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). (National Audobon Society, 1996; Oaks, et al., 1987)
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. (Johnson, 1981; National Audobon Society, 1996; Oaks, et al., 1987; Ortega, 1987; Ortega, 1990a; Ortega, 1990b; Shriner and Stacey, 1991)
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. (Oaks, et al., 1987; National Audobon Society, 1996; Oaks, et al., 1987; Ortega, 1991)
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. (Johnson, 1979; Johnson, 1981; Oaks, et al., 1987; Ortega, 1987; Ortega, 1990a; Ortega, 1990b; Shriner and Stacey, 1991)
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. (Johnson, 1979; Johnson, 1981; Oaks, et al., 1987; Ortega, 1990a; Ortega, 1990b; Shriner and Stacey, 1991)
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. (Johnson, 1979; Johnson, 1981; Oaks, et al., 1987; Ortega, 1990a; Ortega, 1990b; Shriner and Stacey, 1991)
Spermophilus variegatus is a ground squirrel that makes use of burrows. It digs burrows under large rocks, trees, bushes, and other suitable cover with nearby lookout points. The burrows are generally short (0.3 to 1.5 meters), but have been excavated up to 5.8 meters long. The burrow consists of a main chamber, one to three outside openings, and a connecting nest chamber. Inside the burrow, rock squirrels line the ground with grasses, bark, and leaves. These burrows are used year after year, but individuals may move seasonally to different burrows. An individual usually has one home burrow and several other foraging burrows.
The sociality of rock squirrels is highly variable and related to population density. Population densities may be related to environmental factors such as food availability. In dense populations rock squirrels form colonies. These colonies are organized into maternal aggregations at a group of dens with one dominant male. Other subdominate or subordinate males may be located peripherally. The dominant male defends the colony against any other males and permits juveniles and females to come and go freely. In colonies females exhibit little or no aggression towards other females. They are often observed sitting and feeding together. After the young are born, male rock squirrels may be expelled from the territory of the females. In sparse populations the degree of sociality is much less and males and females are generally solitary. They usually only come together during the breeding season.
Spermophilus variegatus is diurnal and largely crepuscular. Their peak activity, however, is tied to the seasons. In spring, winter, and fall, squirrels tend to exhibit peak activities at mid-day. In the summer rock squirrels avoid overheating by concentrating activities in the morning or in a combination of the morning and late afternoon. In extreme heat conditions S. variegatus sometimes estivates. Temperature dictates activity, for rock squirrels curtail actvity in high temperatures and are rarely active below latitude specific temperatures. In the northern part of the range S. variegatus reduces activity when the temperature is below 10 degrees Celsius. Towards the south, in Texas, squirrels are rarely active at temperatures below 35 degrees.
Rock squirrels are facultative hibernators and build up fat reserves accordingly. Throughout most of the range of S. variegatus, it enters a circannual torpor during the winter season. During torpor rock squirrels maintain a body temperature between five and ten degrees C. In the far southern range of the species S. variegatus remains active throughout the year. In anticipation of the winter hibernation rock squirrels put on as much as 200 grams of reserves. Rock squirrels can go long periods of time without water. Individuals have been observed surviving up to 100 days on dry food alone. (Johnson, 1979; Johnson, 1981; National Audobon Society, 1996; Oaks, et al., 1987; Ortega, 1987; Ortega, 1990a; Ortega, 1990b; Ortega, 1991; Shriner and Stacey, 1991)
The home range of rock squirrels varies as the seasons change. It also depends on the population density of the squirrels. In less dense populations, individual home ranges are much larger than those in colonies. The range of adult males is greatly enlarged during the breeding season in less densely populated areas (usually May to July) and decreases towards the end of the active season before hibernation. At that time home-ranges of adult males during the breeding season may be as high as 8.0 ha, and after the breeding season they are significantly smaller (0.7 to 1.6 ha). Adult female home ranges are smaller than those of males during most of the year (about 2.8 to 4.5 ha), but stay relatively constant throughout the year. Studies of colonial populations of rock squirrels indicate much smaller home ranges. The home range of adult males averages 0.24 to 0.40 ha, and adult female home ranges average 0.15 to 0.43 ha. In sparse populations the adult male home range size is a good indicator or reproductive condition.
The home ranges of rock squirrels overlap to a high degree. The males often have home ranges separated from those of the females, save during the breeding season. In dense colonies the ranges of males and females may overlap during the entire year. (Johnson, 1979; Johnson, 1981; Ortega, 1990a; Ortega, 1990b; 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. (Johnson, 1979; Johnson, 1981; Oaks, et al., 1987; Ortega, 1990a; Ortega, 1990b; Ortega, 1991; Shriner and Stacey, 1991)
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. (Christensen and Whitman, 1993; Johnson, 1979; National Audobon Society, 1996; Oaks, et al., 1987)
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. (National Audobon Society, 1996; Oaks, et al., 1987; Owings, et al., 2001)
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. (Johnson, 1979; National Audobon Society, 1996; Oaks, et al., 1987; Ortega, 1987; Ortega, 1990a; Ortega, 1990b; Shriner and Stacey, 1991)
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. (National Audobon Society, 1996; Oaks, et al., 1987)
Presently, rock squirrels are abundant enough throughout their range not to warrant protection.
