A Natural History of the Rubber Boa, Charina bottae.

Andy Sudkamp

July 19, 2002

PROJECT OUTLINE: Natural History of Charina bottae.

I. Nomenclature & Taxonomy

A) Family, Latin & common names

B) One two,or three subspecies?

1) Original description of the species

2) Splitting into three subspecies

3) Dropping of two of the subspecies

4) Current state of affairs, new evidence

II. Morphology

A) General appearance (with original illustration)

1) variability in adult color, size, scalation, eyes

a) regional differences

2) juveniles and subadults

B) Sexual dimorphism

1) size, head:tail ratios, spurs

III. Range(s)

A) Northern populations

B) California populations

IV. Habitat

A) Diversity of habitats/range

V. Habits

A) Fossorial

B) Activity periods

1) daily

2) seasonal

E) Thermoregulation

B) Feeding habits & preferences

1) feeding trials in captivity vs. wild reports

2) Use of the tail to ward off mice / tail scarring

C) Predators and predator avoidance tactics

D) Reproduction

1) courtship and mating

2) litter size and timing

VI. Life Span

The following information on Charina bottae, commonly known as the rubber boa, has been synthesized from a number of different sources. Although it is a common snake which inhabits a large part of the western United States, including Oregon, the rubber boa is seldom-seen. It is also sometimes known as the rubber snake or two-headed snake, owing to its peculiar "rubbery"-looking and oddly wrinkled skin and its thick, blunt tail which resembles a second head.

I. Nomenclature and Taxonomy:

Charina bottae is a monotypic genus, one of only two endemic species of boas in the United States, the remaining members of the family Boidae being tropical and subtropical. Charina bottae occurs in western North America, while the rosy boa, Lichanura trivirgata, is found only in the southwestern U.S. (Nussbaum, et al, 1983, Pickwell, 1947).

The etymology of the genus comes from the Greek word Charina, meaning graceful - a rather odd appellation, given the stocky appearance of this species. The specific epithet was named for the Italian explorer Paolo Emilio Botta, who served as a doctor aboard the Heros, which visited the coast of California in the 1820s. Botta collected a specimen and published a description in his Observations on the Inhabitants of California (Hoyer, 2001).

Despite its current status in field guides as a monotypic genus, the taxonomic classification of the species is anything but straightforward, and has underone repeated changes over the past 60 years. Current research has rendered its nomenclature open to new interpretation at this time.

Prior to 1890, three separate genera were described, based upon the extreme variability of the few specimens known. In 1890, Stejneger consolidated the known specimens into a single genus, but named three species, separated on the basis of scale row counts and head scalation (Nussbaum & Hoyer, 1974).

From 1920 to 1935, a flurry of taxonomic work on the genus by Van Denburgh, Cope, Ruthven, Tanner, Stejneger & Barbour, and Stull separated the species into two, then three subspecies, and finally back to a single species. These authors were trying to make consistent sense out of what all described as a considerable amount of variation in scalation of the specimens known from different localities in California, Idaho, Utah and Nevada (Nussbaum & Hoyer, 1974).

Klauber (1943) described C.b. umbratica from southern California and revived the separation of C.b. utahensis and C.b. bottae. He separated the subspecies on the basis of their ranges, scale rows, divided or entire parietals and ventral scale counts, but acknowledged that considerable variation existed within these groupings, especially those from the Northwest. Also, his proposed subspecies ranges were rather vague (Nussbaum & Hoyer, 1974). Charina bottae bottae, (Pacific rubber boa) was said to inhabit the northwest, C. b. utahensis, (Great Basin rubber boa) lived in Idaho, Utah, and Nevada, and C. b. umbratica (Southern California rubber boa) was isolated in the mountains of southern California.

