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Spermophilus mohavensisMohave ground squirrel

By Caleb Eckloff

Geographic Range

Mohave ground squirrels (Spermophilus mohavensis) have a patchy distribution throughout approximately 20,000 square kilometers of the northwestern Mohave Desert in California. Their range includes portions of Inyo, Kern, Los Angeles and San Bernardino counties. A small subpopulation of Mohave ground squirrels can also be found in 39 square kilometers of the Coso Hot Springs area, about 13 km northeast of Inyo county. (Grinnell and Dixon, 1910; Hafner and Yates, 1983; Harris and Leitner, 2004; Zembal and Gall, 1980)

Habitat

Mohave ground squirrels occupy an area with hot, rainless summers and moderate winters, although temperatures may occasionally fall below freezing. These squirrels are most often found at elevations from 600 to 1,800 m above sea level, occupying the Lower Sonoran zone. Mohave ground squirrels are found in perennial plant communities, with plant cover ranging from 10 to 19%. Their range seems to be restricted to lower elevation deserts, but these squirrels can also be found in the Joshua tree belt, at elevations from 400 to 1,800 m. Their preferred habitat seems to include sandy soil or a sandy gravel mix, with infrequent sage brush (Artemisia) growth. Mohave ground squirrels are often seen in areas with level topography and few ravines. However, they have been documented in all major scrub habitats in the western Mojave Desert including Mojave creosote scrub (dominated by Larrea tridentate and Abrosia dumosa), desert saltbush scrub (dominated by genus Atriplex), desert sink scrub, desert greasewood scrub, shadscale scrub (dominated by Atriplex confertifolia or Atriplex spinescens) and Joshua tree woodlands (dominated by Joshua trees and various shrub species). (Best, 1995; Burt, 1936; Gossard, 1992; Grinnell and Dixon, 1910; Gustafson, 1993; Zembal and Gall, 1980)

  • Range elevation
    600 to 1,800 m
    1968.50 to ft
  • Average elevation
    1,200 m
    ft

Physical Description

The overall body length of Mohave ground squirrels ranges from 210 to 230 mm in adults, their tail lengths range from 42 to 72 mm. They weigh about 85 to 130 grams. These squirrels are uniformly brown and have no distinguishing spots or stripes. They have a short, broad, flat, furred tail. The underside of their tail is white with some black hairs near the tip. Mohave ground squirrels' winter pelage is a drab cinnamon color, with a silvery white underside. Their summer pelage is “browner”, with shorter hair length. Seasonal molts occur in early May and in autumn. Mohave ground squirrels are cryptically colored to match their sandy environment. This is very advantageous because when they are threatened, they often crouch and wait. Their forefeet are hairless, but their rear feet have long hairs. Their ears tend to blend into the rest of their head and they have white eyelids. Sexual dimorphism is not displayed by this species. ("The Animal Ageing and Longevity Database", 2012; Burt, 1936; Grinnell and Dixon, 1910; Jameson and Peeters, 1988)

  • Sexual Dimorphism
  • sexes alike
  • Range mass
    85 to 130 g
    3.00 to 4.58 oz
  • Range length
    210 to 230 mm
    8.27 to 9.06 in
  • Average length
    224 mm
    8.82 in
  • Average basal metabolic rate
    0.6290 cm3.O2/g/hr

Reproduction

Little is known about the reproductive behavior of Mohave ground squirrels. All knowledge on the subject is based strictly on above-ground observations.

Mohave ground squirrels emerge from hibernation in February; the males emerge up to two weeks prior to the females and may establish and defend territories, with an average size of 6.7 ha, in an attempt to mate with multiple females. Copulation is believed to take place in the male’s burrow. The annual mating season takes place during February and March. They have about a one month gestation period and birth generally occurs in late March or early April. Litters may range from 4 to 9 offspring. Females are capable of reproduction at 1 year of age (if conditions allow), while males do not generally reach sexual maturity until two years of age. ("The Animal Ageing and Longevity Database", 2012; Bartholomew and Hudson, 1960; Best, 1995; Burt, 1936; Harris and Leitner, 2004; Harris and Leitner, 2005; Leitner and Leitner, 1998)

