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Ammospermophilus nelsoniNelson's antelope squirrel

By Divya Balaji

Geographic Range

The distribution of Ammospermophilus nelsoni is restricted to the floor of the southern San Joaquin Valley, California, the Cuyama and Panoche valleys in San Luis Obispo County, and the Carrizo and Elkhorn Plains of the United States. The species is Nearctic and endemic to the above range. (Best, et al., 1990; Taylor, 1918)

Habitat

Nelson's antelope squirrels are found in hot deserts that comprise the Lower Sonoran life zone. Lower Sonoran deserts of North America include areas in Texas, New Mexico, Arizona, Nevada, and California. Nelson's antelope squirrels are found in arid grasslands and shrub lands. They have been recorded in areas where shrub cover ranges from light to medium density and ranges from xerophytic, alkali desert scrub and annual grassland receiving less than 15 cm annual precipitation, to halophytic, alkali desert scrub and annual grassland receiving 18 to 23 cm of annual precipitation. Nelson's antelope squirrels prefer alkaline, loamy soils from 50 to 1100 meters elevation. Nelson’s antelope squirrels depend on kangaroo rat burrows, so areas they inhabit may be limited to areas with kangaroo rat popuations. (Grinell and Dixon, 1918; Hawbecker, 1953; Merriam, 1898; Whitaker, et al., 2008)

  • Range elevation
    50 to 1100 m
    164.04 to 3608.92 ft

Physical Description

Nelson's antelope squirrels have fusiform bodies, small, rounded ears, short legs, and short tails. The dorsal head and neck and the outer surfaces of the legs are a dull yellowish-brown or buffy-tan. The tail has thick fringes of hair and the underside is light grey to white. A distinctive light-colored stripe runs along the side of the body from behind the shoulder to the rump. Males are slightly larger than females, with total length of males ranging from 234 to 267 mm (average 249 mm). Female length ranges from 230 to 256 mm (average 238 mm). Nelson's antelope squirrels have distinct summer and winter pelages, with the autumn or winter pelage being darker than the summer pelage. (Best, et al., 1990; Brown and Williams, 2006; Merriam, 1983)

Nelson's antelope squirrels can be distinguished from white-tailed antelope squirrels by its larger size and grey coloring of the pelage. Nelson's antelope squirrels have wider zygomatic arches, more inflated auditory bullae, and larger nasal bones. They also have larger upper incisors and first upper molars. (Best, et al., 1990; Brown and Williams, 2006; Merriam, 1983)

  • Sexual Dimorphism
  • male larger
  • Range mass
    142 to 179 g
    5.00 to 6.31 oz
  • Average mass
    155 g
    5.46 oz
  • Range length
    230 to 267 mm
    9.06 to 10.51 in
  • Average length
    249 mm
    9.80 in
  • Average basal metabolic rate
    0.8 cm3.O2/g/hr

Reproduction

Nelson's antelope squirrels have a promiscuous mating system in which females and males mate with multiple partners. Not much is known about specific mating behaviors, but an interesting observation was made regarding mate searching behavior. Mates are typically found within the home range, but there have been instances of females travelling up to 1 km from their home range in search of a mate. Mate guarding and mate defense have not been observed in Nelson's antelope squirrels. ("Five Year Status Report- Ammospermophilus nelsoni", 1987; Hawbecker, 1947)

The breeding season of Nelson's antelope squirrels extends from late winter to early spring and this species breeds once annually. Males achieve reproductive maturity earlier than females. Brood size varies from 6 to 11 young; with the average being 9. The average gestation period is 26 days. Females give birth to offspring in burrows. The young come above ground approximately on the 30th day after birth. Weaning can start or be completed before the young emerge from their burrow, but this is highly variable. Females wean the young by refusing to nurse and visiting the burrow less frequently. (Best, et al., 1990; Best, 1999; Macdonald, 2009)

  • Breeding interval
    Nelson's antelope squirrels breed once annually.
  • Breeding season
    Nelson's antelope squirrels breed from late winter to early spring.
  • Range number of offspring
    6 to 11
  • Average number of offspring
    9
  • Range gestation period
    25 to 30 days
  • Average gestation period
    26 days
  • Range weaning age
    30 (high) days
  • Average weaning age
    15 days
  • Range time to independence
    3 to 4 weeks
  • Average age at sexual or reproductive maturity (female)
    377 days
  • Average age at sexual or reproductive maturity (male)
    377 days

Nelson's antelope squirrel young are born in an altricial state. Males do not play a large role in caring for the young, as females perform all activities of nursing and weaning. When weaning, the female distances herself from the young and does not respond even when young make attempts to nurse. The female maintains contact with the young by visiting them sometimes or by just using calls to communicate. In captivity, at times when foraging opportunities were limited, instances of cannibalism have been observed. (Best, et al., 1990)

