Natural History (and other info) of the Rubber Boa
Genus: Charina - Derived from Greek
Species: bottae - Named after Paolo Emilio Botta, an Italian explorer serving aboard the ship Heros as doctor, visited California in the 1820's, and published his book Observations on the Inhabitants of California.
Rubber boas are one of the smallest members of the boa family, and one of the northern most ranging. Adults are generally a uniform color dorsally, ranging from tanned leather brown (southern population are almost always light tan), olive brown, medium brown, to a dark chocolate brown.1 . Their ventral surface (belly) is most often a light yellow with brown mottling in some adults. Babies are born pink and slightly transparent, and gradually darken with age. Color variations occur between AND within each locality. In the Pacific Northwest, adult males generally reach an average length of about 21 inches, with the females slightly longer - on average 26 inches. Adults from different regions may vary slightly, such as the Southern Rubber Boa (C. b. umbratica), the smallest subspecies, where males typically max out at 18 inches, and females at 22 inches. 2 See Photos Page for photos of colors and variations.
RAAB Mill site female #1 was found, surveyed, and released by Richard Hoyer in 1971. Over many years she was found over and over again, until finally kept and retained in captivity. Thirty years later she is still alive. When first found she was an adult with extensive scarring. Extrapolation of likely age at time of capture based on growth rates of other wild Rubber Boas from the same locality indicate that she was at minimum 15 years old, and likely more than 20+ years old. 3 In the year 2001 she turned at least Fifty years old! And possibly as much as 70!! More than half of her life was spent in the wild, with the remainder in captivity. She produced a litter of young in the summer of 2000 at the ripe old age of ~49-69 years! See RAAB #1 History for further details of her life history. Other specimens have not lived so long, but demonstrate that Rubber Boas commonly live beyond twenty or thirty years, even in the wild. 3 There are several other wild individuals that Richard Hoyer has found every few years for at least two decades.
Rubber Boas are fossorial (definition), or at least semi-fossorial. They do spend a lot of time underground, but like other snakes, primarily use existing rodent tunnels or rock fractures. The are also nocturnal/crepuscular (definition), and therefore are usually not active during the day, and prefer to hide underground or under pieces surface cover than bask in the open. Rubber Boas hibernate during the winter months over their entire range, sometimes from mid October to to mid March. During Spring and Fall they may be found under surface objects thermoregulating in the warm sun. During Summer when the weather is warm, Rubber boas rarely are found on the surface, and remain underground to keep cool and moist. However, given optimal conditions, they can be found active in the summer indicating that they do not avestate. In almost every description found of Rubber Boas, it is mentioned that they are good swimmers. There does not seem to be a firm basis for this conclusion. Young boas are able to float, and all boas can keep their head above the water, but they are poor swimmers when compaired to some other species.
Rubber boas are incredibly docile snakes and are ideal for handling by children and those trying to overcome a fear of snakes. Unlike many other snakes, they never use striking as a defense mechanism, although if handled too roughly, they will musk the holder (excrete very smelly substance from their vent), but absolutely will not strike in defense. Upon being picked up, a Rubber boa will gently wrap around the holder's wrist for upwards of an hour or more before seeking to crawl around. Even when warm and active, Rubber Boas rarely move swiftly.
Rubber boas can be found from as far south as the San Bernardino and San Jacinto Mountains to the east of Los Angeles, northward in a nearly continuous distribution to British Columbia, and eastward through Idaho, northern Nevada, Utah, central Montana, and western Wyoming. 4 There are also reports of sightings in S.W. Alberta, and N.W. Colorado. Although seldom seen, they are usually common where the habitat is adequate, 1, 3 with population densities at least as great as 20 per hectare! 8 See range map.
Rubber boas can be found in a very wide range of different habitats from the open pockets in coniferous rainforests of the Pacific Northwest to the dry arid mountains of southern California. They generally are not as heat tolerant as many other snakes, and disappear in warmer weather to find cooler surroundings and moisture. In all of these habitats, Rubber Boas are not often found on top of the ground and vegetation. They spend the vast majority of time under logs, rocks, or underground in rock openings or rodent burrows. See Photos Page for examples of habitat used by Rubber Boas. See additional discussion on the Habitat page.
