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© 2011 Museum of Southwestern Biology. All rights reserved.

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Key to the Rodents

1a.     Tail and body covered with quills …......................................… North American Porcupine, Erethizon dorsatum
1b.     Tail and body not covered with quills ............................................................................................................. 2

2a.     Hind foot webbed …..................................................................................................................................... 3
2b.     Hind foot not webbed …............................................................................................................................... 4

3a.     Tail large, scaly, flattened dorsal-ventrally, paddle-shaped …..................... American Beaver, Castor canadensis
3b.     Scaly tail much smaller, flattened laterally ……....................................... Common Muskrat, Ondatra zibethicus

4a.     Tail long-haired and bushy; prominent postorbital process on skull …............................................................… 5
4b.     Tail not bushy (or if bushy, underside of tail white); postorbital process weak or absent …...............................… 8

5a.     Loose fold of skin, or gliding membrane, on each side from wrist to hind leg ........................................................
         .............................................................................................. Northern Flying Squirrel, Glaucomys sabrinus
5b.     No gliding membrane along sides …….......................................................................................................…. 6

6a.     Size large (total length greater than 600 mm), body stout …............................. Hoary Marmot, Marmota caligata
6b.     Size smaller (less than 500 mm), body more slender ……...........................................................................…. 7

7a.    Total length usually greater than 350 mm; top of head cinnamon, black flecked with white; underparts yellowish or tawny ................................................................ Arctic Ground Squirrel, Spermophilus parryii
7b.    Total length usually less than 350 mm; rust-red to grayish-red above, white or grayish-white below.............................................................................................. Red Squirrel, Tamiasciurus hudsonicus

8a.    Hind legs considerably longer than front legs; tail very long in relation to body size, usually greater than 125 mm; infraorbital canal of skull large and oval; 4 upper cheek-teeth that includes a small premolar... 9
8b.    Front legs and hind legs equal in size; tail length variable relative to total size; infraorbital canal  smaller, being much wider at the top than at the bottom; 3 upper cheek-teeth ................................................. 10

9a.    Length of upper cheek-tooth row greater than 3.7 mm; incisive foramina longer than 4.7 mm; posterior portion of septum dividing the incisive foramina very thin ......... Western Jumping Mouse, Zapus princeps
9b.    Length of upper cheek-tooth row less than 3.7 mm; incisive foramina shorter than 4.7 mm; posterior portion of septum dividing the incisive foramina broad .......... Meadow Jumping Mouse, Zapus hudsonius

10a.  Mouse- and rat-like with a slender body, pointed snout, well-developed hind legs, large eyes, prominent ears, and a long tail; cheek-teeth cuspidate or, if cusp pattern not apparent, flat-crowned and prismatic not arranged as alternating triangles ..................................................................................................... 11
10b.  Vole-like with a stout, furry body, blunt snout, short legs, small eyes, ears frequently hidden by long pelage, and tail relatively short; cheek-teeth without rows of cusps on crown; crowns flat with alternating triangles or “puddles” filled with dentine and surrounded by enamel .................................. 14

11a.  Cheek teeth appearing prismatic and flat-crowned; tail bushy, underside white ..................................................................................................... Bushy-tailed Woodrat, Neotoma cinerea
11b.  Cheek teeth clearly cuspidate; tail without hair ...................................................................................... 12

12a.  Two rows of cusps running down the crowns of the tooth row ............................................................................................ Northwestern Deermouse, Peromyscus keeni
12b.  Three rows of cusps running down the crowns of the tooth row ............................................................ 13

13a.  Total length greater than 300 mm; 1st upper cheek-tooth (M1) about equal to or less than combined length of M2 and M3 ..................................................................................... Brown Rat, Rattus norvegicus
13b.  Total length less than 200 mm; M1 length greater than combined length of M2 and M3 ...................................................................................................................... House Mouse, Mus musculus

14a.  Tail very short, about as long as the hind foot; lower incisors set inward from the cheek-teeth, and ending in a horizontal projection opposite or in front of the socket of  the last lower molar ................. 15
14b.  Tail length variable but clearly extending past the hind feet when legs outstretched; lower incisors passing from the tongue to the lip sides of the cheek-teeth and ascending back to within or near the condylar process .................................................................................................................................... 16

