The rosy boa (Lichanura trivirgata) is a snake of the Boidae family, one of only two members of that family native to the United States. The other is the rubber boa (Charina bottae). The rosy boa is native to the American Southwest, and Baja California and Sonora, Mexico.
These small, attractive snakes normally attain a length of 17–34 inches, although some Coastal specimens from California reach 36–44 inches. A large adult has a body width about the diameter of a golf ball. Coloration in rosy boas is highly variable, and usually locale-specific. The common name is derived from the rosy or salmon coloration that is common on the belly of rosy boas originating from coastal southern California and Baja Mexico. Most rosy boas do not have this ventral coloration but instead have a series of dark to orange spots on a light-colored background.
Almost all rosy boas have at least some trace of three longitudinal stripes, one down the center of the back, and two on the lower sides. The appearance of these stripes varies widely, from extremely straight and having high contrast with the interspaces, to extremely broken with almost no contrast with the interspaces. Stripe colors can be orange, maroon, rust, brown, or black. Interspace colors can be shades of light to dark gray, yellow, or tan.
The rosy boa is found in the southwestern United States in the states of California and Arizona, and northwestern Mexico in the states of Baja California and Sonora. In California, the rosy boa ranges throughout the Colorado and Mojave deserts and also occupies the coastal areas of Los Angeles, Orange, Riverside, and San Diego counties. In Arizona, the rosy boa occupies the Mojave Desert and the western areas of the Sonoran Desert. It is absent from the eastern and northern halves of the state. In Sonora, the rosy boa ranges from the border with the United States south throughout the Sonoran Desert to at least as far south as Ortiz. In Baja California, the rosy boa is almost ubiquitous ranging throughout the entire peninsula except in areas of extremely dry or rockless desert.
Rosy boas spend most of their lives concealed beneath rocks and in crevices to escape the elements and natural predators. Granite outcroppings are the most common geologic association inhabited by the rosy boa. Less often they are found in association with volcanic or other rock types. Only in rare places do rosy boas inhabit rockless environments. In areas with few rocks rosy boas will use rodent burrows for concealment.
Rosy boas’ activity season follows local weather patterns; however, they are generally dormant during the winter, and active during the spring, summer and fall. Like all snakes, they are dependent on external temperatures to promote such normal bodily functions as digestion and gestation. Throughout most of their range the winter is too cold for these functions and the rosy boas go into a dormant state called brumation. The spring is breeding season for Rosy Boas, resulting in their highest rate of activity. Most Rosy Boas are encountered in spring as they leave the security of their rock piles and crevices to seek mates. Another reason rosy boas may be active on the surface of the ground is to find prey or new territory.
The surface activity of rosy boas can take place during any hour of the day, but during hot weather they are primarily nocturnal. In the spring, they are often abroad in the afternoon and early evening. In the late spring and summer, this activity period switches from dusk to late into the night. Because most populations of rosy boas live in exceedingly dry habitats, their activity is often highly moisture dependent. During dry periods they remain deep underground to assist in remaining hydrated. Recent rainfall often results in a flurry of surface activity.
These snakes forage mainly for small mammals but have occasionally been known to take other prey items such as birds and lizards. Pack rats, baby rabbits, deer mice, and kangaroo rats make up a large portion of their diet. Rosy boas are one of the slowest-moving species of snake in the world. They are unable to pursue prey and must either wait in ambush or stalk their meals. When a meal is within reach, usually a few inches, a rosy boa will strike with surprising speed and accuracy. Prey is secured with tiny rows of needle-sharp teeth, then suffocated through constriction.
Rosy boas are extremely docile when encountered by humans. When disturbed they usually roll into a compact ball with the head in the center. The species is not prone to bite in defense, and when human bites have occurred they have usually been the result of a feeding response with a captive animal. All rosy boa bites are nonvenomous. Their extreme docility and their attractive coloration have made rosy boas very popular with herpetoculturists.
Rosy boas bear live young, about six in a brood, with newborns about 30 cm (12 in.) in length.
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