HOW TO USE THIS KEY
A dichotomous key is based upon the use of paired, mutually-exclusive statements called couplets through which the user proceeds, making choices about the object at hand until finally an identification is obtained. The term dichotomous refers to the paired nature of the statements, and comes from the Greek word dichotomia meaning "dividing in two." The progression is not unlike use of a flow chart to make decisions involved in diagnosing a car malfunction or a medical condition.
Thus, each couplet below consists of A and B parts. In practice, you would examine your unknown and compare its features with the information provided, following along to the next couplet to which you are referred if identification is not provided by the one you are using.
It is important to remember that any key is only useful within the constraints imposed by the authors. Thus, this key is for Kansas mammals. If you used it in Utah, you might be led to incorrect identification of an animal, even though it might fit into the descriptions provided. This would certainly be the case if you used this key in Venezuela! You might get something to "fit," but it would not be a correct identification.
Another point to remember about keys is that they are artificial constructs -- tools. They are not indicators of phylogenetic relationship. An everyday example of a similar situation is the way grocery stores group merchandise. In the dairy section, you find milk, eggs, cheese, yogurt, egg substitutes, margarine, cake yeast, etc. This grouping of generally unrelated items allows users to locate the items conveniently. It really does not provide information about the relatedness of the items.
| 1A. Tail prehensile, apposable thumb; 5 pair of upper incisor teeth present, braincase small relative to skull | Didelphis virginiana |
| 1B. Tail not prehensile, thumb not apposable; less than 5 pair of incisors present, braincase large relative to skull | 2 |
| 2A. Moveable muzzle long, pointed, extending beyond ventrally placed mouth; canine teeth about the same size as adjacent teeth | 3 |
| 2B. Muzzle not long, pointed, and extending beyond mouth; canine teeth lacking, or when present noticeably larger than adjacent teeth | 5 |
| 3A. No visible external ear, forefeet much broader than hind feet; zygomatic arches and auditory bullae present | Scalopus aquaticus |
| 3B. Visible external ear, forefeet not broader than hind feet; zygomatic arches and auditory bullae absent | 4 |
| 4A. Total length more than 90 mm; total teeth 32; 5 unicuspids, only 4 usually visible in side view | Blarina hylophaga |
| OR Total length more than 90 mm; total teeth 32; tail long, usually more than a third of the total length of body plus tail | Sorex haydeni |
| 4B. Total length less than 90 mm; total teeth 30; 4 unicuspids, only 3 usually visible in side view | Cryptotis parva |
| 5A. Forelimbs lacking claws (except thumb), modified to form membranous wing; teeth with high cusps on cingulum | 6 |
| 5B. Forelimbs with claws or hooves, no membrane forming a wing, not capable of true flight; teeth without high cusps on cingulum | 20 |
| 6A. Tail not extending well beyond tail membrane (5 mm at most); lower incisors trifid, with 2 clefts | 7 |
| 6B. Tail extending well beyond tail membrane; lower incisors bifid, with 1 cleft | 19 |
| 7A. Two pairs of upper incisors; total teeth 32, 34, 36, or 38 | 8 |
| 7B. One pair of upper incisors; total teeth 28, 30, or 32 | 16 |
| 8A. Pelage blackish frosted with white or if not, ear very large (30 mm or more); premolars 2/3; total teeth 36 | 9 |
| 8B. Pelage not blackish frosted with white, and ear 23 mm or less; premolars 1/2, 2/2, or 3/3; total teeth 32, 34, or 38 | 10 |
| 9A. Upper surface of tail membrane 1/3 to 1/2 furred, pelage blackish frosted with white; ear less than 19 mm; rostrum as wide as braincase | Lasionycteris noctivagans |
