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Spermophilus tereticaudusround-tailed ground squirrel

By Carl Flink

Geographic Range

Round tailed ground squirrels, Spermophilus tereticaudus, live in desert areas of the southwest United States and Mexico, including Arizona, California and northern Mexico. They are present in portions of the Mojave, Yuma, and Colorado deserts (Cockrum, 1982). (Cockrum, 1982)

Habitat

Spermophilus tereticaudus "inhabits sandy arid regions of the Lower Sonoran Life Zone (Ernest and Mares, 1987)." It often occupies dunes and shrubs in lower flatter areas. Its burrows have been found among shrubs, and in sand of dunes, especially in areas with dense sand. Its habitats are normally characterized by extreme temperatures with low humidity (eg. -5 C to 39 C during one week)(Ernest and Mares, 1987). (Ernest and Mares, February 27 1987)

Physical Description

Spermopilus tereticaudus is a small ground squirrel. It has a long round tail; and long, broad, and hairy hind feet (Ernest and Mares, 1987). Their fur is uniform and without markings or stripes. The underside or their body is paler, and it appears uniform from tip to tail (Hall, 1981). The summer fur color is paler and brighter than the winter pattern. Spermophilus tereticaudus molts two times per year, once in the spring and once in fall.

The skull is rounded with a short rostrum, and a post orbital process is present. Body mass fluctuates according to season, however, the average mass is 125 g (Ernest and Mares, 1987).

  • Range mass
    110 to 170 g
    3.88 to 5.99 oz
  • Average mass
    125.00 g
    4.41 oz
  • Average basal metabolic rate
    0.501 W
    AnAge

Reproduction

The breeding season of Spermophilus tereticaudus starts in mid January with the enlargement of the male's testes. Females were observed to be pregnant from mid-March to late April. In mid April the testes regress, and the males are no longer capable of insemination. Gestation ranges from 25 to 35 days, and the average number of young is 6.5, with the largest litter observed containing 12. (Reynolds and Turkowski, 1972). The average mass at birth is 3.7 g, and the neonates are hairless, with eyes and ears closed. When they are 25 days old, they are capable of coordinated running, and are weaned at 5 weeks. They are sexually mature when they reach 10 to 11 months of age (Neal, 1965). (Neal, 1965; Reynolds and Turkowski, 1972)

  • Breeding interval
    These ground squirrels breed once per year.
  • Breeding season
    Breeding occurs from January to April.
  • Range number of offspring
    12 (high)
  • Average number of offspring
    6.5
  • Average number of offspring
    5.4
    AnAge
  • Range gestation period
    25 to 35 days
  • Average weaning age
    5 weeks
  • Range age at sexual or reproductive maturity (female)
    10 to 11 months
  • Range age at sexual or reproductive maturity (male)
    10 to 11 months

Parental care seems to be primarily the responsibility of females. As in other ground dwelling sciurids, the neonates are altricial.

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

Lifespan/Longevity

  • Range lifespan
    Status: captivity
    8.9 (high) years
    AnAge

Behavior

Spermophilus tereticaudus does not hibernate. Instead, they go into torpor during the winter months. They emerge in January and February, during the precopulatory phase, and after the juvenile disperal in June and July they resume their inactive phase (Aug. or Sept until January). Short periods of activity have been noted during the inactive season, (Ernest and Mares, 1987). In laboratory settings, food deprivation can trigger torpor (Ernest and Mares, 1987).

While active, these squirrels show two peaks in their activity, one in the morning, and one in the late afternoon. This pattern may lower the amount of time S. tereticaudus is exposed to the heat. These squirrels have been shown to have long periods of activity on overcast afternoons. When active, they spend about 50% of their time foraging (Ernest and Mares, 1987).

Spermophilus tereticaudus communicates using whistles. Their warning is a single whistle. When emitted, it causes the other animals in the area to run to their burrows and then look around. Females sound warning whistles more often than males. Since the females have been found to be more closely related to the their neighbors than the males, it is suspected that this behavior enhances inclusive fitness (Dunford, 1977).

