The Rosy Boa
by Jay Sharp
The rosy boa, which makes its home in southern California, western Arizona, northern Baja and western Sonora, holds the distinction of being one of the smallest members of the boa constrictor family. Variably colored, it has created confusion among herpetologists, who have proposed five or more subspecies, including, for instance: the coastal rosy boa, the Arizona rosy boa, the desert rosy boa, the Baja rosy boa, and the Mexican rosy boa.
Characteristics of the Rosy Boa
The adult rosy boa measures only a tenth the length of the family's largest member, the anaconda, which may reach more than 30 feet. Reflecting the family's evolutionary debt to lizards, the rosy boa has retained two functional lungs (most snakes have only one), and it has remnants of a pelvis, with vestigial hind limbs, or external claw-like spurs, near its vent.
- Body, Length and Weight: The adult rosy boa, with a fairly stocky and powerfully-muscled body, a stubby tail and smooth scales (actually tough folds of skin), typically measures about three feet in length. It weighs one to two pounds. The female is somewhat larger than the male.
- Body Temperature Regulation: Like all snakes, the rosy boa is "ectothermic," which means that it takes its body heat from an external source, like the sun or surrounding soil and rocks. It must avoid extreme temperatures to maintain the proper body temperature.
- Color and Pattern: The snake has three distinct stripes – one down the center of its back and one on either side of the body – that run its entire length. The colors of the stripes, which have serrated margins, vary among the subspecies. The coastal rosy boa, for instance, may have rose to orange stripes set against a cream-colored or gray background. The desert rosy boa may have chocolate-colored stripes set against a cream-colored background. The rosy boa's typical overall reddish hue likely gave rise to its common name.
- Head: The snake's head is slightly larger than its neck. It has small eyes with vertical pupils. Like other non-poisonous snakes, it has two rows of small sharp re-curved, or "hooked," teeth on the top jaw and one row on the bottom; these facilitate seizing, holding and swallowing prey—not chewing. Its jaws are attached, not by bone, but by ligaments, which permit the snake to stretch open its mouth wide enough to swallow surprisingly large prey. Like other snakes, the rosy boa has a bifid (or forked) tongue and a chemical sensory organ (called a "Jacobsen's organ") on the roof of its mouth. It uses its tongue to collect minute scent particles from the air and deposit them on the Jacobsen's organ for detecting possible prey, predators or a potential mate. It has sensitive thermal receptors, or heat sensors, near its lips.
- Senses: The snake has good vision. With its tongue and Jacobsen's organ, it has an acute sense of "smell." With its heat sensors, it can distinguish between temperature differences of no more than a small fraction of a degree. It has no external ears and therefore no hearing, but thanks to remnants of an inner ear, it can sense low frequency sounds and ground vibrations. The snake's senses equip it to track prey, especially warm-blooded prey, in either sunlight or darkness.
- Movement: Unlike, for instance, a gopher snake or a king snake, which travel forward with an undulating lateral S-shaped movement, the rosy boa travels with a rectilinear motion, something like a caterpillar or an inchworm. It can climb smooth surfaces or tree limbs, said the San Diego Zoo, but it "can't move very fast, only about 1 mile per hour…on open ground." It is, in fact, one of the slowest moving snakes in the world.
Distribution and Habitat
The rosy boa, including the subspecies, occupies the southern tip of California, the southwestern corner of Arizona, the northwestern corner of Sonora, and the entire length of the Baja Peninsula. It also occurs on some offshore islands. It occupies habitats from low desert basins to 7000-foot high mountain slopes, said specialist J. A. Perrett. It flourishes in "coastal desert canyons, rocky shrubland, desert slopes and creek-beds, and boulder strewn hillsides."
Behavior and Life Cycle
The rosy boa spends most of its life sequestered in rocky crevices or abandoned animal burrows, secreted from predators and protected from temperature extremes. Across its range, it spends the coolest months in brumation, its body effectively dormant, similar to mammalian hibernation.
Emerging in the spring from dormancy, it looks for a mate. Encountering a female, a "male slowly and deliberately flicks his tongue rapidly over most of [her] body," said Seattle's Woodland Park Zoo. If he finds her receptive, he gradually mounts her, stroking her tail with his vestigial hind limbs. She lifts her tail, and the pair couple. The two then go their separate ways.
About 130 days later, typically in October, the female gives birth to as many as a dozen live 6- to 12-inch-long young, which, said the San Diego Zoo, are enveloped in a protective membrane that they must split open to find freedom. The mother promptly abandons them, and "they are on their own to start protecting themselves (usually by hiding at first) and to find food," depending on instinct for survival. If a rosy boa survives to maturity, it may live for 18 to 22 years.
During the moderate months, the rosy boa may emerge from its hideaways to forage almost any time of day. During the hotter months, it emerges to hunt primarily in the evening and nighttime hours. Because it moves so slowly, it cannot chase down prey. It either ambushes or stalks small mammals, reptiles, frogs or birds. Once within reach, the usually sluggish snake strikes with stunning swiftness, seizing its prize with its re-curved teeth. It envelops and suffocates the victim within its powerful coils—hence its family name, "constrictor." It swallows its catch head first and whole. Its esophagus muscles, with rhythmic contractions, draw the catch slowly into the stomach, where it will be digested by very strong acids over several days. Because the snake has a slow metabolism, it may not eat again for weeks.
If threatened by a predator, for instance, a hawk or other predatory bird, the rosy boa rolls itself into a ball, with its head enveloped for protection and its tail exposed and wiggling as a distraction. It emits an evil-smelling odor from glands near the base of its tail. It may suffer bites to its tail, which may be scarred for the rest of the snake's life.
The International Union for Conservation of Nature and Natural Resources rates the rosy boa as a species of "least concern" and says its population trend is "stable." The U. S. Geological Survey, however, points out that "impacts from roads, habitat fragmentation, and urbanization are greatly impacting rosy boa populations, even within natural reserves." The snake may have to be protected in its natural habitat in the near future if it is to survive long term.
- The rosy boa is one of only two species of boas in the United States. The other, the rubber boa, is widely distributed across the northwestern United State, extending northward into British Columbia.
- The boa can inhale and exhale a large amount of air through its windpipe, creating a hissing sound that serves as a warning to potential enemies.
- The rosy boa, colorful, gentle, moderate-sized and easy to feed and shelter, has become a favored pet among many enthusiasts.
You might also be interested in reading about the Ball Python (also called Royal Python) or Rattlesnakes. To see a list of pages about snakes and other reptiles, click here.
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