  • Breeding interval
    Mohave ground squirrels breed once annually.
  • Breeding season
    Mating generally occurs during late March or early April.
  • Range number of offspring
    4 to 9
  • Average number of offspring
    6
  • Average gestation period
    30 days
  • Average weaning age
    36 days
  • Average age at sexual or reproductive maturity (female)
    1 years
  • Average age at sexual or reproductive maturity (male)
    2 minutes

Offspring are altricial at birth; they are generally blind and unable to hear, with scarce hair, found only on their heads. Young are able to make high-pitched squeaking noises, presumably associated with suckling. Litters emerge from the burrows from late April to mid May. Lactation may persist through May. During years with substantial drought and reduced resources, Mohave ground squirrels may fail to reproduce. Suspending reproduction may occur for several years if conditions are not adequate. This may help maintain fat reserves for aestivation and hibernation. (Best, 1995; Harris and Leitner, 2005)

Lifespan/Longevity

There is limited information regarding the longevity of wild Mohave ground squirrels, but their lifespan is speculated to be about 5 years or more. Their maximum lifespan in captivity, however, is 7.8 years. ("Basic Facts about Mohave ground squirrels", 2013; Weigl, 2005)

  • Average lifespan
    Status: wild
    5 years
  • Typical lifespan
    Status: captivity
    7.8 (high) years

Behavior

Mohave ground squirrels exhibit diurnal activity patterns. After emerging in the morning, they have been observed basking in the sun, periodically rotating to warm different parts of their bodies. Feeding occurs continuously during daylight hours, from three hours after sunrise until one hour prior to sunset. The extreme heat during the middle of the day dictates that foraging occurs in shaded areas, well protected from the sun. Choice foods, such as Joshua tree seeds (Yucca brevifolia), were collected and stored in the animal's burrow. Zembal and Gall (1980) observed the behavior of Mohave ground squirrels and reported that these ground squirrels make frequent trips to their burrows to stash seeds, at intervals of 15 to 20 minutes. (Best, 1995; Zembal and Gall, 1980)

Mohave ground squirrels are generally solitary. Zembal and Gall (1980) only observed one squirrel harvesting seeds from a particular Joshua tree (Yucca brevifolia) at a time. Related species, such as antelope ground squirrels (Ammospermophilus leucurus) are social organisms by comparison, and are found in coinciding regions with Mohave ground squirrels. However, Mohave ground squirrels are dominant, as antelope ground squirrels have been observed retreating from them. Although solitary and very intolerant of their own species, Mohave ground squirrels are described as placid, docile and readily approached. (Bartholomew and Hudson, 1960; Zembal and Gall, 1980)

Observations of Mohave ground squirrels' activities have revealed that they are more subdued during cloudy weather than on warm, sunny days. If threatened, Mohave ground squirrels typically carry their tail over their back when running. However, they rarely run any extended distances, as most of their time is spent near burrows. Their cryptic coloration offers them another option when danger is perceived, remain still and blend into the environment instead of fleeing. If disturbed while feeding, Mohave ground squirrels stand erect on their hind legs to better survey the area. (Burt, 1936)

Mohave ground squirrels' burrows enter the ground at a 35 degree angle. Excavated dirt is presumably scattered, as none was observed at the entrance of a burrow system by Burt (1936). However, it was noted that when squirrels entered a burrow, a plug of dirt and grasses was used to cover the entrance from within. (Burt, 1936)

Mohave ground squirrels typically enter aestivation in July or September, after building up their fat reserves, when temperatures decrease to between 20 and 30 degrees Celsius and their food sources diminish due to lack of water. This period of dormancy continues until February. During hibernation (a descriptor used interchangeably with aestivation in the literature when describing S. mohavensis) Mohave ground squirrels exhibit reduced body temperature, reduced oxygen consumption, prolonged periods of apnea, a state of deep sleep greater than torpor and arousal stimulated by heat. (Bartholomew and Hudson, 1960)

  • Range territory size
    67,299.22 (high) m^2
  • Average territory size
    12,221.51 m^2