Spermophilus tereticaudus have a semicolonial social organization, but their burrows are individual, and other individuals are chased away if they get too close. It is interesting to note that males are dominant from January until March (during breeding season) and females are dominant from March to April (during raising of young)(Dunford, 1977). (Dunford, 1977; Ernest and Mares, February 27 1987)

Communication and Perception

Spermophilus tereticaudus communicates using whistles. Their warning is a single whistle. When emitted, it caused the other animals in the area to run to their burrows and then look around (Dunford 1977). (Dunford, 1977)

Food Habits

Spermophilus tereticaudus is an omnivore like many other sciurids. Their diet includes a large proportion of green vegetation, but also seeds, and to a lesser extent, insects. During the spring, 80% of their diet consists of green vegetation, while 15% is represented by seeds, and 5% is represented by insects. In the summer, all of their diet is composed of green vegetation. In winter, however, their reliance on vegetation drops, and they again use seeds. When they do eat insects, they consume mainly ants, termites and grasshoppers (Ernest and Mares, 1987). They focus their diet on foods that have a high water concentration. This is necessary because they live in desert conditions, and cannot survive on dry foods. The average water content of the food they eat is 80% (Ernest and Mares, 1987). (Ernest and Mares, February 27 1987)

  • Animal Foods
  • insects
  • Plant Foods
  • leaves
  • roots and tubers
  • seeds, grains, and nuts
  • flowers

Predation

Although not specifically reported upon, it is likely that these squirrels are prey for other animals. Because of their diurnal and semi-fossorial habits, likely predators include canids, felids, snakes, and hawks. This species uses alarm signals, and these may help the squirrels evade predators.

Ecosystem Roles

The role of these ground squirrels in their ecosystems has not been documented. Because they are omnivorous, it is likely that they have some impact on plant and insect populations. As a possible prey species, S. tereticaudus is likely to have some influence on predator populations as well. Because they dig burrows, these animals contribute to soil aeration.

Economic Importance for Humans: Positive

These animals are not documented to have any positive economic effect on humans.

Economic Importance for Humans: Negative

They tend to live near cultivated fields, and may eat the alfalfa and dates grown. Thus, they come into conflict with humans, but it is not much of a problem (Ernest and Mares, 1987) (Ernest and Mares, February 27 1987)

Conservation Status

Spermophilus tereticaudus seems to be doing relatively well. Its habitat is not being destroyed significantly, and it seems to have a relatively good hold in its native areas (Ernest and Mares, 1987). (Ernest and Mares, February 27 1987)

Contributors

Nancy Shefferly (editor), Animal Diversity Web.

Carl Flink (author), University of Michigan-Ann Arbor, Phil Myers (editor), Museum of Zoology, University of Michigan-Ann Arbor.

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.

chaparral

Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.

chemical

uses smells or other chemicals to communicate

colonial

used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.

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.

fertilization

union of egg and spermatozoan

fossorial

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

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

pheromones

chemicals released into air or water that are detected by and responded to by other animals of the same species

scent marks

communicates by producing scents from special gland(s) and placing them on a surface whether others can smell or taste them

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

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

Cockrum, E. 1982. Mammals of the Southwest. Tuscon: University of Arizona Press.

Dunford, C. 1977. Kin selection for ground squirrel alarm calls. American Naturalist, 58: 782-785.

Ernest, K., M. Mares. February 27 1987. Mammalian Species. The American Society of Mammalogists.

Hall, E. 1995. Mammals of Nevada. New York: John Wiley and Sons.

Neal, B. 1965. Reproductive habits of round-tailed and Harris antelope ground squirrels. Journal of Mammalogy, 46: 200-206.

Reynolds, H., F. Turkowski. 1972. Reproductive variations in the round-tailed ground squirrel as related to winter rainfall. Journal of Mammalogy, 53: 893-898.

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