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Ammospermophilus harrisiiHarris's antelope squirrel

By Matthew Haloostock

Geographic Range

Harris's antelope squirrels (Ammospermophilus harrisii) are an endemic species found in the deserts of the Nearctic region. They are found throughout southwestern parts of the United States and northwestern Mexico. Harris' antelope squirrels are not found north of the Grand Canyon or on the other side of the Colorado River. (Best, et al., 1990; Escalante, et al., 2007; Mantooth, et al., 2013)

Habitat

Harris's antelope squirrels are found in multiple types of desert habitats, including deserts with cacti and desert shrubs to open plains with gravel and sand. They are also known to inhabit valleys, canyons, and river bottoms. These squirrels favor areas with dense vegetation. (Best, et al., 1990; Nowak, 1999)

  • Range elevation
    1350 (high) m
    4429.13 (high) ft

Physical Description

Harris' antelope squirrels are medium-sized squirrels with medium to short tails and no differences between the sexes. Their average total length ranges from 229.0 to 245.8 mm, with tail lengths of 70.9 to 84.6 mm, and hindfoot lengths 37.2 to 41.6 mm. Harris' antelope squirrels have a small head with small ears. The length of their skull ranges from 39.2 to 41.0 mm and their ears range from 11.5 to 13.9 mm. Their dental formula is: i 1/1, c 0/0, p 2/1 and m 3/3, with 22 total teeth. Harris' antelope squirrels have a grayish dorsum, while the rest of their pelage is dark-brown at the roots, white in the middle, and has brownish-white tips. There is also one narrow white stripe on both sides of the body, running from the back of the shoulders down to the middle of the hips. The under surface fur on the tail is a mix of white and black. Winter pelage is longer and softer than summer pelage. Summer pelage is pale pinkish-cinnamon, as opposed to the gray winter tone. Harris's antelope squirrels have two subspecies, Ammospermophilus harrissii harrisii and A. h. saxicolus. Ammospermophilus harrissii saxicolus, which is found in the desert plains of southern Arizona, tends to have paler fur coloration, however, the differences in fur colors is slight between the subspecies. (Best, et al., 1990; Nowak, 1999)

  • Sexual Dimorphism
  • sexes alike
  • Average mass
    122 g
    4.30 oz
  • Average mass
    126 g
    4.44 oz
    AnAge
  • Range length
    229.0 to 245.8 mm
    9.02 to 9.68 in

Reproduction

Harris' antelope squirrels typically mate during February and March, but they may breed as early as December or January. There are also a few cases of reproductively active Harris' antelope squirrels into late spring. Male spermatogenesis begins and ends within a couple months of mating season. The earliest recorded spermatogenesis was on November 4, while the latest live sperm was recorded on July 10. Female readiness for reproduction is characterized by an enlarged labia and an engulfed uterus, which has been observed as early as mid-February. After copulation, the formation of a vaginal plug is typical. In males, testes sizes begin to shrink in June and by July, they become less than 10% of their size in February and March. Females first show reproductive changes in mid-February. Formation of embryos typically begins within a few days of female reproductive activity. Their gestation period lasts 29 to 30 days. They typically only give birth to one litter per year; however, it is possible for a second litter to be raised. Harris' antelope squirrels show no signs of mating with the same partner within the same or different mating seasons. (Best, et al., 1990; Neal, 1965)

Before giving birth, captive females made a round nest out of cotton material, with a covered top that had only one hole for entry and exit. Litter sizes average 6.5 young, with an average newborn mass of 3.6 grams. On their first day, newborns lose their umbilical cord and their sex can be determined, although they lack sight and hearing. As they develop, newborns are hairless and typically have clear, pink skin, and initially cannot crawl. Within 1 week, they develop a black coloration in their fur and they are capable of defecation, producing small yellow fecal pellets. By 2 weeks, the white stripes observed in adults of this species become distinct. By week 3, claws have emerged along with incisors on the lower jaw. By week 4, their ears open and they are completely covered in fur. They also make noise, especially while being handled. Eyes finally open between weeks 4 and 5 and young leave their nest for the first time. The young achieve 50% of their total adult mass in 56 days and are fully grown in 217 days. Males reach reproductive maturity in the autumn of their first year. They obtain scrotal testes within 14 to 19 weeks of birth. The testes are large and remain in this position until 28 weeks after birth, before shrinking and returning to the abdomen. Females do not reach reproductive maturity until 10 to 11 months after birth and the vagina does not open until the female is in heat. (Best, et al., 1990; Levenson, 1979; Neal, 1965)