  • Parental Investment
  • altricial
  • female parental care
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female

Lifespan/Longevity

Nelson's antelope squirrels usually live for less than a year in the wild because of high infant and young mortality rates. They have been documented to live for about 4 years in captivity, with the longest captive lifespan being 5.7 years. Adult mortality rate is 80% and the average lifespan is 8 months. (Best, et al., 1990; Hawbecker, 1975)

  • Range lifespan
    Status: captivity
    5.7 (high) years
  • Average lifespan
    Status: wild
    8 months
  • Average lifespan
    Status: captivity
    4 years

Behavior

Nelson's antelope squirrels live in small family groups consisting of 6 to 8 individuals. Although the exact nature of their social system of dominance is unknown, the closely-related species Ammospermophilus leucurus has observed dominance hierarchies. Ammospermophilus leucurus individuals form linear hierarchies that are maintained by visual and tactical cues. Dominance contests result in agonistic behaviors, starting with lunging and moving to boxing and fighting. ("Five Year Status Report- Ammospermophilus nelsoni", 1987; Belk and Smith, 1991; Best, et al., 1990; Best, 1999; Grinell and Dixon, 1918; Hawbecker, 1947; Hawbecker, 1953; Hawbecker, 1975)

Nelson's antelope squirrels have a keen sense of territory. When removed from their territory and released at a distance, adults are helpless and unable to navigate. Family groups live in extensive burrow systems, which are sometimes adapted from kangaroo rat burrows. ("Five Year Status Report- Ammospermophilus nelsoni", 1987; Belk and Smith, 1991; Best, et al., 1990; Best, 1999; Grinell and Dixon, 1918; Hawbecker, 1947; Hawbecker, 1953; Hawbecker, 1975)

Thermoregulation plays a key role as these squirrels do not hibernate or estivate. Nelson's antelope squirrels forage actively and are diurnal, usually appearing above ground when the temperature is between 10°C and 32°C. Their daily period of activity varies seasonally so that most activity occurs within this temperature range. Nelson's antelope squirrels are observed most frequently during dawn or dusk when it has been extremely hot during daytime. However, they are active throughout the day at temperatures exceeding 42°C in the Elkhorn Plain Ecological Reserve. They retreat to burrows when temperatures are too cold or hot. ("Five Year Status Report- Ammospermophilus nelsoni", 1987; Belk and Smith, 1991; Best, et al., 1990; Best, 1999; Grinell and Dixon, 1918; Hawbecker, 1947; Hawbecker, 1953; Hawbecker, 1975)

  • Average territory size
    0.044 km^2

Home Range

Males and females have home ranges of about 4.4 hectares, but they are not evenly distributed throughout their range and occur in uneven densities due to niche preferences. The maximum observed dispersal from the home range was 1,260 m by a male and 900 m by a female. (Best, et al., 1990)

Communication and Perception

The main senses used by the Nelson's antelope squirrels are hearing and olfaction, but the species also relies on vision and will scan its environment before making the decision to fully exit its burrow. Olfaction is used to detect danger, to find food and to communicate. Nelson's antelope squirrels also twitch their tails rapidly, in fore and aft movements when frightened or excited. (Best, et al., 1990; Best, 1999; Bolles, 1988; Taylor, 1918)

The Nelson’s antelope squirrel gives out alarm calls, which are indicative of altruistic behavior as it compromises the safety of the individual. These calls are in the form of trills that are low-pitched and are characteristic of antelope squirrels that live in closed habitats. In fact, the call is accomplished by a convulsive motion of its body rather than by vocal stress. The female uses this call to communicate with her family when she is weaning. (Best, et al., 1990; Best, 1999; Bolles, 1988; Taylor, 1918)

Food Habits

Nelson’s antelope squirrels are omnivorous and active foragers. They feed primarily on insects, green vegetation, and seeds, but will sometimes also feed on small vertebrates. With regards to the latter, the San Joaquin antelope squirrel consumes rodents, lizards and members of its own species as carrion. Their diet preferences are largely dependent on the amount of moist vegetation available. With respect to green material, red-stemmed filaree (Erodium cicutarium) and the red brome (Bromus rubens) are preferred. Other food sources include the ephedra, a species of clover and locoweed. When moist food becomes very scarce turpine weed is eaten. A preference for a particular type of insect has not been documented. The San Joaquin squirrel eats seeds only when easily obtainable green material and insects are not available. (Hawbecker, 1947; Macdonald, 2009)

