Texas Tech University

ORDER RODENTIA

RODENTS

The name Rodentia is derived from the Latin verb rodere (to gnaw), an allusion to the gnawing habits of the group.  Among North American mammals, rodents are unique in that the incisors are reduced in number to one on each side above and below, the canines are absent, and the premolars are present with never more than two premolars in each jaw above and one below.  The dental formula varies from I 1/1, C 0/0, Pm 0/0, M 3/3 X 2 = 16 to I 1/1, C 0/0, Pm 2/1, M 3/3 X 2 = 22.  Most animals assigned to the order are small in size; some, for example the beaver, may exceed 25 kg in weight.  Rodents constitute more than one-third of the known kinds of mammals, and individually they are the most abundant mammal in many sections of the world.  Seventy-one species of rodents occur in Texas, making this the most diverse group of mammals in our state.

Members of this order are diverse, especially concerning natural history attributes.  For example, most species are nocturnal or crepuscular; ground squirrels and tree squirrels are strictly diurnal; and some are active both day and night.  In addition, many species have adapted specific characteristics (adaptive radiation) to exploit different habitats, environments, and food sources.  These characteristics have allowed rodents to display many lifestyles and traits and occupy every ecoregion in the state; for example, pocket gophers are fossorial, beavers and muskrats are aquatic, tree squirrels are arboreal, flying squirrels are volant, and many common rats and mice are terrestrial.  Most rodents feed on seeds and vegetation, but a few species, such as the grasshopper mouse, feed extensively upon other animals and insects.  Most rodents are active throughout the year, but others (i.e., ground squirrels) may hibernate for several months. 

Rodents are the most commonly encountered mammal in Texas.  Their rapid rate of reproduction can result in multiple litters per year (over six in some species) and litter sizes of more than eight individuals.  Consequently, rodents provide an important food source for many predators, including other mammals, birds, and reptiles.  In some years, rodent populations may "explode", causing local infestations and associated problems.  Rodent damage to agricultural crops, food supplies, and human structures (e.g., irrigation systems, power lines, and dwellings) is well documented.  Historically, rodent-borne diseases (i.e., bubonic plague) have caused significant human fatalities; by some estimates more humans have perished from the bubonic plague than have died from all combined wars.  In recent years several other rodent-borne diseases (e.g., hantaviruses, chronic waste syndrome, arenaviruses, and Lyme) have generated concern in the southwestern United States; therefore, caution should be displayed when handling or consuming rodents. 

Four species have been introduced and commonly occur statewide (Rattus norvegicus, Rattus rattus, Mus musculus, and Myocastor coypus).  Another species, the woodchuck (Marmota monax) is known from a single record and is not included in the key to rodents. 

 

KEY TO THE RODENTS OF TEXAS

1. Presence of external, fur-lined cheek pouches ................................. 2

Absence of external, fur-lined cheek pouches ..................................... 16

2. Front feet much larger than hind feet; ear (pinna) short and inconspicuous; tail about half the length of head and body (pocket gophers) ................ 3

Front feet much smaller than hind feet; ear (pinna) conspicuous; tail as long as (or longer than) head and body (pocket mice and kangaroo rats) .......... 5

3. Upper incisors not grooved on outer face; claws on front feet relatively small and slender.  Thomomys bottae (Botta's pocket gopher).

Upper incisors distinctly grooved on outer surface; claws on front feet large and long (longest ones about 15 mm) ..................................................... 4

4. Upper incisors with one deep groove; feet blackish.  Cratogeomys castanops (yellow-faced pocket gopher).

Upper incisor with two distinct grooves; feet whitish.  Species of the genus Geomys.

Nine species of the genus Geomys occur in Texas.  These are cryptic species, identifiable primarily on the basis of geographic distribution and characters of the karyotype and genes.  Only specialists working with prepared study specimens can identify them using morphological features. 

(1) Geomys arenarius (desert pocket gopher) occurs in El Paso County in far western Texas.

(2) Geomys attwateri (Attwater's pocket gopher) occurs in the south-central part of eastern Texas.

(3) Geomys breviceps (Baird's pocket gopher) occurs in eastern and north-central Texas.

(4) Geomys bursarius (plains pocket gopher) occurs in northwestern and north-central Texas.

(5) Geomys jugossicularis (Hall's pocket gopher) occurs in the northernmost counties of the Texas panhandle.

(6) Geomys knoxjonesi (Jones's pocket gopher) occurs on the southwestern plains of Texas.