Cunningham (1966) pointed out that Klauber had based his description of C.b. umbratica on the basis of only two specimens, and presented data from twenty-eight specimens to "provide additional information concerning the distribution, variation, and taxonomic status of this rare and protected animal" (Cunningham, 1966). Twenty four of the specimens were collected in the San Bernadino Mountains, three from the San Jacinto range, and one from Newport Beach (which, he noted, was not its natural range). He modified Klauber's key somewhat, agreeing with him on the scutellation patterns, but proposed elevating umbratica to species status, because "Klauber was not aware that umbratica is a much smaller snake than bottae or utahensis. Furthermore, evidence based on available museum specimens obtained since 1943 indicate that umbratica is geographically isolated from both bottae and utahensis. Therefore, intergradation between umbratica and other forms of Charina is doubtful"(Cunningham, 1966).

Eight years later, Nussbaum and Hoyer undertook to sort through the confusion. They noted that many of the specimens they collected around Corvallis, Oregon did not key to the proper subspecies according to Klauber's geographic relationships. They reviewed the literature and pointed out that most of the original descriptions were based on a very limited number of museum specimens, and that subsequent studies had often re-examined the same specimens, thus "there has been little chance of increasing our knowledge of variation." The total number of specimens examined in the various studies for the separation into subspecies was only about 150. Furthermore, almost three fourths of the specimens reported were from California, Nevada and Utah, with only about one fourth representing the greatest (northern) part of the snake's range. They focused their study on the differences between the subspecies bottae and utahensis in the northern part of their purported respective ranges because, as they pointed out "We had no reason to doubt the distinctness of C.b. umbratica, the southern isolate...."(Nussbaum & Hoyer, 1974).

After careful examination of over 500 specimens from the Northwest, Nussbaum and Hoyer determined that the subspecies designations for utahensis and bottae were invalid, and that the remarkable degree of variation in scalation that occured in the species was probably due more to latitude and altitude differences. As they pointed out, other researchers had found that temperatures had an effect on the number of ventral and caudal scales formed during embryogenesis, and this explanation seemed to reasonably fit their data on C. bottae populations. They also stated that, although umbratica was no doubt distinctive in terms of its low numbers of ventral and caudal scales and hence vertebrae, and its relatively small size, these variations were also probably due more to clinal variation from cold to warm regions. (Nussbaum and Hoyer, 1974). In conclusion they stated "The subspecies of C.b. bottae and C.b. utahensis cannot be reasonably recognized because the distinguishing characters break down under close scrutiny. The subspecies C.b. umbratica is no doubt distinguishable by at least one character, but clinality and non-concordance of characters across the species range render the concept "umbratica" meaningless. We recommend that subspecies of C. bottae not be recognized." (Nussbaum and Hoyer, 1974).

This, however, was not adopted. The Great Basin subspecies (utahensis) was dropped because it appeared it could not be distinguished from the Pacific subspecies (bottae). There is a great deal of variation in scale patterns between and within each locality. However, the southern subspecies umbratica was retained, and is currently listed as threatened in California.

Current work on the genus seems to support this separation. Rodriguez-Robles, Stewart and Papenfuss (2001) studied geographic variation in mitochondrial DNA (mtDNA). a phylogenetic analysis of mtDNA sequences revealed a deep split corresponding to the separation of umbratica from all other populations of C. bottae. Using this evidence in combination with previously published datas on morphology and allozymes, they elevated umbratica to the status of a distinct species. The lineages for C. bottae and umbratica appear to have diverged between 12.3 and 4.4 million years ago, when present-day southern California was separated from the north by a large inland sea. This barrier was in place until relatively recently during the Pliocene era. Based on earlier work by this team, this barrier was also speculated to have been responsible for genetic breaks within Lampropeltis zonata and Diadophis punctatus. Thus, "The umbratica clade has been an independent unit for a considerable period of time." They further state "We do not advocate making taxonomic changes solely on the basis of mtDNA data, but we recognize that assessments of a species' genetic structure can alert us to documented and/or overlooked levels of morphological diffentiation among allopatric populations, and sometimes the taxonomic implications of this variation have not been fully considered. Despite noticeable variation and overlap in the characters that define C. b. bottae and C. b. umbratica, the two forms can still be separated using a suite of morphological traits." (Rodriguez-Robles, Stewart & Papenfuss, 2001).