Nelson’s antelope squirrel assumes a distinctive posture while feeding. It squats on its rear limbs with the tail cocked behind the back and holds its food in the forepaws. Its enlarged incisors and abrasive cheek teeth help in breaking down food. (Hawbecker, 1947; Macdonald, 2009)

The Nelson’s ground squirrel does not live near a source of water and so water does not constitute a great part of its diet. It can be inferred that its principal source of moisture is from the vegetation eaten. Although it accepts large amounts of water in laboratory conditions, it can survive for seven months in the shade, without water. (Hawbecker, 1947; Macdonald, 2009)

  • Animal Foods
  • reptiles
  • carrion
  • insects
  • Plant Foods
  • leaves
  • seeds, grains, and nuts

Predation

Nelson's antelope squirrels employ a variety of techniques to guard against predators. The complex burrow systems serve as protection against predators. When emerging from a burrow, they exercise extreme caution and do not exit quickly. They rely on olfaction and use this capability while foraging and fleeing danger. They use characteristic alarm calls to communicate danger and may also rely on the warning calls of birds like horned larks and white-crowned sparrows for detection of predators. Nelson's antelope squirrels use their surroundings while evading predators. They use shrubs and burrows of kangaroo rats as sites of refuge. Their buffy or tan pelage make them difficult to see in their arid habitats. When foraging, these squirrels move close to the ground in a very distinctive manner, by a series of short, rapid jumps. American badgers are the most important predators of these squirrels. They dig into the burrows to prey on young and adults. They are also eaten by coyotes (Canis latrans) and (kit foxes Vulpes macrotis). ("Five Year Status Report- Ammospermophilus nelsoni", 1987; Best, et al., 1990; Brown and Williams, 2006)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

Nelson's antelope squirrels are preyed upon by mid-sized predators, and serve as host to varius endoparasites and ectoparasites. They are hosts to cestodes (Hymenopelis citelli), nematodess (Spirura infundibuliformis and Physaloptera spinicauda), and also acanthocephalans (Moniliformes dubius). The ectoparasites they are host to include fleas (Siphonaptera) and ticks (Ixodes). Nelson's antelope squirrels have a symbiotric relationship with kangaroo rats Dipodomys deserti, taking refuge in their burrows. They further impact their habitat by continuing to burrow. Because they are seed collectors, they also disperse seeds in their environment. (Best, et al., 1990)

  • Ecosystem Impact
  • disperses seeds
  • creates habitat
Mutualist Species
Commensal/Parasitic Species
  • cestodes (Hymenopelis citelli)
  • nematodes (Spirura infundibuliformis)
  • nematodes (Physaloptera spinicauda)
  • acanthocephalans (Moniliformes dubius)
  • fleas (Siphonaptera)
  • ticks (Ixodes)

Economic Importance for Humans: Positive

Nelson’s antelope squirrels do not generally live close to human settlements. They may contribute to controlling insect populations by eating them and they may help to disperse the seeds they gather and store. (Belk and Smith, 1991; Hawbecker, 1947)

Economic Importance for Humans: Negative

In laboratory conditions, Nelson's antelope squirrels have been positive for western equine encephalomyelitis, St. Louis encephalitis, Powassan virus, and Modoc virus. However, there is no documented instance their role in spreading disease to humans or domestic animals. (Best, et al., 1990)

Conservation Status

Nelson’s, or San Joaquin, antelope squirrels are classified as endangered on the IUCN Red List. According to the California Department of Fish and Game, the biggest threat to Nelson’s antelope squirrels is habitat destruction due to crop cultivation and urbanization. Excessive grazing by livestock affects these squirrels and also causes soil erosion. Uses of rodenticides and insecticides, which destroy prey populations, negatively impact Nelson’s antelope squirrels. The Endangered Species Recovery Program (2006) of California has suggested a list of conservation measures that stress the importance of land use practices and management strategies. A detailed conservation program for San Joaquin antelope squirrels includes determining habitat management, protecting additional habitat in surrounding areas of the squirrel’s range, habitat enhancement in areas like Kern County, and reintroduction to protected areas, such as Pixley National Wildlife Refuge. ("Five Year Status Report- Ammospermophilus nelsoni", 1987; Best, et al., 1990; Brown and Williams, 2006; Grinell and Dixon, 1918; Whitaker, et al., 2008)

Other Comments

The genus Ammospermophilus has a rich fossil record. Its divergence can be dated to the Miocene as post-Pleistocene fossils resembling Nelson’s antelope squirrels have been found near Kern County, California. They are thought to have invaded the area over the passes of southern Sierra Nevada and Tehachapi mountains. Due to the change in the moisture levels to less arid conditions in the corridor between Mojave Desert and San Joaquin valley, Nelson’s antelope squirrels became restricted to the San Joaquin valley. (Best, et al., 1990; Hawbecker, 1953)

Contributors

Divya Balaji (author), Yale University, Eric Sargis (editor), Yale University, Rachel Racicot (editor), Yale University, Tanya Dewey (editor), University of Michigan-Ann Arbor, Catherine Kent (editor), Special Projects.

