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Spermophilus brunneusIdaho ground squirrel

By Naly Vang

Geographic Range

Spermophilus brunneus is found only in west-central Idaho. This area consists of five counties which have an elevation between 1150 and 1550 m. (Gravin, et al., 1999)

Habitat

The habitat of Idaho ground squirrels mainly consists of meadows, dominated by grasses and broad-leaved forbs, which are mostly surrounded by coniferous forest. (Yensen and Sherman, 1997; Yensen, 1991)

  • Range elevation
    1150 to 1550 m
    3772.97 to 5085.30 ft

Physical Description

Spermophilus brunneus has a small head and body that is between 209 and 258 mm; the hind foot is less than 40 mm; skull length is 36.1 to 42.5 mm; ear length is 13 to 18 mm; and tail length is 39 to 65 mm. This species is sexually dimorphic, with males about 2.5% larger than females. Weight varies seasonally, and can be between 109 and 258 g. (Yensen and Sherman, 1997)

The dorsal pelage of S. brunneus is dark reddish-gray in color, which is the result from a mixture of black unbanded, and yellowish-red banded guard hairs. It has an off-white eye ring. (Yensen and Sherman, 1997)

Young Idaho ground squirrels do experience a diffuse molting in pelage. The molting season usually occurs in May and early June; however, adult S. brunnesus does not molt and tends to have longer pelage. (Yensen and Sherman, 1997)

The rostrum of a S. brunneus is relatively short and these animals have a broad braincase. The dental formula of Idaho ground squirrels is i 1/1 c 0/0 p 2/1 m 3/3 = 22. (Yensen and Sherman, 1997)

  • Sexual Dimorphism
  • male larger
  • Range mass
    109 to 290 g
    3.84 to 10.22 oz
  • Range length
    209 to 258 mm
    8.23 to 10.16 in

Reproduction

Spermophilus brunneus is very unique in that it shows sexual behavior for at least 12 to 13 days before mating. The yearling males rarely breed, and the older males are polygynous. (Sherman, 1989; Yensen and Sherman, 1997; Yensen, 1991)

Males first emerge from their hibernation burrows 1 to 2 weeks before females emerge. Females are sexually attractive to males for the first couple of hours on the first or second afternoon after females emerge from hibernation. The relatively early emergence of males ensures that males are awake and ready for the females when they come out from hibernation. (Sherman, 1989; Yensen and Sherman, 1997; Yensen, 1991)

Newly emerged females remain near their hibernacula, where they are courted by adult males that are at least 2 years old. Receptive females are scattered around, so males have to search for them in order to mate. Searching for mates is time consuming and dangerous, because this species inhabits the open meadow. Looking for mates puts males at risk of being spotted by hawks, which are one of the major predators of these small squirrels. So, the probability of getting sexual access is low for most males. (Sherman, 1989; Yensen and Sherman, 1997; Yensen, 1991)

Once a male finds a female, he will guard that female until mating occurs. Males compete for access to receptive females, and heavier males are able to displace lighter males. There are times when multiple males sequentially guard one female, and the male who guards the female the longest sires the most offspring. Copulation occurs underground so it is not observed. (Sherman, 1989; Yensen and Sherman, 1997; Yensen, 1991)

There are four events which occur during mating: a male 1) follows a female closely and sniffs or licks her genitalia, then 2) accompanies her into a burrow, where 3) the pair remains for more than 5 minutes, after which 4) a copulatory plug is observed in the female's vagina. All these criteria are fulfilled in just one afternoon of the year. (Sherman, 1989; Yensen and Sherman, 1997; Yensen, 1991)

Idaho groung squirrels reach sexual maturity at approximately 2 years of age. Most of courtship occurs above ground right after females emerge from hibernation in the early spring. Actual copulations occur under ground. (Gravin, et al., 1999; Sherman, 1989)

After fertilization, a female constructs her burrow and nest. Spermophilus brunneus females produce one litter per year. The litters usually emerge in late May to early June, about 50 to 52 days after copulation. The litter size is from two to seven with an average of 5.2 young per litter. Within 2 to 3 days after the pups emerge from their natal burrows, they disperse. (Gravin, et al., 1999; Sherman, 1989)

  • Breeding interval
    Idaho Ground Squirrels breed once yearly
  • Breeding season
    Mating occurs in the early spring, when females emerge from hibernation.
  • Range number of offspring
    2 to 7
  • Average number of offspring
    5.2
  • Average age at sexual or reproductive maturity (female)
    1 years
  • Average age at sexual or reproductive maturity (male)
    approximately 2 years minutes

Birthing happened undergroud so parental care was not observed. But based on their mating system, females likely care for the pups with little paternal care. Females provide young with milk, grooming, and protection in the burrow. The young disperse shortly after they emerge, so parental care is not lengthy. (Yensen and Sherman, 1997)

  • Parental Investment
  • no parental involvement
  • altricial
  • pre-fertilization
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female
  • pre-independence
    • protecting
      • female

Lifespan/Longevity

These animals are not thought to live very long. Most mortality occurs during hibernation, with 75 to 90 percent of juveniles dying. About half of adults also fail to emerge from hibernation. (Yensen and Sherman, 1997)

