"Rock squirrels, with their long, bushy tails, look very much like tree squirrels, but seldom climb trees. They are most commonly found in rocky habitats—canyons, cliffs, and hillsides. Occasionally a nest is found in a tree, but they usually dig burrows, choosing a place that offers a near-by lookout where they can watch for danger. Other mammals and even burrowing owls are known to use their dens if the squirrels abandon them. The rock squirrel's geographic range is large, and it is found at elevations from sea level to 2,900 m. Females produce one litter a year in places or at elevations where winter weather lasts longer, and two in warmer parts of their range."

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This species distribution constitutes an extensive range in southwestern United States and Mexico, from eastern Nevada, Utah, and Colorado, south through west Texas (in the east) and southeast California (in the west) to southernmost Puebla, Mexico. It has been found from sea level to 2,900 m (Wilson and Ruff 1999).

Rock squirrels (Spermophilus variegatus) range throughout much of the southwestern United States and Mexico. In the U.S. they can be found thoughout Arizona, New Mexico, Colorado, and Utah, and they inhabit much of western Texas, southern Nevada, and the panhandle of Oklahoma. At the western edge of its range, S. variegatus occupies the southeasternmost corner of California. At the northern boundary S. variegatus has even been observed in extreme southern Idaho. In Mexico the range of rock squirrels encompasses much of Pueblo, Colima, Guerrero, Mexico, and Morelos, but they are not found in eastern coastal lowlands.

The former range of S. variegatus was more extensive than its present distribution. Fossil records show that it was present in tar pits near Los Angeles (320 km west of its present range) and in Little Box Elder Cave just west of Douglas, Wyoming (200 km north of its present range).

Biogeographic Regions: nearctic (Native )

occurs (regularly, as a native taxon) in multiple nations

United States

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

Global Range: Southwestern North America, from southeastern Nevada, Utah, Colorado, and extreme western Oklahoma south through the southwestern U.S. to Jalisco, Michoacan, and Puebla, Mexico. From near sea level to 2900 m (Arizona).

Spermophilus variegatus is the largest ground squirrel found in its range and is the largest member of the subgenus Otospermophilus. Its pelage is mottled along its back, grayish brown in front, and grayish black behind. The dorsum carries indistinct wavy markings of brown and black. The underside is usually buff white to pinkish. The head color ranges from light cinnamon to darker brown. The pelage color is highly variable between and within populations and subspecies. Rock squirrels have large heads and large eyes, and the ears extend above the top of the head and are longer than wide. The neck is stout and long for a squirrel and the tail is long and bushy. Overall, S. variegatus more closely resembles typical tree squirrels than typical ground squirrels.

Rock squirrels attain adult body lengths of 430 mm to 540 mm. The tail of S. variegatus is very long for ground squirrels, and may be 73 to 82% of the body length (172 mm to 252 mm) and as much as 44% of the total length. The adult weight of S. variegatus is highly seasonal and closely associated with annual activity cycles. Typical adult body masses are from 600 to 800 grams. The weight cycles of rock squirrels are tied to breeding activities and seasonal hibernation. Adult males and females are significantly lighter during their respective mating periods. In Arizona, males begin attaining heavy weights in early August to prepare for hibernation, whereas females begin putting on reserves later in August. The fact that males begin preparing for hibernation earlier than females may have resulted in early reports that rock squirrels display sexual dimorphism in size, where males are larger than females. However, this is probably not true, and is just merely a consequence of different peaks in breeding times and preparation for hibernation.

Range mass: 600 to 850 g.

Range length: 430 to 540 mm.

Sexual Dimorphism: sexes alike

Length: 53 cm

Weight: 800 grams

Sexual Dimorphism: Males are larger than females.

Length:
Range: 466-503 mm

Weight:
Range: 450-875 g

Habitat and Ecology

Systems

• Terrestrial

As the common name implies, rock squirrels almost always inhabit rocky locations including cliffs, canyon walls, talus slopes, boulder piles, and steep rocky hills. Within the wide range of S. variegatus, the presence of rocky terrain is critical. In places without rocky landscapes, rock squirrels may make use of old buildings and rock walls, as well as other man-made structures. Spermophilus variegatus inhabits a variety of habitat covers, but is most closely associated with upland grasslands and oak savanna. It is usually not found in open plains, deserts, and upper montane forests.

Perhaps the most important habitat features for rock squirrels are those near den sites. Within occupied habitats, den sites are usually placed in oak savanna or riparian areas. Dens are almost always on steeper slopes, closer to washes, and have more potential lookout points. The slope direction that the den faces seems to be important too. In the north, dens are usually placed on south or west-facing slopes so that they will have earlier snow melt in the spring. In the south, dens are usually on north-facing slopes to provide less stressful summer radiation. Den sites have more shade, are closer to shade trees, and closer to oaks. The amount of ground cover is usually less and the woody debris is usually greater at den locations.