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.

carnivore

an animal that mainly eats meat

carrion

flesh of dead animals.

chemical

uses smells or other chemicals to communicate

crepuscular

active at dawn and dusk

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.
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.

fossorial

Referring to a burrowing life-style or behavior, specialized for digging or burrowing.

granivore

an animal that mainly eats seeds

herbivore

An animal that eats mainly plants or parts of plants.

insectivore

An animal that eats mainly insects or spiders.

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.

omnivore

an animal that mainly eats all kinds of things, including plants and animals

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

scavenger

an animal that mainly eats dead animals

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

social

associates with others of its species; forms social groups.

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

visual

uses sight to communicate

viviparous

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

References

California Department of Fish and Game. Five Year Status Report- Ammospermophilus nelsoni. EW87. Turlock: Calif. Dep. of Fish and Game, Nongame Bird and Mammal Sec. rep. 1987.

Belk, M., D. Smith. 1991. Ammospermophilus leucurus. Mammalian Species, 368: 1-8.

Best, T. 1999. Ammospermophilus nelsoni. Pp. 407-408 in D Wilson, S Ruff, eds. Smithsonian Book of North American Mammals, Vol. 1999, 1999 Edition. British Columbia: UBC press.

Best, T., A. Titus, C. Lewis, K. Caesar. 1990. Ammospermophilus nelsoni. Mammalian Species, 367: 1-7.

Bolles, K. 1988. The Evolution and variation of antipredator vocalizations of antelope squirrels. Zeitschrift für Säugetierkunde, 53: 129-147.

Brown, N., D. Williams. 2006. "Endangered Species Recovery Program" (On-line). San Joaquin antelope squirrel Ammospermophilus nelsoni. Accessed April 30, 2012 at http://esrp.csustan.edu/speciesprofiles/profile.php?sp=amne.

Cypher, B. 2001. Spaciotemporal Variation in Rodent Abundance in the San Joaquin Valley. The Southwestern Naturalist, 46/1: 66-75.

Grinell, J. 1933. Review of the Recent mammal fauna of California. Berkeley, California: University of California.

Grinell, J., J. Dixon. 1918. Natural History of the Ground Squirrels of California. Sacramento, California: California State Printing Office.

Hawbecker, A. 1975. The Biology of Some Desert Dwelling Ground Squirrels. The Hague, Netherlands: Dr. Junk, b.v.

Hawbecker, A. 1953. Environment of the Nelson Antelope Ground Squirrel. Journal of Mammalogy, 34/3: 324-334.

Hawbecker, A. 1947. Food and Moisture Requirements of the Nelsons Antelope Ground Squirrel. Journal of Mammalogy, 28/2: 115-125.

Hawbecker, A. 1958. Survival and Home Range in the Nelson’s antelope squirrel. Journal of Mammalogy, 39: 207-215.

Heller, C., J. Henderson. 1976. Hypothalamic Thermosensitivity and Regulation of Heat Storage Behavior in a Day-active Desert Rodent Ammospermophilus nelsoni. Journal of Comparative Physiology, 108: 255-270.

Macdonald, D. 2009. Rodentia. Pp. 128-166 in D Macdonald, ed. The Encyclopedia of Mammals, Vol. 2009, 2nd revised Edition. Oxford: Oxford University Press.

Merriam, H. 1898. Life Zones and Crop Zones of the United States. Washington D.C: Government Printing Office.

Merriam, H. 1983. Descriptions of eight new ground squirrels of the genera Spermophilus and Tamias from California, Texas, and Mexico. Proceedings of the Biological Society of Washington, 8: 129-138.

Taylor, W. 1918. A New Spermophile from the San Joaquin Valley California with notes on Ammospermophilus nelsoni nelsoni Merriam. Berkeley, Califronia: University of California Press.

Tevis, L. 1953. Stomach Contents of Chipmunks and Mantled Squirrels in Northeastern California. Journal of Mammalogy, 34/3: 316-324.

Whitaker, J., G. Hammerson, D. Williams. 2008. "Ammospermophilus nelsoni" (On-line). IUCN Red list of Threatened Species. Accessed April 30, 2012 at http://www.iucnredlist.org/apps/redlist/details/1149/0.

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