Behavior

Idaho ground squirrels are diurnal mammals which are active above ground for about five months before retreating to underground burrows. They live at higher elevations and usually emerge in late March or early April and remain active until late July or early August. (Gravin, et al., 1999; Yensen and Sherman, 1997)

Spermophilus brunneus constructs three types of burrows; the nest burrow, auxiliary burrow, and the hibernation burrow. The nest burrow in which females rear their young is a complicated structure. It is 50 to 121 cm deep, has 3 to 11 openings, 2 to 13 branching tunnels, and 1 to 7 chambers. The nest is built on the well drained soil and deepest burrow. (Gravin, et al., 1999; Yensen and Sherman, 1997)

The auxiliary burrow is a burrow that has no nest. It is shallower and is constructed far away from the nest burrows. The hibernation burrow, used for hibernating during winter, consists of a single tunnel that leads to a single nest chamber. Burrow openings are usually placed under rocks, shrubs, or fallen timber, but also occur on open meadows. (Gravin, et al., 1999; Yensen and Sherman, 1997)

Males do not live near females or their young. Females are more social than are males, because they interact with their young. Females with young are known to give alarm calls to warn their offspring of predators. (Yensen and Sherman, 1997)

Home Range

Home range size for these animals has not been reported.

Communication and Perception

Idaho ground squirrels communicate by making high-pitched calls. These calls are usaully alarm calls that are used to warned other ground squirrels that there are pedators in the area. This type of call is used for both terrestrial and aerial predators. (Yensen and Sherman, 1997)

In addition to accoustic communication, these small mammals use visual signals, such as body postures, tactile communication, such as nosing, butting, biting, and chasing, and chemical communication (males sniff and lick a female's genitals prior to copulation). (Yensen and Sherman, 1997)

Food Habits

Spermophilus brunneus is primarily herbivorous and its diet consists of 40 to 50 species of plants. In spite of this overall variety, only 5 to 7 species plants make up more than half of their diet. They eat grasses (Poa bulbosa, Bromus commutatus), dicot leaves (Microseris nigrescens, Lupinus), flowers, roots and bulbs and seeds (Asteraceae, Madia). Some insects may also be consumed. Ingestion of seeds apparently increases as hiberation nears. Because of hibernation, these animals must store enough fat to sustain them through the long months of winter. Weight increases throughout the growing season. (Yensen and Sherman, 1997)

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

Predation

Predators of S. brunneus include prairie falcons, Cooper's hawks, goshawks, red-tailed hawks, northern harriers, badgers, and sometime long-tailed weasels. Idaho ground squirrels use alarm calls to warn others of predators. They are also reported to remain still when threatened, apparently because their dirt-colored backs are often undetected by predators. (Yensen and Sherman, 1997)

Ecosystem Roles

Idaho ground squirrels serve as prey for other larger animals such as hawks, badgers, prairie falcons, and weasels. (Yensen and Sherman, 1997)

Economic Importance for Humans: Positive

No information could be found on the economic importance of Idaho ground squirrels.

Economic Importance for Humans: Negative

No information could be found on the economic importance of Idaho ground squirrels.

Conservation Status

Spermophilus brunneus is considered to be "threatened or endangered" by the United States Fish and Wildlife Service. In 1998, 12 of the 36 populations that they studied were extinct due to loss of habitat. A study done in 1999 showed that since the populations of S. b. brunneus are small and isolated that they are prone to extinction. Apparently, the major threat to these animals is the loss of habitat due to encroaching conniferous forests. (Gravin, et al., 1999; Yensen and Sherman, 1997)

Other Comments

There are ectoparasites of S. b. brunneus, and it included ticks, fleas and nematode eye-worms. (Yensen and Sherman, 1997)

Contributors

Nancy Shefferly (editor), Animal Diversity Web.

Naly Vang (author), University of Wisconsin-Stevens Point, Chris Yahnke (editor, instructor), University of Wisconsin-Stevens Point.

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.

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.

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.

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.

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.

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.

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.

polygynandrous

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

seasonal breeding

breeding is confined to a particular season

sexual

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

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.

threatened

The term is used in the 1994 IUCN Red List of Threatened Animals to refer collectively to species categorized as Endangered (E), Vulnerable (V), Rare (R), Indeterminate (I), or Insufficiently Known (K) and in the 1996 IUCN Red List of Threatened Animals to refer collectively to species categorized as Critically Endangered (CR), Endangered (EN), or Vulnerable (VU).

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

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

Gravin, T. A., P. W. Sherman, E. Yensen, B. May. 1999. Population Genetic structure of the Northern Idaho Ground Squirrel (Spermophilus brunneus brunneus). Jounal of Mammalogy, 88 (1): 156-168.

Sherman, P. 1989. Mate guarding as paternity insurance in Idaho ground squirrels. Nature, 338: 418-420.

Yensen, E. 1991. Taxonomy and distribution of the Idaho ground squirrel, Spermophilus brunneus. Journal of Mammalogy, 72: 583-600.

Yensen, E., P. Sherman. 1997. Spermophilus brunneus. Mammalian Species, 560: 1-5.

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