(7) Geomys personatus (Texas pocket gopher) occurs along the southern coastal plain and inland areas of South Texas.

(8) Geomys streckeri (Strecker's pocket gopher) occurs in parts of two South Texas counties (Dimmit and Zavala) in the vicinity of Carrizo Springs and Crystal City.

(9) Geomys texensis (Llano pocket gopher) occurs in the Llano Basin region of the Hill Country in central Texas and in an isolated area on the northern border of the South Texas Plains.

5. Hind legs more than twice as long as front legs; tail long and bushy at end; head broad, >25 mm in width (kangaroo rats) ................................ 6

Hind legs less than twice as long as front legs; head about 15 mm in width (pocket mice) ............................................................................. 10

6. Large size, total length of adults >300 mm; tip of tail with conspicuous white banner ........................................................................................ 7

Smaller, total length of adults usually <250 mm; tip of tail usually dusky, not white .......................................................................................... 8

7. Hind foot (from tip of longest claw to heel) >50 mm in length; length of tail about 200 mm.  Dipodomys spectabilis (banner-tailed kangaroo rat).

Hind foot <50 mm; tail normally <200 mm.  Dipodomys elator (Texas kangaroo rat).

8. Hind foot with five toes (one toe is very small and difficult to detect) .. 9

Hind foot with only four toes.  Dipodomys merriami (Merriam's kangaroo rat).

9. Pelage long and silky, brownish; mastoid bullae greatly inflated, giving skull a triangular appearance; interparietal narrow and triangular in shape.  Dipodomys ordii (Ord's kangaroo rat).

Pelage short and coarse, with orangish cast; mastoid bullae less inflated; interparietal broad and rectangular to roundish in shape.  Dipodomys compactus (Gulf Coast kangaroo rat).

10. Size small, total length 100-130 mm; weight 6-8 g; pelage silky and soft ..................................... 11

Size larger, total length >150 mm; pelage harsh, often bristly, never silky ..................................... 12

11. Length of tail usually >60 mm; total length usually >120 mm; length of skull usually >21 mm; postauricular patch inconspicuous.  Perognathus flavescens (plains pocket mouse).

Length of tail usually <60 mm; total length usually <120 mm; length of skull usually <21 mm; postauricular patch conspicuous.  Silky pocket mice.

Two species of silky pocket mice occur in Texas, but only specialists working with prepared study specimens can identify them. 

(1) Perognathus flavus (silky pocket mouse) occurs in the Panhandle and Trans-Pecos portions of Texas.

(2) Perognathus merriami (Merriam's pocket mouse) occurs in the Great Plains, central, and southern regions of Texas.

12. Upper incisors plain, not grooved, on outer face; pelage spiny to touch.  Liomys irroratus (Mexican spiny pocket mouse).

Upper incisors distinctly grooved on outer face .................................. 13

13. Length of tail less than length of head and body (tail laid forward over back does not reach snout) and without terminal tuft; weight 30-47 g.  Chaetodipus hispidus (hispid pocket mouse).

Length of tail greater than length of head and body (tip of tail extends beyond snout when laid forward) and with terminal tuft ................................ 14

14. Rump with strongly developed and conspicuous black-tipped spines projecting beyond normal guard hairs; small white patch present at base of ear.  Chaetodipus nelsoni (Nelson's pocket mouse).

Rump spines absent or, if present, fewer and less conspicuous and not black-tipped; small white patches at base of ear absent ............................ 15

15. Rump spines absent; interparietal separated from mastoid bulla by thin projections of the parietal and supraoccipital.  Chaetodipus eremicus (Chihuahuan desert pocket mouse).

Weak rump spines present; interparietal in contact with mastoid bulla or nearly so.  Chaetodipus intermedius (rock pocket mouse).

16. Tail paddle shaped, naked, scaly; hind feet webbed; size large.  Castor canadensis (American beaver).

Tail not paddle shaped ................................................................. 17

17. Pelage with intermixed sharp quills; adults large, 5-11 kg.  Erethizon dorsatum (North American porcupine).

Pelage without quills ................................................................... 18

18. Lower jaw with four cheek teeth on each side ........................... 19

Lower jaw with three cheek teeth on each side ............................... 29

19. Hind feet fully webbed; adults weigh up to 12 kg; tail long, naked, and nearly circular in cross section.  Myocastor coypus (nutria).

Hind feet not fully webbed ........................................................... 20

20. Flying membrane between front leg and hind leg on each side; color wood brown above, white below.  Glaucomys volans (southern flying squirrel).