15a.  Pelage uniformly grizzled brown above, grayish below; upper incisors deeply grooved with projecting outer edges ........................................................................ Northern Bog Lemming, Synaptomys borealis
15b.  Pelage chestnut-brown; upper incisors not grooved ................. Brown Lemming, Lemmus trimucronatus

16a.  Pelage rust-reddish above; skull relatively rounded and light, zygomatic arches relatively slender and the mandibles week; outer angles of cheek-teeth rounded, without a “heel” projecting posteriorly on the last upper molar (M3) ............................................................................................................................. 17
16b.  Pelage color not as above; skull relatively angular and more massive, zygomatic arches and mandibles more robust; outer angles of cheek-teeth pointed, with a “heel” projecting posteriorly on the M3 ...... 18

17a.  Tail short, thick, with closely set bristly hairs; post-palatal bridge usually incomplete in adults, and always incomplete up through first year ............................................. Northern Red-backed Vole, Myodes rutilus
17b.  Tail longer and more slender, with short hairs except at tip where hairs longer; post-palatal bridge always complete, even in half grown young .......................... Southern Red-backed Vole, Myodes gapperi

18a.  Cheek-teeth rooted in adults; re-entrant angles on the inner side of the lower molars deeper than those on the outer side (Fig. 3) ............................................... Western Heather Vole, Phenacomys intermedius
18b.  Cheek-teeth not rooted in adults; re-entrant angles on the inner side of the lower molars approximately equal in depth ........................................................................................................................................ 19

19a.  Tail averaging 1/3 or more of total length ...................................... Long-tailed Vole, Microtus longicaudus
19b.  Tail averaging less than 1/3 of total length ............................................................................................ 20

20a.  Second upper molar (M2) with four closed triangles and a posterior loop ....................................................................................................... Meadow Vole, Microtus pennsylvanicus
20b.  M2 with four closed triangles and no posterior loop .................................. Root Vole, Microtus oeconomus


Key to the Lagomorphs

1a.    Hind legs scarcely larger than front legs; no visible tail; skull lacking supraorbital process on   frontal ................................................................................................................... Collared Pika, Ochotona collaris  
1b.    Hind legs notably larger than front legs; tail short but obvious; skull with supraorbital process on frontal ............................................................................................................. Snowshoe Hare, Lepus americanus


Key to the Shrews

1a.    Third unicuspid tooth (U3) noticeably smaller than the fourth ................................................................... 2
1b.    Third unicuspid tooth (U3) equal to or larger than than the fourth ........... Cinereus Shrew, Sorex cinereus

2a.    Skull relatively large (usually >19 mm); hind foot more than 18 mm greater than 65 mm;  pelage grayish-black, never distinctly brown ................................... Water Shrew, Sorex palustris (includes S. alaskanus)
2b.    Skull, hind foot smaller;  pelage distinctly brown ...................................... Dusky Shrew, Sorex monticolus


Key to the Bats

1a.     Pelage black-brownish with a frosting of white on back and usually on underside; skull with 36 teeth  ............................................................................................. Silver-haired Bat, Lasionycteris noctivagans
1b.     Pelage brownish without white frosting; skull with 38 teeth ................................................................... 2

2a.     Calcar with well developed keel ............................................................................................................... 3
2b.     Calcar without well developed keel .......................................................................................................... 4

3a.     Underwing furred outward to a line from the elbow to the knee ........ Long-legged Myotis, Myotis volans
3b.     Underwing not furred ........................................................................ California Myotis, Myotis californicus

4a.     Ears long (> 17 mm), when laid forward extends about 4 mm beyond tip of nose; fur dark brown without glossy sheen ................................................................................................. Keen’s Myotis, Myotis keenii
4b.     Ears shorter (< 17 mm), when laid forward reaches only to nostril; fur reddish brown with glossy sheen  ......................................................................................................... Little Brown Myotis, Myotis lucifugus


Key to the Small Carnivores

1a.    Weight more than 50 g; tail long with black tip .................................................... Ermine, Mustela erminea
1b.    Weight less than 50 g; tail short without black tip ....................................... Least Weasel, Mustela nivalis
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Illustrated keys to the bats (PDF 10 MB)...

Illustrated Identification Guide to the Shrews and Small Rodents (PDF 11 MB)...

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