taking cloacal temperatures for two summers (1 June and 31 August) between 1100 and 1400 h. Cloacal temperatures were assumed to be representative of core temperature throughout the study. Temperatures were only taken from non-gravid, nondigesting snakes in order to get the most accurate value for thermal preference. An average core temperature was calculated for each snake and then a single grand mean for all snakes, which represents the thermal preference. The thermal preference was determined to be 27.4 C from 942 measurments in the lab. Oral temperature measurements and cloacal temperature measurements were than taken from free ranging rubber boas found while road hunting between June 1990 and August 1995. A total of 45 free-ranging rubber boa's core (cloacal) and head (oral) temperatures were measured during the extent of this study.
Major Findings--Field measurments indicated a presence of regional temperature differences between the head and body. When body temperatures were below the thermal preference, the avarage head temperature was high compared to body temperatures. When the body temperature was above the thermal prefernce, however, head temperatures tended to be lower than body temperatures. This indicates that head temperatures are held more constant than body temperatures. The mechanism for this control is unknown in rubber boas, but may include both behavioral and physiological mechanisms. It has been noted that gravid females will expose the posterior portion of their body to warm developing embryos , leading to an increased body temperature. Snakes will also shield their head from direct sunlight, leading to lower head temperatures. The question remained, how can rubber boas do this at night? The answer came when soil temperatures were measured. Rubber boas were able obtain heat through conduction from the soil allowing boas to raise their head temperature above environmental and core temperatures. The constant presence of higher head temperatures held within a smaller range further supports the assumption that it may play an important role in central nervous system function.
Futher studies need to be conducted in the area of thermal regulation in reptiles where different regional temperatures are maintained. These studies might shed new light on the mechanism, whether behavioral or physiological, that aids in the maintenance of regional temperatures.
References
Barker, W. 1964. Familiar Reptiles and Amphibians of America. Harper & Row, New York, New York, USA.
Dorcas, M.E. and C.R. Peterson. 1997. "Head-body Temperature Differences in Free-ranging Rubber Boas." Journal of Herpetology 31: 87-93.
Nussbaum, R.A., E.D. Brodie, and R.M. Storm. 1983. Amphibians and Reptiles of the Pacific Northwest. University of Idaho Press, Moscow, Idaho, USA.
Stebbins, R.C. 2003. Peterson Field Guides: Western Reptiles and Amphibians. Third Edition. Houghton Mifflin, New York, New York, USA.
Created by Nicholas Smeenk