Spermophilus variegatus has a diploid chromosome number of 38. Twenty-two of the autosomes are metacentric and fourteen are submetacentric. The fundamental number is 72. The X chromosome is metacentric and the Y chromosome is acrocentric. In New Mexico different populations of rock squirrels show plymorphism in serum proteins. In addition, it has been shown that the genetic makeup of two subpopulations at a location in New Mexico are relatively close. This suggests high gene flow, and is logical since there is no physical barrier between subpopulations here.
Rock squirrels experience a seasonal adjustment of solar heat gain without changing their coat coloration or reflectivity. They do this by altering the structural and optical properties of their pelage. The species' fur consists of a dense inner coat with fine, darker hairs and a sparse outer coat with coarse, lighter hairs. The seasonal differences in coats of rock squirrels allow for a much greater solar heat load capture in winter coats than summer coats. The most important factors contributing to the increased heat gain in winter coats are changes in thermal resistance per unit fur depth in the inner coat and changes in individual hair properties. The changes in optical properties of hairs in the winter coat increase forward scattering, decrease backward scattering, and decrease absorptivity, which together improve radiation penetration. The density of guard hairs in winter coats increase by about 34%, contributing to a 21 % increase in radiation interception in the outer coat and a 28% rise in the inner coat. Also, it has been shown that rock squirrels with summer coats significantly raise metabolic heat production with increasing wind speeds.
The ability to change coat properties to reduce heat loads during the hot and dry summer season allows rock squirrels to decrease the amount of water lost to evaporation for heat dissipation. This is a very important adaptation for a diurnal animal that must endure a long summer drought with little drinking water. (Gustafson-Ropski, et al., 1989; Oaks, et al., 1987; Gustafson-Ropski, et al., 1989; Oaks, et al., 1987; Walsberg and Schmidt, 1989; Walsberg and Wolf, 1995; Walsberg, 1988; Walsberg, et al., 1997)
Matthew Wund (editor), University of Michigan-Ann Arbor.
Lucas Langstaff (author), University of Michigan-Ann Arbor, Phil Myers (editor, instructor), Museum of Zoology, University of Michigan-Ann Arbor.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
uses sound to communicate
young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
uses smells or other chemicals to communicate
used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.
active at dawn and dusk
having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
parental care is carried out by females
A substance that provides both nutrients and energy to a living thing.
the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.
having the capacity to move from one place to another.
the area in which the animal is naturally found, the region in which it is endemic.
an animal that mainly eats all kinds of things, including plants and animals
having more than one female as a mate at one time
Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).
communicates by producing scents from special gland(s) and placing them on a surface whether others can smell or taste them
scrub forests develop in areas that experience dry seasons.
breeding is confined to a particular season
reproduction that includes combining the genetic contribution of two individuals, a male and a female
associates with others of its species; forms social groups.
places a food item in a special place to be eaten later. Also called "hoarding"
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
Living on the ground.
defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement
A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.
A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.
A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.
uses sight to communicate
reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.
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National Audobon Society, 1996. Field Guide to North American Mammals. New York, New York: Alfred A. Knopf.
Oaks, E., P. Young, G. Kirkland, D. Schmidt. 1987. Spermophilus variegatus. Mammalian Species, 272: 1-8.
Ortega, J. 1987. Den site selection by the rock squirrel (Spermophilus variegatus) in southeastern Arizona. Journal of Mammalogy, 68 (4): 792-798.
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Ortega, J. 1990. Reproductive biology of the rock squirrel (Spermophilus variegatus) in southeastern Arizona. Journal of Mammalogy, 71 (3): 448-457.
Ortega, J. 1991. The annual cycles of activity and weight of rock squirrels (Spermophilus variegatus) in southeastern Arizona. American Midland Naturalist, 126 (1): 159-171.
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Shriner, W., P. Stacey. 1991. Spatial relationships and dispersal patterns in the rock squirrel, Spermophilus variegatus. Journal of Mammalogy, 72 (3): 601-606.
Walsberg, G. 1988. The significance of fur structure for solar heat gain in the rock squirrel. Journal of Mammalogy, 138: 243-257.
Walsberg, G., C. Schmidt. 1989. Seasonal adjustment of solar heat gain in a desert mammal by altering coat properties independently of surface coloration. Journal of Experimental Biology, 142: 387-400.
Walsberg, G., T. Weaver, B. Wolf. 1997. Seasonal adjustment of solar heat gain independent of coat coloration in a desert mammal. Physiological Zoology, 70 (2): 150-157.
Walsberg, G., B. Wolf. 1995. Solar heat gain in a desert rodent: unexpected increases with wind speed and implications for estimating the heat balance of free-living animals. Journal of Comparitive Physiology, 165 (4): 306-314.