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Key to the ground squirrels of the USA

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WARNING! This key is currently under construction.

This key to the ground squirrels of the USA is based on Rickart and Yensen (1991). It was modified to accomodate recent taxonomic changes.

1

  • Molars brachydont; parastyle ridge on M1 and M2 joining protocone without an abrupt change of direction; confined to Nearctic region . . . . . 2
  • Molars hypsodont; parastyle ridge on M1 and M2 joining protocone with an abrupt change of direction; occurring in Nearctic or Palearctic region . . . . . 13


2

Return to couplet #1

  • P3 with two cusps and a functional cutting edge, crown area >25% that of P4... Franklin's Ground Squirrel (Poliocitellus franklinii)
  • P3 simple, crown area <25% that of P4... . . . . . 3


3

Return to couplet #2

  • Tail > 128 mm; upper parts of variegated black, white and buff.... genus Otospermophilus . . . . . 4


4

Return to couplet #3

  • Supraorbital foramen closed; pelage with some rufous areas; confined to subtropics ...S. annulatus or S. adocetus
  • Supraorbital foramen open; pelage without distinct rufous areas; not occurring in subtropics . . . . . 5


5

Return to couplet #4

  • Charcoal to blackish-brown triangular patch extending from the nape of neck to about midway down the back (may be faint in immature individuals)... Douglas's Ground Squirrel (Otospermophilus douglasii)


6

Return to couplet #5

  • Tail long (more than 150 mm) and bushy; hind foot long (more than 48 mm); skull large, with greatest length more than 53 mm and upper toothrow more than 10.0 mm... Common Rock Squirrel (Otospermophilus variegatus)
  • California Ground Squirrel (Otospermophilus beecheyi) photographed by Nathan Petersen near Pasadena, California. © Nathan Petersen

7

8

  • Dorsum with white or gray stripe on each side; greatest width of nasals >20% of zygomatic width; greatest length of skull >39 mm... genus Callospermophilus . . . . . 10
  • Dorsum without stripes; greatest width of nasals <20% of zygomatic width; greatest length of skull <40 mm... genus Xerospermophilus . . . . . 11


9

  • Confined to northern Mexico... C. madrensis
  • Occurring north of Mexico . . . . . 10


10

  • Golden mantle on head and neck and median stripes well defined; hind foot ≤44 mm; not occurring in Cascade Range of Washington and British Columbia... Common Golden-mantled Ground Squirrel (Callospermophilus lateralis)
  • Dorsal mantle and median dark stripes poorly defined; hind foot ≥44 mm; confined to Cascade Range of Washington and British Columbia... Cascade Golden-mantled Ground Squirrel (Callospermophilus saturatus)

11

  • Upper parts with numerous, whitish spots (but may be nearly absent if fur is badly worn), tail shorter and with more flattened appearance, bullae less inflated... Spotted Ground Squirrel (Xerospermophilus spilosoma)


12

  • Mohave Ground Squirrel (Xerospermophilus mohavensis) photographed by kliewer in Kern County, California.
  • Mohave Ground Squirrel (Xerospermophilus mohavensis) observed by Venkat Sankar in San Bernardino County, California. Notice the that the underside of the tail is creamy white.

13

  • Metaloph on P4 not continuous; dorsal pelage with stripes or spots; confined to Nearctic region... genus Ictidomys . . . . . 14
  • Metaloph on P4 continuous; dorsal pelage plain, or with white or buffy spots; occurring in Nearctic or Palearctic regions... genus Urocitellus . . . . . 15


14

15

17

  • Dorsum with fine white spots; tail length >40% length of head and body; diploid number 32; occurring in central and northeastern Asia west of the Lena River S. undulatus
  • Dorsum with bold white spots; tail length <40% length of head and body; diploid number 34; occurring in northwestern North America and northeastern Asia east of the Lena River S. parryii

18


Urocitellus armatus Urocitellus beldingi Urocitellus brunneus Urocitellus canus Urocitellus columbianus Urocitellus elegans Urocitellus idahoensis Urocitellus mollis Urocitellus parryii Urocitellus richardsonii Urocitellus townsendii Urocitellus washingtoni

8

Return to couplet #7

  • Underside of tail white, tail short, auditory bullae more inflated... White-tailed Antelope Squirrel (Ammospermophilus leucurus)
  • Underside of tail blackish, tail long, auditory bullae less inflated... Harris's Antelope Squirrel (Ammospermophilus harrisii)

References

Rickart EA, Yensen E. 1991. Spermophilus washingtoni. Mammalian Species (371):1-5. https://doi.org/10.2307/3504272

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