Home Range

Male Mohave ground squirrels establish large territories, averaging 6.7 ha, immediately after emerging from hibernation in February, presumably to mate with as many females as possible. Territories of the males remain consistent until June, at which time dominant individuals take possession of the best foraging areas. This characteristic allows the dominant individuals to occupy smaller home ranges, while juveniles may be forced to roam an area twice the size of their dominant counterparts. Home range sites and sizes are dependent on forage quality, which is directly related to rainfall. Post-mating, the average home range size of Mohave ground squirrels is 1.24 ha for males and 1.20 ha for females; however, there is no significant difference between male and female home range size. (Best, 1995; Harris and Leitner, 2004)

Communication and Perception

Little is known about communication between Mohave ground squirrels, such as scent or physical displays, as much of the year they are aestivating or hibernating. When active, they are generally solitary, which makes observations difficult. Reproductive rituals are also difficult to document as the activity takes place below ground. The call of Mohave ground squirrels is described as a “shrill whistle.” There is a high pitched beep accompanied by a slight rasping sound, similar to a horned lark. Neonatal Mohave ground squirrels produce high-pitched squeaky noises, presumably associated with suckling. (Best, 1995; Burt, 1936; Zembal and Gall, 1980)

Food Habits

Mohave ground squirrels have been seen harvesting seeds of Joshua trees (Yucca brevifolia), which, when available, appear to be their preferred food, however, Joshua trees do not flower annually. Seed harvesting occurs during the day, from about three hours after sunrise until one hour prior to sunset. Mohave ground squirrels demonstrate larder hoarding of seeds, storing them in their burrows. Mohave ground squirrels are omnivorous, the majority of their diet is composed of various plants as well as a smaller percentage of grasshoppers, ants and beetles. Results of a fecal analyses indicated arthropods make up 5 to 8% of Mohave ground squirrels' diet. Plants in their diet include spotted locoweed (Astragalus lentiginosus), white mallow (Eremalche exilis), Arabian schismus (Schismus arabicius), woolly desert marigold (Baileya pleniradiata), desert calico (Langloisia matthewsii), species of genus Gilia, and saltbushes (Atriplex). Various parts of the plants are consumed including the flower, seed or leaves. Their diet varies seasonally; different plants are valued at certain times of year for their water content. Shrub species consumed by Mohave ground squirrels include winterfat, spiny hopesage and saltbrush. (Best, 1995; Grinnell and Dixon, 1910; Laabs, 1998; Morton, 1979; Zembal and Gall, 1980)

  • Animal Foods
  • insects
  • Plant Foods
  • leaves
  • seeds, grains, and nuts
  • flowers
  • Other Foods
  • fungus

Predation

There is little information regarding predators specific to Mohave ground squirrels. Common predators in the Mohave Desert, such as the golden eagles, prairie falcons, red-tailed hawks, badgers, bobcats, coyotes and Mohave rattlesnakes, likely prey upon Mohave ground squirrels. If danger is imminent, Mohave ground squirrels quickly find a burrow, but they have been observed immediately poking their heads back out for observation. Sometimes these ground squirrels freeze in the presence of danger and wait, relying on their cryptic coloration for protection. (Burt, 1936; Gustafson, 1993)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

Little information is available about the ecosystem services provided by Mohave ground squirrels. They may provide seed dispersal, specifically for Joshua trees (Yucca brevifolia), however, this is speculative; this theory is based on the seed dispersal performed by their relative white-tailed antelope squirrels. Compared to population densities of neighboring ground squirrel species (Ammospermophilus and Spermophilus tereticaudus), the Mohave ground squirrel is less common, making them an unlikely prey species upon which their predators are significantly dependent. The burrows constructed by this species may contribute to soil aeration, based on similar practices by round-tailed ground squirrels. No parasites have been reported to be associated with Mohave ground squirrels. (Waitman, et al., 2012)

Economic Importance for Humans: Positive

There are no known positive effects of Mohave ground squirrels on humans.

Economic Importance for Humans: Negative

There are no known negative effects of Mohave ground squirrels on humans.