Although they agreed with Nussbaum and Hoyer that there was no basis for the utahensis designation, they noted two clade groupings of the northern bottae populations. Evidence derived from fossil records and mitochondrial DNA suggest that the ancestors of modern Charina bottae evolved some 29-10 million years ago. "A subsequent cladogenetic event in the bottae clade resulted in two groupings, which we have termed the Sierra Nevada and the Northwestern subclades, based on the geographic distribution of their constituent populations" (Rodriguez-Robles, Stewart & Papenfuss, 2001). The break in the subclades occurs near Mt. Lassen in northern California, and coincides with a "genetic discontinuity" in populations of Ensatina escholtzii oregonensis to the north, and E.e. platensis to the south. It is thought that glaciation during the Pleistocene era and volcanic activity in this region eliminated these species from the the area and allowed some genetic divergence, and the area has subsequently been repopulated from both the north and south.

In summary, the taxonomy of Charina bottae has undergone repeated revisions as new evidence became available, but there are two currently recognized subspecies: C. b. bottae occupying most of the range of the species across the western U.S., and C. b. umbratica occuring only in isolated pockets of mountain ranges in southern California. It remains to be seen if the herpetological community adopts the recommendations of Rodriguez-Robles, et al and elevates the southern subspecies to full species status.

Current Taxonomy:

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Subclass: Diapsida

Order: Squamata

Suborder: Orphidia (Serpentes)

Family: Boidae (boas) Genus: Charina

Species: bottae (Blainville)

C.b. bottae

C.b. umbratica

II. General Morphology and Sexual Dimorphism:

The most striking characteristics of the species, and ones that make identification unmistakable when one is fortunate enough to encounter it, is that the species feels and looks "rubbery" due to its small scales, generally gray-brown coloration and the peculiar wrinkles exhibited in the skin as it coils. The head is small and wedge-shaped, with an indistinct neck. (Brown, et al 1995) It has a short, non-prehensile, blunt tail that gives the appearance of another head, with fused terminal vertebrae in the tail forming a hard cone under the skin. Like other boas, the small eyes have vertical pupils and there are anal spurs on either side of the cloaca, which are used in mating (Nussbaum, Brodie & Storm, 1983).

Coloration varies but is uniform on the dorsal side. Medium to dark brown, gray-brown, olive green, or yellow brown are the usual colors noted. However, the southern subspecies in one study were consistently light brown or tan (Hoyer & Stewart, 2000). Rare lilac-colored individuals have been found in western Oregon (Brown, et al 1995, Hoyer 1974). These colors grade into a uniform lighter yellowish coloration on the ventral surface (occasionally with brown mottling). The young might be tan or pink, and are often translucent. The color of the body darkens with age, while the eyes tend to lighten in color (Nussbaum, et al 1983). Eye color has been reported as mostly greyish-green, yellowish or brown in northern populations (Hoyer, 1974) and black or gray-black in the southern subspecies (Hoyer & Stewart, 2000).

Size: Rubber boas in northwestern Oregon usually measure less than 60 cm, but 70 cm specimens are not unusual, and one large specimen of 83 cm is recorded. Females are generally longer (average 55 cm) than males (average 47 cm) and heavier. Hoyer and Stewart (2000) report that the females of the southern rubber boa averaged 20% longer and 93% heavier than males. The southern subspecies is generally smaller than the northern subspecies, with males averaging 40 cm and the females 49 cm. Although smaller, male rubber boas have longer tails than females relative to their body length.

Scalation Patterns are highly variable, as has been noted in the discussion of taxonomy. These patterns were used to separate three subspecies and even three genera at one point. Generally, rubber boas have dorsal scales in 39-53 rows, 193-219 ventrals, and from 24-43 caudals. Ventral scales are small compared to other snakes in western Oregon. Head scales are large and well-defined. There are 9-11 upper labial scales, and parietals may be entire or divided. There are no enlarged chin scales. Extreme variability and common anomolies are recognized. For detailed descriptions and illustrations of these features in specimens from the Willamette Valley, please see Hoyer (1974), who states: "Considerable scale variability is a natural feature of this species and occurs in body as well as head scalation." Or, as one author puts it "The scales of boids are small and more variable in number than in other families of snakes that occur in the Northwest (Nussbaum et al 1983, p 255).