| 9B. Tail membrane furred only at base, pelage brownish; ear more than 29 mm; rostrum 1/2 as wide as braincase | Corynorhinus townsendii |
| 10A. Upper surface of tail membrane 1/3 to 1/2 furred, pelage reddish brown; 2 pairs of upper premolars, total teeth 34 | Pipistrellus subflavus |
| 10B. Upper surface of tail membrane furred only at base, pelage brownish or yellowish-brown; one or three pairs of upper premolars; total teeth 32 or 38 | 11 |
| 11A. Total length usually more than 110 mm; greatest length of skull more than 18 mm; premolars 1/2, total teeth 32 | Eptesicus fuscus |
| 11B. Total length usually less than 110 mm; greatest length of skull less than 18 mm; premolars 3/3, total teeth 38 | 12 |
| 12A. Pelage woolly and thin; forearm usually more than 40; greatest length skull more than 15.7 mm | 13 |
| 12B. Pelage not woolly, forearm usually less than 40; greatest length of skull less than 15.7 mm | 14 |
| 13A. Pelage dark grayish-brown; wing membrane attached to lower leg instead of foot; greatest length skull less than 16.5 mm | Myotis grisescens |
| 13B. Pelage pale brownish; wing membrane attached to foot; greatest length of skull more than 16.5 mm | Myotis velifer |
| 14A. Pelage pale yellowish-brown contrasting with black ears and membranes, forearm less than 33.0 mm; hind foot usually less than 8 | Myotis ciliolabrum |
| 14B. Pelage brownish, not contrasting with ears or membranes, forearm more than 33.0 mm; hind foot usually more than 8 | 15 |
| 15A. Ear extending well beyond tip of nose when laid forward; maxillary tooth row more than 5.5 mm | Myotis keenii |
| 15B. Ear not extending much beyond tip of nose when laid forward; maxillary tooth row 5.5 mm or less | Myotis lucifugus |
| 16A. Upper surface of tail membrane thickly furred; 2 pairs of upper premolars, total teeth 32 | 17 |
| 16B. Upper surface of tail membrane naked or only furred at base; one pair of upper premolars, total teeth 28 or 30 | 18 |
| 17A. Pelage hoary, dark brownish tipped with grayish white; forearm more than 45 mm; greatest length of skull more than 17.0 mm | Lasiurus cinereus |
| 17B. Pelage reddish-orange to yellowish-brown; forearm less than 45 mm; greatest length of skull less than 14.5 mm | Lasiurus borealis |
| 18A. Pelage dark brownish; 3 pairs of lower incisors, total teeth 30 | Nycticeius humeralis |
| 18B. Pelage yellowish-brown; 2 pairs of lower incisors, total teeth 28 | Antrozous pallidus |
| 19A. Forearm less than 48 mm; greatest length of skull less than 21 mm | Tadarida brasiliensis |
| 19B. Forearm more than 55 mm, greatest length of skull more than 21 mm | Nyctinomops macrotis |
| 20A. Body covered with plates; all cheek teeth peg-like; teeth not present in anterior part of upper or lower jaws | Dasypus novemcinctus |
| 20B. Body not covered with plates; teeth not usually peg-like, present in anterior part of at least lower jaw | 21 |
| 21A. Feet with claws; upper incisors present | 22 |
| 21B. Feet with hooves; upper incisors absent | 83 |
| 22A. Canines absent; incisors 1 pair below and 1 pair above or 1 pair below and 2 pair above and separated from cheek teeth by a wide diastema | 23 |
| 22B. Canine teeth present, incisors 3 pairs below and 3 pairs above, no diastema | 63 |
| 23A. Tail shorter than ears; incisors 1 pair below and 2 pair above (small upper incisor pair lies flat against the rear of the large front upper incisor pair) | 24 |
| 23B. Tail longer than ears (same length in Synaptomys); 1 pair of upper and lower incisors | 28 |
| 24A. Ear from notch more than 90 mm, hind foot 120 mm or more; interparietal fused with parietals | 25 |
| 24B. Ear from notch less than 90 mm, hind foot 110 mm or less; interparietal not fused with parietals | 26 |
| 25A. Top of tail white, sometimes with narrow black mid-dorsal line which does not extend onto lower back; posterior projection of supraorbital process usually not touching skull | Lepus townsendii |