The reason that rock squirrels make their dens on steeper slopes is probably related to better drainage in the summer rainy season. Also, the large amounts of woody debris near dens provide good lookout points. The proximity to washes is most likely tied to the more productive nature of these areas and the associated greater food sources.

Range elevation: 0 to 2900 m.

Habitat Regions: temperate ; terrestrial

Terrestrial Biomes: savanna or grassland ; scrub forest

Other Habitat Features: riparian

Comments: Rocky habitats; cliffs, canyons, hillsides, arroyos, talus slopes, old buildings, bridges, terraced roads, stone walls. Usually not in open plains, wide valleys, deserts, or high montane forest. Burrows under rocks, bushes, trees, etc.

Non-Migrant: Yes. At least some populations of this species do not make significant seasonal migrations. Juvenile dispersal is not considered a migration.

Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).

Locally Migrant: No. No populations of this species make annual migrations of over 200 km.

The daily activities of S. variegatus consist mostly of foraging and feeding. Rock squirrels forage both in the trees and on the ground. They climb bushes and trees in search of food nearly as well as tree squirrels. Most foraging takes place in the morning, where food ventures last about twelve minutes at a time. As temperatures increase, rock squirrels spend less and less time during each foraging trip. Rock squirrels eat some of their food as they collect it, but often bring it to a lookout point to eat. In addition, rock squirrels collect large numbers of food items in their cheek pouches and bury their bounty in small holes outside their burrows. The cheek pouches of rock squirrels are quite large, and one researcher counted 62 Gambel's oak acorns carried in the pouches of one individual.

A very wide range of food items makes up the diet of rock squirrels. S. variegatus is known to eat green vegetation, fruits, nuts, grains, berries, roots, flowers, cacti, invertebrates, and small vertebrates. Some of the most important plant foods include, oak acorns, wild sumac berries, cherries, wild plums, wild rye, assorted grasses, walnuts, pine nuts, mesquite, juniper berries, currants, cacti, and agaves. The primary invertebrate food items are grasshoppers, beetles, and earthworms. The wild vertebrates that rock squirrels prey on include young wild turkeys (Meleagris gallopavo) and other fowl. In captivity S. variegatus has even eaten a bannertail kangaroo rat (Dipodomys spectabilis).

The diet of rock squirrels changes seasonally depending on availability of food items. Mostly green vegetation is consumed in early spring, berries and and other fruits are consumed in the summer, and grains, seeds, and nuts are eaten in the fall.

Some preliminary research has shown that where rock squirrels feed on pinyon pine (Pinus edulis), they compete for seeds against birds and insects. When insect herbivores were removed from pinyon pines, small mammals (including S. variegatus) consumed more of the seeds.

Animal Foods: birds; mammals; eggs; insects; terrestrial worms

Plant Foods: leaves; roots and tubers; wood, bark, or stems; seeds, grains, and nuts; fruit; flowers

Foraging Behavior: stores or caches food

Primary Diet: omnivore

Comments: Feeds on a wide variety of plant material: nuts, seeds, grain, berries, roots, green vegetation, cactus, invertebrates, and fresh and dried meat. Diet changes seasonally, depending on local availability of foods.

Rock squirrels are important plant dispersers. They often gather seeds and nuts and bury them near their den sites. Also, many animals prey upon this species. Rock squirrels may be an important food source for birds of prey, as well as snakes and many others.

Spermophilus variegatus is host to many species of ectoparasites. Lice, larval Diptera, mites, ticks, and fleas are all parasites of rock squirrels. At least nineteen species of flea use the species as a host. Many of these flea species are vectors of the sylvatic plague, and rock squirrels are susceptible to the plague organism (Yersinia pestis) and may serve as reservoirs for the disease. Other ectoparasites are known as vectors of tularemia, brucellosis, and Rocky Mountain spotted fever.

Ecosystem Impact: disperses seeds

Rock squirrels are preyed upon by many different animals. Snakes, birds of prey, and many other animals all feed on S. variegatus. In addition, people have been known to kill rock squirrels for food and because they are regarded as pests. Rock squirrels are sympatric with venomous snake species almost throughout their range. Because of this, S. variegatus has developed a distinct anti-snake behavior, which is similar to that of California ground squirrels. When close to a snake, a rock squirrel will approach the snake and examine it with a catious elongate posture. It then erects its tail and waves it from side to side and harrasses the snake by throwing debris. Sometimes, a rock squirrel even attacks the snake. Spermophilus variegatus also has the capacity to neutralize rattlesnake venom.