  • Key Reproductive Features
  • gonochoric/gonochoristic/dioecious (sexes separate)
  • sexual
  • Breeding interval
    Harris' antelope squirrels breed once every year.
  • Breeding season
    Mating can begin as early as December and January for this species, but it usually occurs during mid-February to early March.
  • Average number of offspring
    6.5
  • Average number of offspring
    6.8
    AnAge
  • Range gestation period
    29 to 30 days
  • Average gestation period
    30 days
  • Average weaning age
    49 days
  • Range age at sexual or reproductive maturity (female)
    10 to 11 months
  • Range age at sexual or reproductive maturity (male)
    14 to 19 weeks

Females raise the young by themselves. Young are unable to open their eyes for about 4 to 5 weeks. Once their eyes open, the young squirrels venture out of the nest for the first time. Weaning takes about 7 weeks and is signified by a change in fecal matter, which becomes darker and more pellet-like, showing a change in diet from milk to solid food. (Best, et al., 1990)

Lifespan/Longevity

In the wild, the typical lifespan of Harris' antelope squirrels ranges from 2 to 4 years. In captivity, these squirrels may live to be up to 11 years. ("Animal Fact Sheet: Harris' Antelope Squirrel", 2014)

  • Range lifespan
    Status: captivity
    11 (high) years
  • Typical lifespan
    Status: wild
    2 to 4 years
  • Average lifespan
    Status: captivity
    10.6 years
    AnAge

Behavior

Harris' antelope squirrels are diurnal animals that are active throughout the day, including the hottest hours. They do not hibernate and remain active above ground for all months of the year. These squirrels are never found in groups, they remain alone, dispersed far from their conspecifics. Adult squirrels are only found together for mating. Harris' antelope squirrels often climb cacti to survey their surroundings. Their method of climbing cacti is unknown, however, they do not show scars on their soft padded feet to indicated damage from cactus thorns. These squirrels regularly run, interspersed by irregular patterns of rest. (Best, et al., 1990; Levenson, 1979)

Home Range

Harris' antelope squirrels often create burrows for shelter. Many of these burrows are found underneath various shrubs. They also tend to situate themselves around rock-bound hills, where they can easily take shelter when necessary. These squirrels travel at an average rate of 274 m per individual. Densities in southern Arizona range from 0.24 to 0.36 per ha during the spring to late summer and 0.08 to 0.24 per ha during the autumn and winter. (Best, et al., 1990)

Communication and Perception

Harris' antelope squirrels produce calls and stamp their forepaws as alarm signifiers. Their calls are usually a trill sound, which is unaffected by sex, temperature, season, or ontogeny. Trills are high-pitched and range in frequency based on the size of the squirrel. (Best, et al., 1990)

Food Habits

Harris' antelope squirrels have an omnivorous diet. They feed on seeds from desert vegetation and on insects. The plants these squirrels eat depend on their geographic locations. Squirrels in New Mexico feed primarily on fruits and seeds of cactus plants. Those in Arizona eat mesquite beans and yucca plant seeds. The fruit produced by prickly pear cacti are a major food source for squirrels found in the Graham Mountains. Harris' antelope squirrels have also been observed burying mesquite beans. These squirrels usually stuff their mouths with seeds to transport them. They eat both within their burrows and above ground. Various other green plants and seeds are consumed by these squirrels as well. Their hands, faces, and digestive tracts are often stained by the juices of their food. Harris' antelope squirrels also feed on insects and small mammals, such as mice, that they have trapped. ("Animal Fact Sheet: Harris' Antelope Squirrel", 2014; Best, et al., 1990; Brown, 1988)

  • Animal Foods
  • mammals
  • insects
  • Plant Foods
  • seeds, grains, and nuts
  • fruit

Predation

Predation risk in Harris' antelope squirrels is a hindrance to foraging. These squirrels have a significantly higher predation risk in open environments as compared to those located in bushes. Their pelage helps camouflage them by resembling the rocky-desert terrain; this helps them avoid predators that rely on sight. When a predator is spotted, Harris' antelope squirrels run quickly back to cover under shrubbery containing burrows, with their tail held high in the air. While running, they let out multiple alarm calls. Before escaping their predator, these squirrels often stop, let out a call and stamp their forepaws. These alarm calls have been adapted to best fit the desert environment. These high frequency calls last about 2.24 seconds and are very similar to alarm calls produced by other Ammospermophilus species. Cactus wrens have been observed attacking Harris' antelope squirrels but do not feed on them. (Best, et al., 1990; Brown, 1988; Brown, 1989; Smith, 1970)