Legs normal, no flying membrane ................................................. 21

21. Upperparts striped or distinctly spotted or both ......................... 22

Upperparts not striped or distinctly spotted .................................... 26

22. Upperparts striped ................................................................. 23

Upperparts spotted ..................................................................... 25

23. One white stripe on each side; underside of tail grayish white (held over back while animal is running); upperparts grizzled grayish.  Ammospermophilus interpres (Texas antelope squirrel).

Three or more white or light stripes on upperparts .......................... 24

24. Six continuous, whitish stripes alternating with seven rows of whitish spots; ground color brown.  Ictidomys tridecemlineatus (thirteen-lined ground squirrel).

Four whitish stripes alternating with five dark-brown stripes; sides of face striped.  Tamias canipes (gray-footed chipmunk).

25. Spots in 10 or more distinct rows; tail narrowly bushy and about three times as long as hind foot.  Ictidomys parvidens (Rio Grande ground squirrel).

Spots scattered, never in distinct rows; tail about twice as long as hind foot.  Xerospermophilus spilosoma (spotted ground squirrel).

26. General color yellowish brown; tail very short (1.5 times length of hind foot) and black-tipped.  Cynomys ludovicianus (black-tailed prairie dog).

General color gray, brown, or blackish; tail long and bushy .............. 27

27. Belly reddish or rusty in color; upperparts grayish; hind foot >70 mm.  Sciurus niger (eastern fox squirrel).

Belly whitish or grayish, not reddish; hind foot <70 mm .................. 28

28. Belly white; upperparts gray, unspotted.  Sciurus carolinensis (eastern gray squirrel).

Belly grayish, back grayish with faint light spots, or shoulders and head black and rump grayish or brownish.  Otospermophilus variegatus (rock squirrel).

29. Tail flattened laterally; sparsely haired and scaly; hind toes fringed with stiff hairs; length of adults about 45 cm.  Ondatra zibethicus (common muskrat).

Tail round, sparingly haired or bushy ............................................ 30

30. Enamel pattern of molar teeth with transverse or oblique folds or triangles ......................................... 31

Enamel pattern of molar teeth with two or three rows of cusps (unworn condition) or roughly circular with slight lateral indentations (worn condition) ......................................... 40

31. Mouse size, total length usually <150 mm; tail <50 mm; ears nearly hidden in the fur ................................................................................... 32

Rat size, total length of adults >225 mm; tail >100 mm; ears conspicuous or partly hidden in the dense fur ....................................................... 34

32. Tail <25 mm in length; hind foot usually <18 mm; color glossy, reddish brown.  Microtus pinetorum (woodland vole).

Tail >25 mm in length; hind foot usually >18 mm; color brownish gray or blackish ..................................................................................... 33

33. Enamel pattern of third upper molar with no more than two closed triangles, often with no closed triangles, hence with three loops; never more than two inner reentrant angles.  Microtus ochragaster (prairie vole).

Enamel pattern of third upper molar with three closed triangles or, if with only two closed triangles, then with three inner reentrant angles.  Microtus mogollonensis (Mogollon vole).

34. Ear conspicuous; tail in adults usually >150 mm; eyes large, black, and bulging in life; fur rather soft; whiskers long, usually >50 mm (woodrats) ....................................... 35

Ears partly hidden in dense pelage; tail 100-125 mm long; pelage rather harsh' whiskers 25-35 mm long (cotton rats) .......................................... 38

35. First upper molar tooth with a relatively well-developed and deep antero-internal fold extending halfway across the crown; tail not sharply bicolored; dorsal color olivaceous, lateral sides buffy in color ......................... 36

Antero-internal reentrant angle of first upper molar poorly developed, shallow, and not across the crown; tail distinctly bicolored; dorsal color steel gray or buffy brown; lateral sides gray or brown in color ................................... 37

36. Total length of adults >375 mm.  Neotoma floridana (eastern woodrat).

Total length of adults <375 mm.  Neotoma mexicana (Mexican woodrat).

37. Upper parts typically steel gray; gular hairs dark at base.  Neotoma micropus (southern plains woodrat).

Upper parts not steel gray, washed with buffy hairs; gular hairs white at base.  Neotoma leucodon (white-toothed woodrat).

38. Underparts buffy to ochraceous; tail entirely black; top surface of feet buffy.  Sigmodon fulviventer (tawny-bellied cotton rat).