Another anti-predator behavior of S. variegatus is that it often rests, feeds, and sunbathes in trees or on high rocks or stumps. This allows it to look out for potential predators and communicate alarm calls to other individuals. When disturbed, S. variegatus emits a strong musky scent from its anal glands. This is probably some type of defense mechanism.

Known Predators:

• golden eagles (Aquila chrysaetos)
• bobcats (Lynx rufus)
• ringtails (Bassariscus astutus)
• gray foxes (Urocyon cinereoargenteus)
• raccoons (Procyon lotor)
• coyotes (Canis latrans)
• American badgers (Taxidea taxus)
• western rattlesnakes (Crotalus viridis)
• bullsnakes (Pituophis melanoleucus)
• domestic cats (Felis silvestris)
• domestic dogs (Canis lupus familiaris)

Anti-predator Adaptations: cryptic

Tends to be colonial (maternal aggregation and dominant male, with subordinate males nearby). Home ranges are large, overlap greatly; some studies found average less than 0.5 ha, but radiotelemetric study in southeastern Arizona found spring-summer home range size averaging about 7.9 ha for males, 3.8 ha for females (Ortega 1990); the same values were reported for a population in central New Mexico (Shriner and Stacey 1991). Population density is about 2-13/ha according to Oaks et al. (1987). Density of adult residents was less than 2/ha in central New Mexico (Shriner and Stacey 1991).

Spermophilus varieagatus communicates with others in a variety of ways. Tactile communication is used in courtship behavior and in greeting and recognition between a mother and her young. Siblings also communicate by greeting with nasonasal contact. Familiar individuals often do this to greet each other too. Spermophilus variegatus has scent glands on the dorsum, posterior to the shoulders, that secrete an oily substance with a faint musky odor. This secretion is used to mark objects within a territory, and during the mating season males rub their backs on rocks and trees. Individuals also rub their cheeks against rocks and smell each other around the different gland concentrations.

The most important means of communication in rock squirrels are posture and vocalization. When unfamiliar squirrels meet they often try to assert dominance by posturing their bodies and tails in different threatening displays. When in proximity to a snake, rock squirrels use a different combination of postures, contorting their bodies in an elongate manner and waving thier tails to appear threatening to the potential predator. Spermophilus variegatus is known to make at least five different types of calls. These are used mostly as warnings. It makes long and short alarm calls that consist of chucks and whistles. These are used to locate the direction from which danger is approaching. Whistle alarms are loud and given from within burrows. It makes squeals when it feels pain or is being handled, and growls when it is involved in aggressive encounters. When threatened, it chatters its teeth as a warning.

Communication Channels: visual ; tactile ; acoustic ; chemical

Other Communication Modes: scent marks

Perception Channels: visual ; acoustic

Comments: May hibernate at higher elevations; active all year at lower elevations (Schmidly 1977). Diurnal, with peak in activity at midday in spring, fall, and winter; in morning and/or late afternoon in summer (Oaks et al. 1987). At 1500 m in southeastern Arizona, adults were relatively inactive from late October through February; juveniles were active throughout autumn (Ortega 1991). Avoids extremes in weather.

Very little is known about the lifespan of S. variegatus. Wild study animals are known to have lived for at least two and a half years.

Average lifespan

Status: captivity: 9.8 years.

Maximum longevity: 9.8 years (captivity)

The mating system of S. variegatus is polygynous and seasonal. In areas where rock squirrels form colonial aggregations, the dominant male of the colony aggressively defends the breeding rights to the females of the colony against any other adult male. Confrontations between males consist of flank-to-flank shoving that escalates into a rolling fight. Agressive encounters often result in individuals being scarred on the head and flanks. Females actively defend the area immediately around their burrows after the mating season.

Males greatly expand their home ranges during the mating season to look for females that are sexually receptive. This happens more in areas where the population of rock squirrels is less dense and less colonial. Courtship is initiated by the male and consists of nasal to nasal contact, followed by the male stroking the female's head with his paws, and then nasoanal contact. If responsive the female usually arches her back and raises her tail. The female resists the male's attempts at mounting and retreats to a burrow while the male chases. Copulation takes place in the burrow. Observations of sub-dominant males attempting to mount females outside of the burrow have been made, but the female resists such efforts.

Mating System: polygynous

The breeding season of rock squirrels is seasonal and the length and time of the breeding season depend on the latitude and altitude of the population. Unlike many other ground squirrels that emerge from hibernation in breeding condition, male rock squirrels become capable of breeding after emergence in the spring. In southeastern Arizona the testes of adult males are in the scrotal position, which indicates breeding capability, from late March until at least early July. Females are in estrus from at least mid-May to mid-June. Lactating females have been observed here in early June to early July, where the peak percentage of females lactating is in early July to late August. Mating in southeastern Arizona lasts about 9 weeks and takes place between mid-April and early July. The mating season is closely associated with the summer rains, so that juveniles will emerge from the dens shortly after the heavy rains have started and increased production of vegetation. This ensures that food for the young will be readily available when they emerge. The breeding season rock squirrels in northern Utah lasts about 4 weeks, and in central Texas it lasts about 6 weeks. In New Mexico courtship behaviors may occur for up to 14 weeks. Longer breeding seasons may be related to lower population densities, where males may have difficulty finding estrous females.

