The Rosy boa is a small nonvenomous snake native to the United States. A large adult has a body width about the diameter of a golf ball. These snakes are highly variable in color. Their 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. Stripe colors can be orange, maroon, rust, brown, or black. Interspace colors can be shades of light to dark gray, yellow, or tan.
Rosy boas are 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, they range throughout the Colorado and Mojave Deserts and also occupy the coastal areas of Los Angeles, Orange, Riverside, and San Diego Counties. In Arizona, Rosy boas occupy the Mojave Desert and the western areas of the Sonoran Desert. In Sonora, they range from the border with the United States south throughout the Sonoran Desert to at least as far south as Ortiz. In Baja California, Rosy boas range throughout the entire peninsula except in areas of extremely dry or rockless desert. These snakes inhabit the desert, arid scrub, brushland, sandy plains, rocky and mountain slopes.
Rosy boas spend most of their life concealed beneath rocks and in crevices to escape the elements and natural predators. In areas with few rocks, these snakes use rodent burrows for concealment. Rosy boas generally brumate (hibernate) during the winter, and are active during the spring, summer, and fall. The spring is the breeding season, resulting in their highest rate of activity. Another reason why Rosy boas may be active on the surface of the ground is to find prey or new territory. These snakes can be active during any hour of the day, but during hot weather, they are primarily nocturnal. In the spring, they are often out in the afternoon and early evening. In the late spring and summer, they are active from dusk to late into the night. Because most populations of the Rosy boa live in exceedingly dry habitats, activity is often highly moisture-dependent. During dry periods they remain deep underground to stay hydrated. Recent rainfall often results in a flurry of surface activity. Rosy boas are one of the slowest-moving snakes in the world. It is unable to pursue prey and must either wait in ambush or stalk its meals. When a meal is within reach, usually a few inches, a rosy boa strikes with surprising speed and accuracy. These snakes are extremely docile when encountered by humans. When disturbed, they usually roll into a compact ball with the head in the center. They don't bite in defense, but rather will release a foul-smelling musk from the base of the tail when threatened.
Rosy boas are carnivores. They hunt mainly for small mammals, but may also take other prey items, such as lizards, birds, and mammals. Pack rats, baby rabbits, deer mice, and kangaroo rats make up a large portion of their diet.
Rosy boas have a polygynandrous (promiscuous) mating system in which both males and females have multiple partners. These snakes are ovoviviparous and give birth to fully-developed live young. Their breeding season occurs from May through July and the gestation period lasts around 103-143 days. Females give birth to about 6 snakelets in a brood; they are 30 cm (12 in.) long and are independent immediately. Young males and females usually reach reproductive maturity at 2-3 years of age.
There are no major threats to Rosy boas at present. However, they suffer from road mortality, urbanization, habitat fragmentation, and over-collecting for the pet trade.
According to IUCN, the Rosy boa is locally common and widespread throughout its range but no overall population estimate is available. Currently, this species is classified as Least Concern (LC) on the IUCN Red List but its numbers today are decreasing.
Rosy boas are important for their ecosystem because due to their diet these snakes help to control populations of small mammals, especially rodents.