Regardless, the overall appearance of the rubber boa is one of smooth small scales of uniform coloration.

Anal spurs (rudimentary hind limbs) are located beside the cloaca in both males and females, a characteristic of the Boidae. The spurs are larger and hooked downward in males, and are used to stroke the female during mating. In females the spurs point straight towards the rear, are smaller, and may not be visible.

III. RANGE:

The range of the northern subspecies is vast, from Southern British Columbia (including Vancouver Island) eastward through Idaho, to western and central Montana and northern Wyoming and northwestern Colorado. It extends southward throughout Washington, Oregon, and California and east to Utah and northern Nevada. Throughout Washington and Oregon it occurs nearly everywhere, except for the coast ranges and some of the sagebrush country of central Washington and southeastern Oregon.(Brown, 1995). It is also absent from the Central Valley of California (Rodriguez-Robles, et al 2001)

In contrast, the range of the southern subspecies is limited and fragmented. It is found in a few locales in the San Bernadino and San Jacinto mountain ranges east of Los Angeles in southern California (Hoyer and Stewart, 2000, Rodriguez-Robles, 2001).

IV. HABITAT:

Rubber boas occupy a variety of habitats depending on locality. These include deserts, chaparral, foothills with open vegetation, grasslands, coastal scrub and montane forests. Usually they are found within several hundred meters of water. They have been found from sea level to 3,050 m. In Oregon, the highest elevation where a specimen was found was 1,890 m on Hart Mtn., Lake County (Brown 1995). In desert canyons they are sometimes found under flat rocks at bottom of talus slopes. Otherwise, they may be found in forest litter or crevices of cliffs (Nussbaum, et al 1983).

In western Oregon, including the Willamette Valley, they are typically found in clearings of cut forest, rotting stumps or logs. According to Hoyer (1974) "In western Oregon, the rubber boa occurs in most habitats except where grazing and cultivation is practiced or in large areas that undergo periodic flooding. Thus, it exists in habitat heretofore not considered typical for the species such as vacant city lots, large grassy fields, railroad and highway right-of-ways, and areas adjacent to industrial plants." Hoyer reported good results in locating them under artificial cover debris such as plywood and old carpet scraps. Hoyer and Stewart (2000) conducted their survey of umbratica in areas above the chaparral belt, in open areas with bracken fern and in coniferous, deciduous and mixed forests and reported that all of these area had abundant woody debris on the ground which made suitable habitat for rubber boas.

In the Siskiyou mountains of southern Oregon I have found them in areas of mixed forest on rocky slopes, and under down logs. In general, Charina bottae may be found by turning over rocks, logs and other protective coverings throughout its range, but success is a matter of luck and timing.

V. HABITS:

Charina bottae are fossorial or semi-fossorial (burrowing), generally using existing rodent burrows for cover. They are mostly nocturnal or crepuscular, slow-moving and secretive creatures. They move around primarily at night to forage for prey. They generally spend most of their time during the day and in hot weather seeking cooler, moister environments underground, under rocks or in rotten logs. Brown (1995) reports that they can also climb and swim well.

Hoyer (2001) states that "rubber boas appear to have a strong fidelity to a small home range where food, cover, and warmth are adequate. Some individuals have been observed year after year in the same location, or within 100 meters of last capture."

Because of their protective coloration and the characteristics mentioned above, Charina bottae is rarely seen and may be considered rare, but may actually be common or even abundant in places. Hoyer (2001) reports personally finding 12 rubber boas in one 15-minute search of .6 hectares (20/hectare) in good habitat.

In the Willamette Valley, they are generally active from mid-March to early November, with peak activity in spring and early summer. It becomes harder to find them as summer heat builds up. "Collecting success is highest in the spring, late summer and early fall." (Hoyer, 1974). They seek a protected area underground and become inactive during the winter months, sometimes from mid-October to mid-March, depending on locale.

Temperament:

Rubber boas are very docile snakes, and have never been known to bite or strike, even if antagonized. However, if handled too roughly, they may excrete a smelly musk on the handler. Upon being picked up, they will usually gently wrap around the holder's wrist and may stay there for an hour or more before seeking to crawl around (Hoyer, 2001).