Conservation Status

Due to the limited distribution of Mohave ground squirrels, this species is threatened. Major threats and causes of decline are development (most notably urbanization, agriculture and military uses), habitat fragmentation and habitat degradation. In 1971, the state of California first listed Mohave ground squirrels as rare, stimulating population assessments and management activities. Population estimates are ineffective due to the inactivity of the Mohave ground squirrel for most of the year and their uneven distribution, which is due partly to the annual variation in food availability. Thus habitat quality is deemed the best indicator for the health of the population. In order to preserve Mohave ground squirrels, preservation of large tracts of land, with a minimum size of 24,281 ha is emphasized. (Gustafson, 1993; Laabs, 1998)

The U.S. Army Corps of Engineers developed an assessment of Mohave ground squirrels at Fort Irwin, California in 1993. They concluded that Mohave ground squirrels may be extinct from numerous sites on the base and strict management plans were proposed to protect the regions where these ground squirrels may still be found. (Krzysik, 1994)

In order to monitor and manage populations of Mohave ground squirrels, a Mohave Ground Squirrel Conservation Strategy has been prepared through the cooperation of several management groups. The overall goals of this strategy were to protect the Mohave ground squirrel by fostering communication among parties interested in the conservation of this species, determining their range, determining their ecological requirements, developing effective long-term conservation methods and developing and implementing an adaptive management plan. The plan outlined preliminary measures to monitor and restore the population of Mohave ground squirrels. This plan has not yet been implemented. (Gustafson, et al., 2006)

Other Comments

Environmental stressors and encroaching urbanization has threatened the population of Mohave ground squirrels, causing a moderate decline in their numbers. Habitat fragmentation is the primary threat to the ground squirrel’s existence but erosion caused by grazing livestock and ORV use has impacted their habitat. Due to the decline in Mohave ground squirrels' numbers round-tailed ground squirrels have opportunistically expanded their range into the range of Mohave ground squirrels. These two ground squirrels are close relatives; the only organisms found in the subgenus Xerospermophilus, and are not found in coinciding ranges, but contact one another along a competitive zone. Due to the close genetic and morphologic relationship of these two ground squirrels, Hafner and Yates (1983) investigated hybridization of the two species but found limited cases. Hafner (1992) investigated the persistence of the contact zone between these two species, despite the presence of appropriate habitat for each on either side of the division and no known reason for their separation. In an effort to explain the distribution of Mohave ground squirrels and round-tailed squirrels, Hafner mentioned a remarkable geographic similarity between the current contact zone of these ground squirrels and the Wisconsinan pluvial maximum, a former aquatic feature present during the Pleistocene epoch. (Hafner and Yates, 1983; Hafner, 1992)

Contributors

Caleb Eckloff (author), Northern Michigan University, John Bruggink (editor), Northern Michigan University, Leila Siciliano Martina (editor), Animal Diversity Web Staff.

Glossary

Nearctic

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.

World Map

acoustic

uses sound to communicate

altricial

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.

bilateral symmetry

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.

chemical

uses smells or other chemicals to communicate

cryptic

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.

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.

diurnal
  1. active during the day, 2. lasting for one day.
dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates

endothermic

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.

female parental care

parental care is carried out by females

folivore

an animal that mainly eats leaves.

granivore

an animal that mainly eats seeds

herbivore

An animal that eats mainly plants or parts of plants.

hibernation

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.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

polygynous

having more than one female as a mate at one time

scrub forest

scrub forests develop in areas that experience dry seasons.

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

soil aeration

digs and breaks up soil so air and water can get in

solitary

lives alone

stores or caches food

places a food item in a special place to be eaten later. Also called "hoarding"

tactile

uses touch to communicate

temperate

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).

terrestrial

Living on the ground.

territorial

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

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

References

2013. "Basic Facts about Mohave ground squirrels" (On-line). Defenders of Wildlife. Accessed May 02, 2013 at http://www.defenders.org/mohave-ground-squirrel/basic-facts.

2012. "The Animal Ageing and Longevity Database" (On-line). Human Ageing Genomic Resources. Accessed February 16, 2013 at http://genomics.senescence.info/species/entry.php?species=Spermophilus_mohavensis.

