1. Tail less than one-third total length, condylobasal length more than 60 mm: 2
1'. Tail more than one-half total length; condylobasal length less than 60 mm: 4
2. Maxillary tooth-rows approximately parallel; condylobasal length more than 80 mm; front feet with four clawed digits: Marmota flaviventris.
2'. Maxillary tooth-rows converging posteriorly; condylobasal length less than 70 mm; front feet with five clawed digits: Cynomys, 3
3. Tip of tail black: Cynomys ludovicianus.
3'. Tip of tail white or light gray: Cynomys gunnisoni.
4. Zygomatic arches nearly parallel to one another: 5
4'. Zygomatic arches convergent anteriorly: 8
5. Total length usually less than 360 mm; P3 vestigial or absent: Tamiasciurus hudsonicus.
5'. Total length usually more than 450 mm; P3 usually well developed: 6
6. Venter orangish, reddish, or buffy: Sciurus niger.
6'. Venter white or black: 7
7. Ears with prominent tufts of hair on ends; usually five cheek teeth in upper tooth-row: Sciurus aberti.
7'. Ears without tufts; usually four cheek teeth in upper tooth-row: Sciurus arizonensis.
8. Sides of head striped; posterior border of zygomatic process of maxillary opposite P3: Tamias, 9
8'. Sides of head not striped; posterior border of zygomatic process of maxillary opposite Ml: 13
9. Dorsal stripes obscure, not clearly delineated: Tamias dorsalis.
9'. Dorsal stripes clearly marked: 10
10. Condylobasal length less than 30.5 mm; hind foot usually 32 mm or less; upper lip buffy: Tamias minimus.
10'. Condylobasal length more than 31 mm; hind foot usually 33 mm or more; upper lip whitish: 11
11. Hind feet grayish dorsally; in Gallinas, Capitan, Sacramento, and Guadalupe mountains: Tamias canipes.
11'. Hind feet buffy dorsally; not distributed as above: 12
12. Neck with grayish dorsal wash; south of U.S. 66 and west of Rio Grande: Tamias cinereicollis.
12'. Neck without grayish wash; not distributed as above: Tamias quadrivittatus.
13. Dorsal pattern incorporating some continuous stripes: 14
13'. Dorsal pattern spotted or mottled: 18
14. Dorsal pattern of stripes and rows of spots: Ictidomys tridecemlineatus.
14'. Dorsal pattern of two white stripes; not spots: 15
15. Tail brownish ventrally, longer than 85 mm: Callospermophilus lateralis.
15'. Tail whitish, grayish, or blackish ventrally, less than 85 mm: 16
16. Tail dark below: Ammospermophilus harrisii.
16'. Tail whitish below: 17
17. East of Rio Grande: Ammospermophilus interpres.
17'. West of Rio Grande: Ammospermophilus leucurus.
18. Tail long and bushy, more than 135 mm; mottled grayish dorsally: Otospermophilus variegatus.
18'. Tail shorter-haired, less than 135 mm; spotted dorsally: 19
19. Spots arranged in many rows: Xerospermophilus mexicanus.
19'. Spots irregularly arranged: Xerospermophilus spilosoma.
Key slightly modified from Findley (1987).
Last Update: 29 Jan 2012