Thermoregulation:

As may be deduced from the preceeding discussion on the habits of the rubber boa, they exhibit behavioral mechanisms to maintain body temperature by seeking shelter from solar radiation during the day and the hottest parts of the year, and become inactive in underground burrows during winter. Physiological as well as behavioral mechanisms are at work in the thermoregulation of Charina bottae, as in other ectotherms.

Rubber boas are cold tolerant, and "active individuals have been measured with body temperatures less than 7 C" (Brown, et al 1995). Dorcas and Peterson (1997) determined that the thermal preference for boas was 27.4 C, but measured oral temperatures ranged from 13.8 to 32.2 C and cloacal temperatures between 11.5 and 34.5 C. They are generally not as tolerant of heat as some snakes, and may burrow into cooler, moister surroundings during warm weather (Hoyer, 2001). However, they are occasionally seen basking in the open on warm humid days (especially pregnant females) (Nussbaum, et al 1983, and personal observations).

In an effort to add to existing knowledge of head-body temperature differences in snakes, Dorcas & Peterson (1997) conducted a 5-year study to measure temperatures in wild rubber boa population in southeastern Idaho. They chose rubber boas because "...they are extremely docile. Thus measurement of their temperature is relatively easy and involves minimal handling. Additionally, rubber boas are often nocturnal, frequently at very low body temperatures and thus provide an interesting comparison to studies of regional temperature differences in diurnal species." By measuring the head and body temperatures of 45 free-ranging boas using oral and cloacal thermocouple probes, they determined that "during the daytime, rubber boas generally exhibited warmer heads at average body temperatures below their thermal preference and cooler heads at temperatures above their thermal preference. At night, active rubber boas exhibited significantly higher head temperatures than body temperatures (mean difference = 2.0 C)". This was consistent with other findings that showed that snakes regulate head temperatures more precisely than body temperatures, through a variety of behavioral and physiological mechanisms (Dorcas and Peterson, 1995).

VI. DIET:

Charina bottae feeds almost entirely on young nestling voles, mice and shrews, but various sources have also reported a diet of salamanders, lizard and snake eggs, lizards, snakes, baby rabbits, small birds, and bats. Like other boas, they constrict their prey before swallowing (Nussbaum, et al 1983, Hoyer, 1974) .

Hoyer and Stewart (2000) conducted laboratory feeding trials on southern rubber boas and found that their food preferences were similar to those reported for the northern populations, with the primary differences being that the southern species generally refused lizards but readily consumed lizard eggs. In fact, lizard eggs seemed to be a preferred food.

Numerous authors (Hoyer, 2001, Hoyer and Stewart, 2000, Nussbaum, et al 1983, Hoyer 1974) report that the tails of wild-caught rubber boas often show considerable scarring, and laboratory trials have also helped to determine the cause of this scarring. The vertebrae of the tail in rubber boas are telescoped into each other, forming a bony club, which the snake has been reported to use to "strike" at a female mouse defending her nest, while the snake constricts and eats the young. The mother snake will often chew on the tail and sides, resulting in heavy scarring, and the snakes ignore this attack while consuming the nestlings. This scarring is quite common in wild snake populations, particularly in females. "As a general observation, adult females also possess a higher incidence of scarring over the entire body and tail as well as the tail tip.... Since female rubber boas grow at a faster rate than males and also require additional food for reproduction, the frequency of these foraging encounters is undoubtably greater for female snakes which, in part, can explain the higher incidence of scarring." (Hoyer, 1974).

VII. DEFENSES:

Due to their slow movements, rubber boas are vulnerable to attacks from almost any predator. Generally, they try to escape if they are near their burrow. If threatened, they may coil the body tightly with their head tucked underneath and the tail elevated and waving in response to a predator attack, even striking out with the tail. If pressed, they release a strong-smelling musk from the anal glands. None of these are particularly effective against a determined predator, however, and rubber boas are eaten by other snakes, raccoons, skunks, owls, ravens, and other birds and mammals (Hoyer & Stewart, 2000).