Upperparts whitish and not buffy or ochraceous; tail bicolor, dark above and light below; top surface of feet whitish .............................................. 39

39. Snout and eye rings yellowish or orangish and conspicuously different from color of back and sides; hind foot usually <30 mm; total length usually <260 mm.  Sigmodon ochrognathus (yellow-nosed cotton rat).

Snout and eye rings not conspicuous and same color as sides and back; hind foot usually >30 mm; total length usually >260 mm.  Sigmodon hispidus (hispid cotton rat).

40. Rat size, total length >230 mm ........................................... 41

Mouse size, total length usually <200 mm .................................. 43

41. Cusps on upper molars in two rows; hind foot narrow and slender.  Rice rats.

There are two species of rice rats in Texas that only a specialist can identify with certainty.

(1) Oryzomys texensis (Texas marsh rice rat), a grayish-brown form characteristic of marshy areas along the coast from Brownsville northward into deep East Texas.

(2) Oryzomys couesi (Coues's rice rat), a tawny form that occurs in marshy areas in extreme South Texas (Hidalgo and Cameron counties).

Cusps on upper molars in three rows (introduced rats) ................ 42

42. Tail slender and as long as or longer than head and body (tail reaches to or beyond nose when laid forward); color brownish or black; weight to 225 g.  Rattus rattus (black rat).

Tail chunkier and shorter than head and body; color brownish; weight to 450 g.  Rattus norvegicus (Norway rat).

43. Outer face of each upper incisor with deep groove (harvest mice) ..................................... 44

Outer face of upper incisors not grooved .................................. 47

44. Tail much longer than head and body (projects beyond nose when laid forward along back); last lower molar with dentine in the form of an S.  Reithrodontomys fulvescens (fulvous harvest mouse).

Tail shorter than or about as long as head and body; last lower molar with dentine in the form of a C ..................................................... 45

45. Color rich brown to blackish brown; a distinct labial shelf or ridge, often with distinct cusplets on first and third lower molars.  Reithrodontomys humulis (eastern harvest mouse).

Color mainly grayish brown or light buff; no distinct labial shelf or ridge on first and second molars .............................................................. 46

46. Tail shorter than head and body; dark dorsal strip on tail narrow, approximately one-fourth diameter of tail; breadth of braincase not exceeding 9.5 mm.  Reithrodontomys montanus (plains harvest mouse).

Tail length about equal to, or slightly longer than, head and body; dark dorsal stripe on tail broader and indistinct, approximately half diameter of tail; breadth of braincase of adults usually >9.5 mm.  Reithrodontomys megalotis (western harvest mouse).

47. Upper incisors with distinct notch at tip when viewed from the side; distinctly musky odor.  Mus musculus (house mouse).

Upper incisors lacking distinct notch at tip .............................. 48

48. Total length of adults <100 mm; tail short, 35 mm, about three times length of hind foot; color blackish and sooty.  Baiomys taylori (northern pygmy mouse).

Total length of adults >125 mm; color not blackish or sooty ....... 49

49. Tail <60% of head and body; coronoid process of mandible extends high above level of condyloid process; soles of feet furred (grasshopper mice) ..................................... 50

Tail >60% of head and body; coronoid process of mandible does not ascend above tip of condyloid process; soles of feet only slightly furred (deermice and relatives) ............................................................................ 51

50. Tail less than half length of head and body; crown length of maxillary tooth row >4 mm.  Onychomys leucogaster (northern grasshopper mouse).

Tail more than half length of head and body; crown length of maxillary tooth row <3.9 mm.  Onychomys arenicola (Chihuahuan or Mearns's grasshopper mouse).

51. General color golden yellow.  Ochrotomys nuttalli (golden mouse).

General color brown, buff, or gray (white-footed mice) ............... 52

52. Tail much shorter than head and body ................................ 53

Tail as long as or longer than head and body ............................ 55

53. Hind foot (of adults) >23 mm.  Peromyscus gossypinus (cotton deermouse).

Hind foot (of adults) <23 mm ................................................. 54

54. Tail with narrow and distinct dorsal stripe; total length of adults usually <170 mm; length of tail usually <75 mm; greatest length of skull usually <26 mm.  Peromyscus maniculatus (North American deermouse).

Tail with broad dorsal stripe and not sharply bicolored; total length of adults usually >170 mm; length of tail usually >75 mm; greatest length of skull usually >26 mm.  Peromyscus leucopus (white-footed deermouse).