| 25B. Top of tail black, extending onto lower back; posterior projection of supraorbital process usually touching skull | Lepus californicus |
| 26A. Hind foot more than 100 mm; basilar length of skull more than 60 mm | Sylvilagus aquaticus |
| 26B. Hind foot usually less than 100 mm; basilar length of skull less than 60 mm | 27 |
| 27A. Ear more than 58; auditory bullae relatively large and inflated, length more than 11.5 mm | Sylvilagus audubonii |
| 27B. Ear usually less than 58 mm; auditory bullae relatively small and not inflated, length less than 10.5 mm | Sylvilagus floridanus |
| 28A. Modified for semi-aquatic life, hind feet fully webbed; tail naked, flattened dorsoventrally, or rounded; lower incisor more than 6 mm wide at base | 29 |
| 28B. Not modified for semi-aquatic life (except Ondatra), hind foot not fully webbed; tail naked or furred; lower incisor less than 5.5 mm wide at base (less than 4 mm in all except Erethizon) | 30 |
| 29A. Tail rounded; infraorbital canal larger than foramen magnum | Myocastor copyus (See "Species Possibly in Kansas"). |
| 29B. Tail flattened dorsoventrally; infraorbital canal smaller than foramen magnum | Castor canadensis |
| 30A. Sharp quills on back and tail, interspersed with long blackish hairs; infraorbital canal larger than foramen magnum | Erethizon dorsatum |
| 30B. Not as above | 31 |
| 31A. Hair on tail bushy; skull with distinct postorbital process | 32 |
| 31B. Hair on tail not bushy; skull lacking distinct postorbital process | 40 |
| 32A. Hind foot more than 75 mm; greatest length of skull more than 30 | Marmota monax |
| 32B. Hind foot less than 75 mm; greatest length of skull less than 70 | 33 |
| 33A. Fold of loose skin on sides; narrow interorbital region with V-shaped notches | Glaucomys volans |
| 33B. No loose skin fold on side; interorbital region lacks V-shaped notches | 34 |
| 34A. Tail usually shorter, moderately to only somewhat bushy; second upper molar wider than long | 35 |
| 34B. Tail very long and bushy; second upper molar as long as wide | 39 |
| 35A. Upper parts striped with black and white, with well-defined stripe above eye; infraorbital foramen pierces zygomatic plate | Tamias striatus |
| 35B. Upper parts striped, spotted, mottled or plain; no stripe above eye; infraorbital canal between zygomatic plate and rostrum | 36 |
| 36A. Tail less than one-fourth of total length; tail tip black, upper third premolar enlarged, cheek teeth convergent posteriorly, laterally expanded | Cynomys ludovicianus |
| 36B. Tail more than one-fourth of total length; upper third premolar simple and peg-like; cheek tooth rows nearly parallel, not laterally expanded | 37 |
| 37A. Upper parts not striped or spotted; total length more than 325 mm; upper molariform tooth-row more than 8.5 | Spermophilus franklinii |
| 37B. Upper parts with large stripes or spots, total length less than 325 mm; upper molariform tooth-row less than 8.5 | 38 |
| 38A. Upper parts mostly striped, with some rows of spots; postorbital constriction less than 12 | Spermophilus tridecemlineatus |
| 38B. Upper parts spotted; postorbital constriction more than 12 mm | Spermophilus spilosoma |
| 39A. Hairs on side of tail tipped with white, pelage mostly gray above and white below; upper third premolar small, but usually present | Sciurus carolinensis |
| 39B. Hairs on side of tail not tipped with white, pelage tawny brown above and orangish below; upper third premolar absent | Sciurus niger |
| 40A. Fur-lined external cheek pouch present; cheek teeth 4/4 | 41 |
| 40B. No fur-lined external cheek pouches; cheek teeth 4/3 or 3/3 | 46 |
| 41A. Tail less than 3/4 length of head and body, hind feet smaller than front feet; tympanic bulla not visible dorsally | 42 |
| 41B. Tail more than 3/4 length of head and body, hind feet larger than forefeet; tympanic bulla visible dorsally | 43 |
| 42A. Feet same color as dorsal pelage or darker; upper incisor with single groove | Cratogeomys castanops |