VIII. REPRODUCTION:

Rubber boas are ovoviviparous. That is, the fertilized eggs are retained in the female's body during development. The eggs hatch in the body and the females then give birth to the young, but there is no nutrient exchange between mother and young as there is in viviparous vertebrates. Males become sexually mature at approximately 45 cm in length, and females at 55 cm (Pickwell, 1947, Nussbaum, et al, 1983).

Adults emerge from their winter dormancy in early spring. Males usually emerge from their burrows before the females and courtship and mating generally occur during April to early May, but the timing will depend upon the location and insolation (Hoyer & Stewart, 2000, Hoyer, 1974).

Gravid females may congregate in areas to maintain stable body temperatures, and may be seen during the summer months exposing their bodies to solar radiation (Dorcas and Peterson, 1997, Brown, et al 1995). "In rubber boas, the larger size of females permits them to bear relatively large young and to store fat reserves adequate to sustain themselves while fasting during a 90-100 day gestation. Females may invest half of their body weight in reproductive efforts" (Hoyer and Stewart, 2000).

They give birth to small litters of 2-8 young in early-late autumn (generally in September around Corvallis, Oregon (Nussbaum, et al, 1983). The young are born approximately 20 cm long, pink and slightly translucent when held up to the light. The coloration gradually darkens with age (Hoyer, 2001). In cooler areas of their range, the activity season for rubber boas is shorter and females may not reproduce every year. (Brown, 1995)

IX. LIFE SPAN:

Rubber boas may be long-lived if they are in favorable habitat and are fortunate enough to escape predation. They may live for twenty or thirty years or more in the wild. Brown et al (1995) reports an individual captured as an adult and living 18 years in captivity. Hoyer (2001) reports an adult female captured in 1971, released, and repeatedly recaptured over the course of several years. She was then kept in captivity for over 30 years, gave birth to a litter of young in the summer of 2000, and believed to be at least 50 years old (or possibly much older) in 2001.

For an excellent pictorial survey of Charina bottae, please see Ryan Hoyer's website at: http://www.kingsnake.com/rubberboa/Photos/comparisons/himalfem.html

Literature Cited:

Brown, H.A. et al. (1995). Reptiles of washington and Oregon. pp. 94-96. Seattle: Seattle Audubon Society.

Cunningham, J.D. (1974). Taxonomic status of the southern rubber boa, Charina bottae umbratica. Copeia: 1974(4): 996-997.

Dorcas, M.E. and Peterson, C.R. (1997). Head-body temperature differences in free-ranging rubber boas. Journal of Herpetology, 31(1): 87-93.

Hoyer, R.F. (1974). Description of a rubber boa (Charina bottae) population from western Oregon. Herpetologica. 30:275-283.

Hoyer, R.F. & Stewart, G.R.. (2000). Biology of the rubber boa (Charina bottae), with emphasis on C. b. umbratica. Part I: Capture, size, sexual dimorphism, and reproduction. Journal of Herpetology. 34:248-354.

Hoyer, R.F. & Stewart, G.R.. (2000). Biology of the rubber boa (Charina bottae), with emphasis on C. b. umbratica. Part II: Diet, antagonists, and predators. Journal of Herpetology. 34:354-360.

Hoyer, R. (2001) Natural History of the Rubber Boa. Retrieved from the World Wide Web June 26, 2002 http://www.kingsnake.com/rubberboa/Content/about.html

Nussbaum, R.A., Brodie, E.D.Jr., & Storm, R.M. (1983). Amphibians & Reptiles of the Pacific Northwest. pp. 255-259. Moscow, ID: University of Idaho Press.

Nussbaum, R. and Hoyer, R.F. (1974) Geographic Variation and the validity of subspecies in the rubber boa, Charina bottae. Northwest Science. 48:219-229.

Pickwell, G. (1947). Amphibians and Reptiles of the Pacific States. pp.40, 192-193. Stanford, CA: Stanford University Press.

Rodriguez-Robles, J.A., Stewart, G.R., & Papenfuss, T.J. (2001). Mitochondrial DNA-based phylogeography of North American rubber boas, Charina bottae (Serpentes: Boidae). Molecular phylogenetics and Evolution: 18(2): 227-237.