Ecosystem Roles

Harris' antelope squirrels coexist with many other desert species. They are sympatric with round-tailed ground squirrels, with which they occasionally share dens created by kangaroo rats. They also compete for food sources with many other desert species due to their large diet range. By feeding primarily on seeds and fruits, Harris' antelope squirrels are good seed dispersers. These squirrels host many different parasites. The fungus, Coccidioides immitis, can be obtained from these squirrels. Parasitic nematodes can be found within their digestive tracts and caecums. External parasites such as fleas, lice, and ticks also make a home on Harris' antelope squirrels. (Best, et al., 1990; Brown, 1989)

  • Ecosystem Impact
  • disperses seeds
Commensal/Parasitic Species

Economic Importance for Humans: Positive

There are no recorded positive impacts of Harris' antelope squirrels on human. However, there is room for research on how these squirrels deal with the parasites that affect them. (Best, et al., 1990)

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

Harris' antelope squirrels have become a nuisance by raiding crops and burrowing through ditch banks in irrigated areas. They also carry parasites and may spread harmful diseases. (Best, et al., 1990; Nowak, 1999)

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

Conservation Status

Harris' antelope squirrels are considered a species of least concern and are currently not endangered. (Timm, et al., 2013)

Other Comments

Evidence shows that Harris' antelope squirrels are the oldest members of their genus (Ammospermophilus). Morphologically and genetically, they are more similar to Texas antelope squirrels than white-tailed antelope squirrels or San Joaquin antelope squirrels. (Best, et al., 1990; Mantooth, et al., 2013)

Contributors

Matthew Haloostock (author), University of Michigan, Joanna Larson (editor), University of Michigan, Priscilla Tucker (editor), University of Michigan, 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

aposematic

having coloration that serves a protective function for the animal, usually used to refer to animals with colors that warn predators of their toxicity. For example: animals with bright red or yellow coloration are often toxic or distasteful.

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

chemical

uses smells or other chemicals to communicate

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

frugivore

an animal that mainly eats fruit

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.

island endemic

animals that live only on an island or set of islands.

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.

sexual

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

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

terrestrial

Living on the ground.

References

2014. "Animal Fact Sheet: Harris' Antelope Squirrel" (On-line). Arizona-Sonora Desert Museum. Accessed April 18, 2014 at http://www.desertmuseum.org/kids/oz/long-fact-sheets/Harris%27s%20Antelope%20ground%20Squirrel.php.

Best, T., A. Titus, K. Caesar, C. Lewis. 1990. Ammospermophilus harrisii. Mammalian Species, 366: 1-7.

Brown, J. 1988. Patch Use as an Indicator of Habitat Preference Predation Risk and Competition. Behavioral Ecology and Sociobiology, 22: 37-48.

Brown, J. 1989. Desert Rodent Community Structure: A Test of Four Mechanisms of Coexistence. Ecological Monographs, 59: 1-20.

Escalante, T., V. Sanchez-Cordero, J. Morrone, M. Linaje. 2007. Deforestation affects biogeographical regionalization: a case study contrasting potential and extant distributions of Mexican terrestrial mammals. Journal of Natural History, 41: 965-984.

Levenson, H. 1979. Sciurid Growth Rates: Some Corrections and Additions. Journal of Mammalogy, Vol. 60: 232-235.

Mantooth, S., D. Hafner, R. Bryson, B. Riddle. 2013. Phylogeographic diversification of antelope squirrels (Ammospermophilus) across North American deserts. Biological Journal of the Linnaean Society, 109: 949-967.

Neal, B. 1965. Reproductive Habits of Round-Tailed and Harris Antelope Ground Squirrels. Journal of Mammalogy, Vol. 46: 200-206.

Nowak, R. 1999. Walker's Mammals of the World. Baltimore, Maryland: Johns Hopkins University Press.

Smith, E. 1970. Cactus Wrens Attack Ground Squirrel. Journal of Mammalogy, Vol. 72: 363-364.

Timm, R., S. Alvarez-Castaneda, I. Castro-Arellano, T. Lacher. 2013. "Ammospermophilus harrisii" (On-line). The IUCN Red List of Threatened Species. Accessed April 20, 2014 at http://www.iucnredlist.org/details/42399/0.

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