55. Nasals decidedly exceeded by premaxillae; two principal outer angles of first and second upper molars simple, without (or at most with rudimentary) accessory cusps or enamel lophs; sole of hind foot naked to end of ankle; no pectoral mammae; inguinal mammae, 2-2.  Peromyscus eremicus (cactus deermouse).

Nasals slightly or not at all exceeded by premaxillae; two principal outer angles of first and second upper molars with well-developed accessory tubercles or enamel lophs; sole of hind foot hairy on proximal fourth to ankle; pectoral mammae, 1-1, inguinal mammae, 2-2 ................................... 56

56. Ear longer than hind foot; tail about as long as head and body (except in P. t. comanche, in which it is longer); bullae unusually inflated.  Peromyscus truei (piñon deermouse).

Ear equal to or shorter than hind foot; tail usually longer than head and body; bullae moderately or less inflated ......................................... 57

57. Hind foot length of adults >24 mm.  Peromyscus attwateri (Texas deermouse).

Hind foot length of adults <24 mm ...................................... 58

58. Tarsal joints of ankles white like upper side of hind foot; baculum with long cartilaginous spine at its terminal end.  Peromyscus laceianus (Lacey's white-ankled deermouse).

Dusky color of hind leg extending to end more or less over tarsal joint; baculum with a short cartilaginous spine at its terminal end ................ 59

59. Dorsal coloration grayish black and often like immature pelage; top of head and flanks of adults predominantly grayish; ears large for size; first two lower molars usually with one or more accessory lophids or stylids.  Peromyscus nasutus (northern rock deermouse).

Dorsal coloration with considerable yellow or buff; top of head same color as back; flanks of adults predominantly bright yellowish brown; first two lower molars usually without any accessory lophids or stylids.  Peromyscus boylii (brush deermouse).

 

Family Castoridae

BEAVERS

The Castoridae family contains only two species worldwide.  The North American species, Castor canadensis, is the largest rodent on the continent and is second in size only to the capybara of South America.  Beavers are easily recognized by their distinctive flattened tails.  They are highly adapted for an aquatic existence.  The fur of the beaver was so highly prized in the past that it played a major role in encouraging the exploration of western North America.

 

Family Cricetidae

NEW WORLD MICE, RATS, AND VOLES

The family Cricetidae includes mice, rats, hamsters, voles, lemmings, and gerbils.  Following the Muridae, this is the largest family of mammals, with 130 genera and 681 species worldwide.  Ten genera and 30 species occur in Texas.  All cricetids are relatively small mammals with short life spans and high reproductive rates.  All cricetids have the same dental formula: I 1/1, C 0/0, Pm 0/0, M 3/3 X 2 = 16.  They occupy all types of terrestrial habitats, and a few are arboreal or aquatic. 

 

Family Erethizontidae

NEW WORLD PORCUPINES

This family contains four genera and 12 species, but only 1 species reaches northward into the United States.  Porcupines are heavyset, large rodents characterized by their outer covering of sharp spines, or quills.  Porcupines are at home on the ground as well as in trees.

 

Family Geomyidae

POCKET GOPHERS

Pocket gophers are named for their fur-lined cheek pouches, or pockets, that they use for food storage and transport.  These strictly fossorial rodents dig extensive underground burrow systems and are rarely seen aboveground. 

 

Family Heteromyidae

POCKET MICE AND KANGAROO RATS

Heteromyids have external, fur-lined cheek pouches for transporting food.  Their diet is predominantly seeds and vegetation but occasionally includes insects and other invertebrates.  Many species are extremely well adapted to desert environments and capable of existing with little or no free water intake.  All heteromyids are nocturnal.  Kangaroo rats are strongly bipedal and have enlarged hindquarters.  Texas is home to four genera and 13 species of heteromyid rodents.

 

Family Sciuridae

SQUIRRELS AND ALLIES

This family includes chipmunks, marmots, ground squirrels, prairie dogs, flying squirrels, and tree squirrels.  Most squirrels are diurnal and are among the commonly encountered rodents.  Most species feed on nuts, fruits, barks, buds, and leaves, although they opportunistically feed on insects and small vertebrates (lizards, birds, and mammals).  Texas is home to eight genera and 10 species of squirrels.  The ground squirrels recently underwent a major taxonomic revision; herein we follow the nomenclature of Helgen and colleagues and no longer apply Spermohpilus to Texas ground squirrels. 

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From The Mammals of Texas, Seventh Edition by David J. Schmidly and Robert D. Bradley, copyright © 1994, 2004, 2016.  Courtesy of the University of Texas Press.

Natural Science Research Laboratory