| 42B. Feet lighter than dorsal pelage; upper incisor with two grooves | Geomys bursarius |
| 43A. Hind foot less than 30 mm, sole naked; interparietal breadth more than 1/4 greatest breadth of skull | Dipodomys ordii |
| 43B. Hind foot less than 30 mm, sole naked; interparietal breadth more than 1/4 greatest breadth of skull | 44 |
| 44A. Total length more than 150 mm; greatest length of skull more than 28 mm | Chaetodipus hispidus |
| 44B. Total length less than 150 mm; greatest length of skull less than 24 mm | 45 |
| 45A. Postauricular patches indistinct, same length as ears; interparietal breadth more than 4.1 mm, distinctly broader than long | Perognathus flavescens |
| 45B. Postauricular patches distinct, twice length of ears; interparietal breadth less than 4.1 mm, roughly quadrate in shape | Perognathus flavus |
| 46A. Tail more than 1 1/2 times length of head and body, hind feet noticeably elongated; cheek teeth 4 above and 3 below | Zapus hudsonius |
| 46B. Tail less than 1 1/2 times length of head and body, hind feet not noticeably elongated; cheek teeth 3 above and 3 below | 47 |
| 47A. Annulations on tail concealed by fur (except Ondatra, in which tail is laterally flattened); maxillary molars with two or fewer longitudinal rows of cusps (or prisms in Neotoma) | 48 |
| 47B. Annulations of tail clearly visible; maxillary molars with 3 longitudinal rows of cusps | 62 |
| 48A. Tail laterally compressed, fringe of stiff hairs on toes of feet; length of skull more than 50 mm | Ondatra zibethicus |
| 48B. Tail round, no fringe of stiff hairs on toes of hind feet; length of skull less than 50 mm | 49 |
| 49A. Tips of ears conspicuous and not partially hidden by hair, cheek-teeth with cusps, occlusal surface lacking prisms of dentine surrounded by enamel (except Neotoma) | 50 |
| 49B. Tips of ears not conspicuous and partially hidden by hair, cheek-teeth without cusps, occlusal surface of dentine prisms surrounded by enamel | 59 |
| 50A. Total length less than 200 mm; greatest length of skull less than 28mm | 51 |
| 50B. Total length more than 200 mm; greatest length of skull more than 28mm | 57 |
| 51A. Tail less than 60 percent of length of head and body; coronoid process long, extending above articular process | Onychomys leucogaster |
| 51B. Tail more than 60 percent of length of head and body; coronoid process short, not higher than articular process | 52 |
| 52A. Body usually less than 80 mm; upper incisor grooved on anterior face | 53 |
| 52B. Body usually more than 80 mm; upper incisor not grooved on anterior face | 55 |
| 53A. Upper parts golden-brown, tail more than 78 mm; dentine of 3rd lower molar with S pattern | Reithrodontomys fulvescens |
| 53B. Upper parts grayish or brownish, length of tail less than 78 mm; dentine of 3rd lower molar with C pattern | 54 |
| 54A. Dorsal surface of tail with a thin dark median line; greatest length of skull less than 20.3 mm | Reithrodontomys montanus |
| 54B. Dorsal surface of tail with a broad dark stripe; greatest length of skull more than 20.3 mm | Reithrodontomys megalotis |
| 55A. Hind foot usually longer than 23 mm; skull usually longer than 27 mm | Peromyscus attwateri |
| 55B. Hind foot usually shorter than 23 mm; skull usually shorter than 27 mm | 56 |
| 56A. Tail sharply bicolored, hind foot usually shorter than 21 mm; greatest length of skull usually less than 25 mm | Peromyscus maniculatus |
| 56B. Tail faintly bicolored, hind foot usually longer than 21 mm; greatest length of skull usually more than 25 mm | Peromyscus leucopus |
| 57A. Guard hairs prominent, grizzled pelage coarse to touch; all cheek-teeth nearly equal in size, square and with low-crowned cusps | Sigmodon hispidus |
| 57B. Guard hairs not prominent, gray to brownish pelage smooth; 1st cheek tooth nearly twice as large as third, rectangular and with dentine prisms surrounded by enamel | 58 |