Estimates of the gestation period are around 30 days, and the pre-emergence period is approximated at 8 weeks. In southeastern Arizona, this means that the young emerge sometime between late July and late September. The young nurse for about two months while in the burrow, and begin foraging for food about 3 days after they emerge. At this point they weigh about 100 grams, and continue to gain weight for more than 2 years after birth. Adult females usually have five pair of mammae, but individuals with four pair have been reported. Juvenile rock squirrels remain within their mother's home range and may use the same burrow for as long as 14 weeks after emergence. Individual rock squirrels achieve sexual maturity at different ages in different locations and populations. In New Mexico few individuals become sexually active as yearlings, but in southeastern Arizona around 50% of yearling females lactate, and several successfully wean young. Similarly, yearling males have been observed with descended testes, but their breeding success is not known.

Breeding interval: Rock squirrels most likely breed only once per year, but some observations of two litters have been made

Breeding season: April to August

Range number of offspring: 3 to 9.

Average number of offspring: 4.8.

Average gestation period: 30 days.

Average weaning age: 2 months.

Range age at sexual or reproductive maturity (female): 1 to 2 years.

Range age at sexual or reproductive maturity (male): 1 to 2 years.

Key Reproductive Features: seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); viviparous

Average birth mass: 7.8 g.

Average gestation period: 30 days.

Average number of offspring: 4.3.

Females actively protect their natal burrows after mating. They ward off both adult males and females. After a gestation period of about 30 days, the female nurses and takes care of her young for about two months. After the young emerge from the natal burrows, females protect them at least until hibernation. Nose touching between the mother and her young is used for greeting and recognition. Female yearlings often remain near their natal burrows and yearling males usually disperse to new areas.

Parental Investment: altricial ; female parental care ; pre-fertilization (Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Provisioning: Female, Protecting: Female); post-independence association with parents

In southeastern Arizona, mating occurred mid-April to early July; percentage of lactating females peaked in July-Aug. (Ortega 1990). Gestation probably lasts about 30 days. Parturition occurs April-August. At 1500 m in southeastern Arizona, young first appeared above ground in July (Ortega 1991). Number of young emerging from natal burrow averages about 4. Lactation lasts about 2 months. One litter/year (possibly 2 in south). In southeastern Arizona, about half of yearling females lactated (Ortega 1990).

Coat changes with the seasons: rock squirrel
 

The fur of rock squirrels serves as a source of long-term, seasonal thermoregulation due to changeable hair optics and coat structure.

       
  "Physical theory predicts that animals with fur or feather coats can adjust solar heat gain independently of surface coloration or environmental factors by altering coat structure or hair optical properties. This hypothesis is tested by examining seasonal changes in the solar heat load transferred to the skin by the pelage of a desert-dwelling mammal, the rock squirrel (Spermophilus variegatus). Although coat colour remains constant, solar heat gain at low wind speeds is about 20% greater in winter coats than in summer coats. This change is in an apparently adaptive direction and is predicted to have a major effect on the animal's heat balance in nature. The determinants of these alterations in solar heat gain are explored using an empirically validated biophysical model and are found to result from changes in hair optics and coat structure. These results suggest the existence of a previously unknown mode of long-term thermoregulation that allows adjustment of solar heat gain without affecting the animal's external appearance." (Walsberg 1989:387)
  Learn more about this functional adaptation.

Presently, rock squirrels are abundant enough throughout their range not to warrant protection.

US Federal List: no special status

CITES: no special status

IUCN Red List of Threatened Species: least concern

United States

Rounded National Status Rank: N5 - Secure

Rounded Global Status Rank: G5 - Secure

Population

Population Trend

Major Threats

Conservation Actions

Spermophilus variegatus is sometimes considered a pest because it occasionally damages crops. The effect on crops is usually not significant though. Rock squirrels also carry many ectoparasites that are known vectors of tularemia, Rocky Mountain spotted fever, brucellosis, and sylvatic plague.

Negative Impacts: injures humans (carries human disease); crop pest

The rock squirrel is an important disperser of many plant seeds and fruits. It also may have been a source of food for Native Americans.

Positive Impacts: food

Comments: See Jones and Manning (1989) for decription of new subspecies tiburonensis from Isla Tiburon, Sonora, Mexico. Thorington and Hoffmann (in Wilson and Reeder 2005) did not recognize tiburonensis as a valid subspecies.