MO Portrait

Rubber Boa

Rubber boaCharina bottae
By Sam Curtis. Photo by Michael H. Francis

Steve Moore was hiking in the Bridger Mountains last spring when he came across what looked like a giant brown worm, about 2 feet long, lying across the trail. “It was blunt at both ends,” says Moore. “I couldn’t tell its head from its tail.” He knew from past experience it was a rubber boa.

Not wanting dogs or other hikers to bother the snake, Moore tried to move the boa off the trail. “It curled into a tight ball with what looked like its head sticking up in the air, but it really was its tail,” he says. “This is a classic defensive posture for rubber boas, sort of a sacrificial pose, that seems to say to potential predators, ‘Bite me if you want, but you won’t get anything important.’”

Moore slipped his hand under the balled snake, which didn’t budge from its defensive posture, and moved it well off the trail where it wouldn’t be further disturbed.

Most Montanans have never seen this unique snake, but Moore, an avid hiker, has encountered about a dozen, mainly on forest trails at elevations of 5,000 to 9,000 feet. He once saw a boa with two large lumps in its body. “It looked like it had eaten a couple of mice, whole,” Moore says.

Montana’s rubber boa is the northernmost relative of the giant boa constrictor and anaconda of South America. Like those snakes, it kills prey by squeezing it to death with its muscular body and swallows the meal whole.

The rubber boa’s common name comes from its rubbery appearance, the result of small, smooth scales and somewhat loose skin. Its scientific name, Charina bottae, is derived from the Greek word charieis, meaning graceful, and from the Italian explorer Paolo Emilio Botta, who collected the first specimen in California in the 1820s.

Uniformly colored on its back, the adult snake ranges from tan to brown to olive green. Its yellow underside may be mottled with brown or black. Newborns are lighter in color, appearing tan or even pink and slightly translucent.

Rubber boas are one of the smallest members of the boa family, reaching only 14 to 28 inches long. They have a uniformly thick body, blunt head with small eyes, and blunt tail. Because the head and tail look similar, it is sometimes called the two-headed snake.

Snakes in the primitive Boidae family, of which the rubber boa is a member, have vestiges of hind legs in the form of small “spurs” on each side of their vent, passed down from lizardlike ancestors that lived 135 million years ago.

Distribution and habitat
Rubber boas are found from southern California north to British Columbia and east to the Rocky Mountains. They adapt to widely varying habitats, living in arid mountains of California, rainy forests of the Northwest, and inland grasslands. In Montana, rubber boas mainly inhabit forested foothills and rocky areas near water, ranging from the eastern slopes of the Continental Divide westward to the Idaho border.

Because rubber boas move slowly and are most active at night and twilight, people rarely see them. The snakes spend much of their time under rocks, logs, and forest litter and in rodent burrows. The only time they come out in the open is when air temperatures are between 65 and 75 degrees.

A rubber boa’s bite is rare and harmless. When frightened, the snake coils into a ball with its head tucked underneath and tail sticking out. Many rubber boas have scars on their tails indicating how well the tactic has fooled hawks, coyotes, raccoons, skunks, and other predators.

Because rubber boas are relatively slow, they seek defenseless newborn mice, voles, and shrews still in the nest. (Some tail scarring may be from adult rodents attacking the tail while the snake consumes the young.) Boas also consume small adult rodents, as well as lizards and an occasional bird.

Some scientists believe a rubber boa’s home range could be as small as 100 square yards if the snake can find adequate food, cover, and hibernation sites there.

Little is known about the breeding habits of this snake. Mating likely occurs in April or May, and two to eight live young are born in late August or September. The young are thought to hibernate with adults and disperse the following spring. Rubber boas reach sex­ual maturity when they’re about 14 inches long. They may live 15 to 20 years in the wild.

Rubber boas are doing well. Globally, populations are considered secure. In Mon­tana, rubber boa populations appear secure, though the species may be rare in parts of its range, especially at the periphery.Bear bullet