Ammospermophilus harrisii

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.

• Biogeographic Regions
• nearctic nearctic
  • native native

• Other Geographic Terms
• island endemic island endemic

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.

• Habitat Regions
• terrestrial terrestrial

• Terrestrial Biomes
• desert or dune desert or dune

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.

• Other Physical Features
• endothermic endothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• sexes alike

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.

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

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.

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

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.

• Parental Investment
• female parental care female parental care

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.

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.

• Key Behaviors
• diurnal diurnal
• motile motile
• solitary solitary

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.

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.

• Communication Channels
• tactile tactile
• acoustic acoustic

• Perception Channels
• tactile tactile
• chemical chemical

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.

• Primary Diet
• carnivore carnivore
  • eats terrestrial vertebrates
  • insectivore insectivore
• herbivore herbivore
  • frugivore frugivore
  • granivore granivore
• omnivore omnivore

• Animal Foods
• mammals
• insects

• Plant Foods
• seeds, grains, and nuts
• fruit

• Foraging Behavior
• stores or caches food stores or caches food

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.

• Anti-predator Adaptations
• aposematic aposematic

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.

• Ecosystem Impact
• disperses seeds

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.

• 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.

• 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.

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 .

Additional Links

Encyclopedia of Life

BioKIDS Critter Catalog

Contributors

Matthew Haloostock (author), University of Michigan, Joanna Larson (editor), University of Michigan, Priscilla Tucker (editor), University of Michigan, Leila Siciliano Martina (editor), Texas State University.

References

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 .

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 .