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 after year in the same location, or within 100 meters of last capture. 2, 3 Yet there is at least some small portion of the population that is migratory for one reason or another including excessive competition, too dense of tree canopy growth, lack of prey, etc. Young born in the fall disperse the following spring shortly after emergence from hibernation. 3
Rubber boas are slow, small snakes that primarily prey on young nestling mammals (voles, shrews, deer mice, etc). When nestling rodents are encountered, they will eat the entire litter if possible, deflecting any attacks from the mother mouse with their blunt tail. Adult wild Rubber boas often have extensive scarring on their tails. Although baby rodents are the preferred prey, they will eat lizard and snake eggs; and to a lesser extent, lizards, baby birds, baby bat, and there is one recorded instance of eating another snake. My personal opinion is that the instances where another snake has been consumed is more due to accidents than intent to eat another snake. Rubber Boas have been observed constricting multiple prey simultaneously. 3,5
Rubber Boas are vulnerable to most any carnivorous predator due to their slow
nature and lack of active defenses. Other snakes, birds of prey, Ravens,
Coyotes, Raccoons, Skunks, Moles, and Cats are only a few of the known
predators. The Rubber Boas primary defense against predatation is its secretive
It was previously thought that the reason a Rubber Boa's tail was often scarred is due to their tendency to ball up, and use the tail as a false head when attacked by a predator. Common reasoning though tells us that such a defense is not useful against the predators listed above. Tail tip scarring occurs more commonly from deflecting the attacks of a mother mouse trying to defend her nest as the Rubber Boa consumes the nestling mice. Aggression trials have demonstrated that a Rubber Boa, once feeding, may be undeterred by repeated attacks from a mouse, simply holding the mouse at bay with "false strikes" of the tail as it is flipped back and forth.
As with many other animals, the taxonomic classification is uncertain and debated. Paolo Emilio Botta collected the first specimen of this snake in the 19th century. In 1920, Van Denburgh, described three subspecies which was confirmed by Klauber in 19436 ; Pacific (C.b. bottae), Great Basin (C.b. utahensis), and Southern Californian (C.b. umbratica). Initial subspecies were designated based on scale variations noted. In 1974, the Great Basin subspecies was dropped when it was noted that individuals in Western Oregon did not fit the guidelines established for identification between Pacific and Great Basin subspecies.7 Initial confusion over differences occurred due to an apparent misunderstanding of the wide variation in scale patterns between and within each locality. In fact, each individual snake may be identified by the "fingerprint" of their head and ventral scales unique to each snake. Scalation features can be extremely variable within a single locale bringing into question the ability to use such morphological traits to differentiate between subspecies of the Rubber Boa. Some sources list utahensis as a valid subspecies, while other sources do not. The Southern Rubber Boa subspecies is currently listed as threatened by the CA DF&G, and therefore protected by California law (although evidence exists that they are abundant, and range further than currently published).2 3 In 2001, a paper9 was published in which it was proposed that the Southern Rubber Boa is a separate species. This may be the case, but is seriously doubted by other experts on the Rubber Boa, due to numerous errors and assumptions made in this paper that may not be appropriate. Certain scalation features of umbratica consistently differ from other Rubber Boas, and other morphological traits such as diminutive size do indicate the validity of subspecies status. Further work, building off of the mtDND work already performed may be necessary to sort all this out.
The closest relative to the Rubber Boa is the Rosy Boa, also native to Western North America. In the southern most part of the Rubber Boa's range, and the northern most part of the Rosy Boa's range, they live in the same mountain ranges, although generally at differing elevations. There is at least one place I am aware of where a Rosy Boa and a Rubber Boa were found to be sympatric. See Jordan's excellent page regarding Rosy Boas.
Sand Boas also appear to be closely related to Rubber and Rosy Boas. See Chris's excellent site about Sand Boas He also has devoted quite a bit of time discussing the taxonomic relationships of these species Here
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1 - Hoyer, R. F. 1974. Description of a rubber boa (Chrina bottae) population from western Oregon. Herpetologica. 30:275-283
2 - Hoyer, R. F. and 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
3 - Personal interviews with Richard Hoyer.
4 - Stebbins, R. C. 1955. A Field Guide to Western Reptiles and Amphibians, 2nd ed. Houghton, Mifflin, Boston.
5- Hoyer, R. F. and Stewart, G. R. 2000. Biology of the Rubber Boa (Chrina bottae), with Emphasis on C. b. umbratica. Part II: Diet, Antagonists, and Predators. Journal of Herpetology. 34:354-360
6- Klauber, L. M. 1943. The subspecies of the rubber boa, Charina. Trans. San Diego Soc. Natur. Hist. 10:83-90
7 - 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
8 - Personal observation in Spring of 2001 from one search of 15 minutes. 12 boas found in an area of .6 hectare, or 1.5 acres. Note: weather was unfaborable for finding Rubber Boas, yet a population of 8 per acre was found in this one short search! Density could be higher, but this represents minimimum density at a given point in time in good habitat.
9 - Rodrigues-Robles, J. A., Stewart, G. R., Papenfuss, T. J. 2001. Mitochondrial DNA Based Phylogeography of North American Rubber Boas, Charina bottae. Molecular Phylogenetics and Evolution. Vol. 18 No. 2 pp.227-237.
© 2001 by Ryan Hoyer.