| 58A. Color gray above without rufous tinge; length of incisive foramen more than 11 | Neotoma micropus |
| 58B. Color brownish above with rufous tinge (except juveniles); length of incisive foramen less than 11 | Neotoma floridana |
| 59A. Tail about same as hind foot; upper incisor grooved on anterior surface | Synaptomys cooperi |
| 59B. Tail distinctly longer than hind foot; upper incisors smooth | 60 |
| 60A. Dorsal pelage blackish to brownish-black; fourth triangle of upper second molar having prominent posterior "loop" that appears as a fifth triangle | Microtus pennsylavanicus |
| 60B. Dorsal pelage brownish-gray to reddish, fourth triangle of upper second molar lacking posterior "loop" | 61 |
| 61A. Dorsal pelage coarse, brownish or brownish-gray; skull not broad and flat, width generally less than 60 per cent of length | Microtus ochrogaster |
| 61B. Dorsal pelage smooth, reddish to chestnut; skull broad and flat, width generally greater than 60 per cent of length | Microtus pinetorum |
| 62A. Hind foot less than 22 mm; greatest length of skull less than 30 mm | Mus musculus |
| 62B. Hind foot more than 22 mm; greatest length of skull more than 30 mm | 63 |
| 63A. Tail longer than head and body; distinct outer notches on anterior row of cusps on first molar tooth | Rattus rattus |
| 63B. Tail shorter than head and body; no distinct notches on anterior row of cusps on first molar tooth | Rattus norvegicus |
| 64A. Claws blunt, not completely retractile and concealed; at least some cheek teeth with broad crushing surfaces; total teeth 34-42 | 65 |
| 64B. Claws sharp, completely retractile, and concealed in fur; cheek teeth without crushing surfaces; total teeth 28-30 | 82 |
| 65A. Body shape dog-like; forefeet and hind feet digitigrade; longitudinal septum in tympanic bulla; total teeth 42 | 66 |
| 65B. Body shape bear-like to weasel-like; at least hind feet plantigrade, no longitudinal septum in tympanic bulla; total teeth 34-42 | 71 |
| 66A. Total length more than 1050 mm; greatest length of skull more than 160, postorbital process thickened and convex dorsally | 67 |
| 66B. Total length less than 1050; greatest length of skull less than 160, postorbital process thin and concave dorsally | 69 |
| 67A. Diameter of nose pad less than 30; anteroposterior diameter of base of canine less than 11.5; length of first lower molar less than 25 | Canis latrans |
| 67B. Diameter of nose pad more than 30; anteroposterior diameter of canine more than 11.5; length of first lower molar more than 25 | 68 |
| 68A. Weight more than 31 kg in males and 24 kg in females; breadth of zygomatic arch more than 131 in males and more than 121 in females | Canis lupus |
| 68B. Weight less than 31 kg in males and 24 kg in females; breadth of zygomatic arch less than 131 in males and 121 in females | Canis rufus (See "Species Possibly in Kansas"). |
| 69A. Upper parts reddish, tail tip white; maxillary tooth row more than 55 | Vulpes vulpes |
| 69B. Upper parts not reddish, tail tip not white; maxillary tooth row less than 55 | 70 |
| 70A. Upper parts yellowish to buffy brown, tail tip black; temporal ridge not lyre-shaped | Vulpes velox |
| 70B. Upper parts grizzled grayish, tail with black dorsal mane and tip; temporal ridge lyre-shaped | Urocyon cinereoargenteus |
| 71A. Total length usually more than 1250 mm; skull length more than 20 mm; total teeth 42 (except when small premolars are lost) | 72 |
| 71B. Total length usually less than 1250 mm; skull length less than 170 mm; total teeth 40 or less | 73 |
| 72A. Longest claw on forefoot less than 55 mm; upper second molar broadest midway along its length; upper third molar less than 31 mm long | Ursus americanus |
| 72B. Longest claw on forefoot more than 55 mm; upper second molar broadest anteriorly; upper third molar at least 31 mm long | Ursus arctos |
| 73A. Conspicuous rings on tail; total teeth 40 | 74 |
| 73B. Tail lacking rings; total teeth 34-38 | 75 |