Bartholomew, G., J. Hudson. 1960. Aestivation in the Mohave ground squirrel, Citellus mohavensis. Bulletin of the Museum of Comparative Zoology, 124: 193-208.

Best, T. 1995. Spermophilus mohavensis. Mammalian Species, 509: 1-7.

Burt, W. 1936. Notes on the habits of the Mohave ground squirrel. Journal of Mammalogy, 17: 221-224.

Gossard, G. 1992. The Joshua Tree, A Controversial, Contradictory Desert Centurion. Tehachapi, CA: Yellow Rose Publications.

Grinnell, J., J. Dixon. 1910. Natural History of the Ground Squirrels of California. Monthly Bulletin of the State Commision of Horticulture, 7/11-12: 597-708. Accessed January 31, 2013 at http://archive.org/stream/naturalhistoryof00grin/naturalhistoryof00grin_djvu.txt.

Gustafson, J. 1993. A status review of the mohave ground squirrel (Spermophilus mohavensis). Nongame Bird and Mammal Section Report, 93:9: 1-234. Accessed February 10, 2013 at https://nrm.dfg.ca.gov/FileHandler.ashx?DocumentVersionID=17416.

Gustafson, J., S. Harris, R. Jones, S. Juarez, T. Moore, D. Racine, A. Tenneboe, J. Vance, T. Dayak, C. Bernis, P. Leitner, T. Recht, L. Oviatt, R. Scott, C. Wilkerson, C. Everly, S. Collis, B. Wood, M. Quillman, B. Shomo, S. Ellis, L. LaPre, B. Parker, C. Sullivan, R. McMorran, C. Gonzalez, M. Joia, T. Campbell, J. O'Gara. 2006. "Mohave Ground Squirrel Conservation Strategy" (On-line). Desert Mangers Group. Accessed March 21, 2013 at http://www.dmg.gov/documents/DFT_MGS_Consv_Strategy_DMG_082906.pdf.

Hafner, D. 1992. Speciation and persistence of a contact zone in Mohave Desert ground squirrels, subgenus Xerospermophilus. Journal of Mammalogy, 73/4: 770-778.

Hafner, D., T. Yates. 1983. Systematic status of the Mohave ground squirrel, Spermophilus mohavensis (subgenus Xeropermophius). Journal of Mammalogy, 64: 397-404.

Harris, J., P. Leitner. 2005. Long-Distance Movements of Juvenile Mohave Ground Squirrels, Spermophilus mohavensis. The Southwestern Naturalist, 50/2: 188-196.

Harris, J., P. Leitner. 2004. Home-Range Size and Use of Space by Adult Mohave Ground Squirrels, Spermophilus mohovensis. Journal of Mammalogy, 85/3: 517-523.

Jameson, E., H. Peeters. 1988. Mammals of California. Berkeley, CA: University California Press.

Krzysik, A. 1994. The mohave ground squirrel at Fort Irwin, California. USACERL Technical Report, 94/09: 1-40. Accessed March 14, 2013 at http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA299946.

Laabs, D. 1998. Mohave Ground Squirrel, Spermophilus mohavensis. Bureau of Land Management, 1: 1-7. Accessed January 31, 2013 at http://www.blm.gov/ca/pdfs/cdd_pdfs/Mgs1.pdf.

Leitner, P., B. Leitner. 1998. Coso grazing exclosure monitoring study: Mohave ground squirrel study, Coso Known Geothermal Resource Area: Major findings, 1988- 1996. Orinda, CA: CalEnergy Company, Inc..

Morton, S. 1979. Diversity of Desert-Dwelling Mammals: A Comparison of Australia and North America. Journal of Mammalogy, 60/2: 253-264.

Waitman, B., S. Vander Wall, T. Esque. 2012. Seed dispersal and seed fate in Joshua tree (Yucca brevifolia). Journal of Arid Environments, 81: 1-8.

Weigl, R. 2005. Longevity of mammals in captivity; from the Living Collections of the world. Stuttgart, Germany: Kleine Senckenberg-Reihe, Band 48.

Zembal, R., C. Gall. 1980. Observations on Mohave ground squirrels, Spermophilus mohavensis, in Inyo County, California. Journal of Mammalogy, 61: 347-350.

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