Animal Diversity Web

Ge­o­graphic Range

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)

• Biogeographic Regions
• nearctic
  • native

Habi­tat

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)

• Habitat Regions
• temperate
• terrestrial

• Terrestrial Biomes
• savanna or grassland
• scrub forest

• Other Habitat Features
• riparian

• Range elevation

  0 to 2900 m

  0.00 to 9514.44 ft

Phys­i­cal De­scrip­tion

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)

• Other Physical Features
• endothermic
• bilateral symmetry

• Sexual Dimorphism
• sexes alike

• Range mass

  600 to 850 g

  21.15 to 29.96 oz

• Range length

  430 to 540 mm

  16.93 to 21.26 in

Re­pro­duc­tion

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)

• 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. (John­son, 1979; John­son, 1981; Oaks, et al., 1987; Or­tega, 1990a; Or­tega, 1990b; Shriner and Stacey, 1991)

• Key Reproductive Features
• seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual
• viviparous

• 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 number of offspring

  4.3

  AnAge

• Average gestation period

  30 days

• Average gestation period

  30 days

  AnAge

• 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

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)

• 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

Lifes­pan/Longevity

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. (John­son, 1979; Oaks, et al., 1987)

• Average lifespan
  Status: captivity

  9.8 years

  AnAge

Be­hav­ior

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)

• Key Behaviors
• terricolous
• diurnal
• crepuscular
• motile
• hibernation
• aestivation
• territorial
• social
• colonial

Home Range

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)

Com­mu­ni­ca­tion and Per­cep­tion

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)

• Communication Channels
• visual
• tactile
• acoustic
• chemical

• Other Communication Modes
• scent marks

• Perception Channels
• visual
• tactile
• acoustic
• chemical

Food Habits

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)

• Primary Diet
• omnivore

• 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

Pre­da­tion

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)

• Anti-predator Adaptations
• cryptic

• 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)

Ecosys­tem Roles

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)

• Ecosystem Impact
• disperses seeds

Eco­nomic Im­por­tance for Hu­mans: Pos­i­tive

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. (John­son, 1979; Oaks, et al., 1987)

• Positive Impacts
• food

Eco­nomic Im­por­tance for Hu­mans: Neg­a­tive

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)

• Negative Impacts
• injures humans
  • carries human disease
• crop pest

Con­ser­va­tion Sta­tus

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

• IUCN Red List

  Least Concern
  More information

• IUCN Red List

  Least Concern
  More information

• US Federal List

  No special status

• CITES

  No special status

Other Com­ments

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)

Con­trib­u­tors

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.

Ref­er­ences

Chris­tensen, K., T. Whit­man. 1993. Im­pact of in­sect her­bi­vores on com­pe­ti­tion be­tween birds and mam­mals for pinyon pine seeds. Ecol­ogy, 74 (8): 2270-2278.

Gustafson-Rop­ski, M., S. Rop­ski, P. Stacey. 1989. Ge­netic dif­fer­en­ti­a­tion in a pop­u­la­tion of rock squir­rels (Sper­mophilus var­ie­ga­tus) in cen­tral New Mex­ico. South­west­ern Nat­u­ral­ist, 34 (3): 424-427.

John­son, K. 1981. So­cial or­ga­ni­za­tion in a colony of rock squir­rels (Sper­mophilus var­ie­ga­tus, Sci­uri­dae). South­west­ern Nat­u­ral­ist, 26 (3): 237-242.

John­son, K. 1979. Ecol­ogy, be­hav­ior, and so­cial or­ga­ni­za­tion of the rock squir­rel, Sper­mophilus var­ie­ga­tus . M. S. the­sis, Trin­ity Univ., San An­to­nio, Texas, 107pp.: Un­pub­lished.

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.

Or­tega, J. 1990. Home-range size of adult rock squir­rels (Sper­mophilus var­ie­ga­tus) in south­east­ern Ari­zona. Jour­nal of Mam­mal­ogy, 71 (2): 171-176.

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.

Ow­ings, D., R. Coss, D. Mck­er­non, M. Rowe, P. Ar­rowood. 2001. Snake-di­rected an­tipreda­tor be­hav­ior of rock squir­rels (Sper­mophilus var­ie­ga­tus): Pop­u­la­tion dif­fer­ences and snake-species dis­crim­i­na­tion. Be­hav­iour, 138 (5): 575-595.

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.