| 74A. Black rings on tail incomplete; palate does not extend markedly beyond posterior end of tooth row | Basariscus astutus |
| 74B. Black rings on tail complete-palate extends far beyond posterior end of tooth row | Procyon lotor |
| 75A. Upper parts brown, buffy-yellow or silver-gray, never black and white; palate extending posteriorly far behind last upper molars; total teeth 34, 36, or 38 | 76 |
| 75B. Upper parts black and white; palate extending posteriorly only to last upper molars or slightly beyond; total teeth 34 | 81 |
| 76A. Total length more than 840 mm; 4 pairs of upper and 3 pairs of lower premolars; total teeth 36 | Lontra canadensis |
| 76B. Total length less than 840 mm; total teeth 34 or 38 | 77 |
| 77A. Upper parts some shade of brown or buffy-yellow (or white in winter); basilar length of skull less than 80 mm; 4 pairs of upper and lower premolars; total teeth 38 | 78 |
| 77B. Upper parts silver-gray with single median stripe on head extending posteriorly toward shoulders; basilar length of skull more than 80 mm; 3 pairs of upper and lower premolars; total teeth 34 | Taxidea taxus |
| 78A. Tail lacks black tip (at most, only a few scattered black hairs); mastoid breadth less than 17.2 mm | Mustela nivalis |
| 78B. Tail with well-defined black tip, or else body entirely dark brown; mastoid breadth more than 17.2 mm | 79 |
| 79A. Total length less than 475 mm; pelage all white (northern Kansas, winter), or brown above, yellowish to whitish below; condylobasal length of skull less than 52 mm | Mustela frenata |
| 79B. Total length more than 475 mm; pelage color not as above; condylobasal length of skull more than 55 mm | 80 |
| 80A. Upper- and under parts yellowish-brown with prominent black mask on face; interorbital breadth more than 17 mm in males and more than 16 mm in females | Mustela nigripes |
| 80B. Upper- and under parts dark brown, no face mask; interorbital breadth less than 17 mm in males and 16 mm in females | Mustela vison |
| 81A. Pelage black with four or more rows of broken white stripes or spots; first lower molar less than 8 mm | Spilogale putorius |
| 81B. Pelage black, usually with two white stripes on back; first lower molar more than 8 mm | Mephitis mephitis |
| 82A. Tail more than twice the length of hind foot; 3 pairs of upper premolars | Felis concolor |
| 82B. Tail shorter than hind foot; 2 pairs of upper premolars | Lynx rufus |
| 83A. Rump patch lacking; permanent unbranched horns in both sexes, not shed annually; bony horn cores are round, project laterally from frontals; lacrimal bone articulating with nasal | Bison bison |
| 83B. Rump patch present; branched horns (Antilocapra) or antlers in males only (Antilocapra females may have small unbranched horns); shed annually (outer sheath only in Antilocapra); lacrimal bone not articulating with nasal | 84 |
| 84A. Lateral digits absent; horns consist of unbranched, flattened, dorsally projecting bony core covered with an annually shed sheath of fused hair; horn sheath with one branch (prong) in males, simple in females | Antilocapra americana |
| 84B. Paired lateral digits above hooves; branched antlers in males only (when growing covered with soft skin) | 85 |
| 85A. Tail and large rump patch straw-colored; belly and sides both dark brown; upper cheek tooth row more than 110 mm | Cervus elaphus |
| 85B. Tail white above and below with black tip and white rump patch, or tail brown above, white below and no rump patch; belly much lighter in color than sides; upper cheek tooth row less than 110 mm | 86 |
| 86A. Tail short, usually less than 200 mm, white with black tip; metatarsal gland about 25 mm long; dichotomously branching antlers | Odocoileus hemionus |
| 86B. Tail long, usually more than 200 mm, brown on upper side and white on underside; metatarsal gland 100-150 mm long; antlers with one main beam and smaller tines branching dorsally from it | Odocoileus virginianus |