Wild Things: Rock squirrel (Otospermophilus variegatus)
The Rock squirrel is a ground-dwelling squirrel about the size of a prairie dog, approximately 10-12 inches in body length, with a tail of about 8 inches. They are mottled light brown to gray in color with a white eye ring and ears that extend over the top of the head.
They are native to North America, occurring in the arid regions of Colorado, Utah, Arizona, Nevada, New Mexico, Texas, and down into Mexico. The preferred habitat is rocky terrain, cliffs, talus slopes, and disturbed areas along roadsides with brushy vegetation. They dig burrows that serve as shelter, birthing, and food storage chambers.
The diet of Rock squirrels is mostly vegetarian, consisting of leaves and shoots of plants. They also consume seeds, nuts, acorns, and fruits. They will take bird eggs and insects, and may also scavenge carrion. While foraging, Rock squirrels will eat some foods immediately, but they will also collect large numbers of hard mast items, such as acorns, for later consumption. They carry these items back to the burrow in cheek pouches, which can be greatly distended and carry numbers of acorns and other food items.
Though primarily ground squirrels, Rock squirrels will climb up into shrubby vegetation and even taller trees to forage for leaf shoots and other foods, or to escape a terrestrial predator if they are away from a burrow. They also bask on promontories such as rocks or up in trees, where they can at the same time be alert for approaching predators and warn colony members by their loud, piercing alarm chirp. Rock squirrels are rather drought tolerant, being able to go long periods without access to water.
Their tree climbing ability is impressive. In addition to observing rock squirrels foraging on leaves and berries high in roadside bushes, I watched one a few months ago climb about 20 feet up the trunk of a ponderosa pine tree and perch on a limb there. And this past weekend, I watched one running fast across a trail and continue up a small sapling oak on the other side without breaking stride. It seemed to run up the tree with the same strides it used on flat ground, rather than climbing it with the kind of lurching jumps of tree squirrels.
The social structure of Rock squirrels is termed polygyny, in which a dominant male mates with a group of females in a colony. The male will defend his female mates and offspring against other males, ensuring that all young are sired by him. Females produce two litters per year, one in the spring from April to June and a second one in late summer, August to September. After a month-long gestation period, the 3-9 young nurse in the burrow for a couple of months.
Once they emerge and begin aboveground activity, the young are able to forage on their own. They remain near the natal burrow during the first summer and fall, protected by their mother. During the next season female offspring normally stay close to the home colony while males often disperse to establish their own colony. Members of a Rock squirrel family colony maintain contact and reduce stress by tactile greetings, touching noses and sniffing to confirm identity.
Rock squirrels may hibernate over winter in colder northern regions, but are active year-round in warmer southern habitats. In especially hot weather they will go into the burrow and become dormant. This estivation state is similar to hibernation, but takes place to avoid hot conditions rather than freezing temperatures. The resting squirrels will adopt a slower metabolic rate and wait until temperatures moderate before becoming active again.
As with all ground dwelling small mammals, a variety of predators will take Rock squirrels when they can catch them. Hawks, snakes, and carnivores like coyotes, bobcats, foxes, and raccoons all take some. As diurnal mammals, Rock squirrels are primarily vulnerable to daytime predators, though snakes hunting at night can take them in their burrows. They do have some defensive capability, sometimes rushing an attacker or tossing litter at it. Rock squirrels may also be aggressive to other species, including humans, chasing them if they feel their colony territory is being invaded. Their ecological role as potential food for a variety of predatory species makes Rock squirrels a vital part of the arid southwestern ecosystems where they occur. They also serve to disperse the seeds of many plant species and their burrows, when abandoned, provide refuge for other wildlife species.
The IUCN lists the Rock squirrel population as stable and of Least Concern at this time.
James Taulman is a semi-retired research wildlife biologist who worked with the U.S.F.S. research branch and taught zoology, ecology, and more in several university positions. He lives in the East Mountains, and explores natural areas observing native wildlife and conducting independent research projects. Search for James Taulman on Youtube to see wildlife videos.