Subspeciation in the Kangaroo Rat, Dipodomys ordii

Chapter I: Part 1

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Subspeciation in the Kangaroo Rat,
Dipodomys ordii

BY
HENRY W. SETZER

University of Kansas Publications
Museum of Natural History

Volume 1, No. 23, pp. 473-573, 27 figures in text, 7 tables
December 27, 1949

University of Kansas
LAWRENCE
1949

UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY

Editors: E. Raymond Hall, Chairman, A. Byron Leonard,
Edward H. Taylor, Robert W. Wilson

Volume 1, No. 23, pp. 473-573, 27 figures in text, 7 tables

December 27, 1949

UNIVERSITY OF KANSAS
Lawrence, Kansas

PRINTED BY
FERD VOILAND, JR., STATE PRINTER
TOPEKA, KANSAS
1949

22-6114

Subspeciation in the Kangaroo Rat,
Dipodomys ordii

HENRY W. SETZER

CONTENTS

PAGE

Introduction 477

Methods and Acknowledgments 478

Paleontology 480

Phylogeny of the Species of the Genus 484

Dispersal of the Several Species 498

Subspeciation 499

Accounts of Subspecies 511
_Dipodomys ordii_ 511
_Dipodomys ordii richardsoni_ 511
_Dipodomys ordii oklahomae_ 514
_Dipodomys ordii compactus_ 515
_Dipodomys ordii sennetti_ 517
_Dipodomys ordii evexus_ 518
_Dipodomys ordii medius_ 519
_Dipodomys ordii obscurus_ 521
_Dipodomys ordii terrosus_ 523
_Dipodomys ordii fremonti_ 524
_Dipodomys ordii uintensis_ 525
_Dipodomys ordii sanrafaeli_ 526
_Dipodomys ordii panguitchensis_ 527
_Dipodomys ordii monoensis_ 528
_Dipodomys ordii ordii_ 530
_Dipodomys ordii luteolus_ 533
_Dipodomys ordii extractus_ 534
_Dipodomys ordii chapmani_ 536
_Dipodomys ordii montanus_ 538
_Dipodomys ordii cinderensis_ 540
_Dipodomys ordii fetosus_ 541
_Dipodomys ordii utahensis_ 543
_Dipodomys ordii columbianus_ 544
_Dipodomys ordii idoneus_ 546
_Dipodomys ordii priscus_ 547
_Dipodomys ordii celeripes_ 549
_Dipodomys ordii cineraceus_ 550
_Dipodomys ordii marshalli_ 551
_Dipodomys ordii inaquosus_ 552
_Dipodomys ordii attenuatus_ 553
_Dipodomys ordii fuscus_ 555
_Dipodomys ordii longipes_ 556
_Dipodomys ordii pallidus_ 558
_Dipodomys ordii nexilis_ 559
_Dipodomys ordii cupidineus_ 561
_Dipodomys ordii palmeri_ 562

Conclusions 563

Tables of Measurements 565

Literature Cited 571

INTRODUCTION

The geographic range of the kangaroo rats, genus _Dipodomys_, extends from southern Canada south to the southern limits of the Mexican Tableland and from the Pacific Coast east to the eastern limits of the Great Plains in Kansas, Oklahoma and Nebraska. These animals are usually restricted to sandy soils in semiarid regions. The species _Dipodomys ordii_, with which this account is primarily concerned, is, to the best of my knowledge, almost exclusively confined to sandy areas.

Since 1841, when Gray gave the generic name _Dipodomys_ to the kangaroo rats, basing the name on the four-toed species _Dipodomys phillipsi_, several other generic names have been applied. Fitzinger, in 1867, used the name _Perodipus_ for those animals with five toes on the hind foot, designating _Dipodomys agilis_ as the type of his genus. In 1890, Merriam proposed the generic name _Dipodops_ with _Dipodomys agilis_ as the type, apparently being unaware of Fitzinger's name, _Perodipus_. Trouessart, in 1897, through what was an apparent _lapsus calami_, applied the generic name _Cricetodipus_ Peale to all of the species of the then known genera _Perodipus_ and _Dipodomys_, but Trouessart later, 1904 or 1905, in his Supplementum, corrected this _lapsus_ and used the names _Dipodomys_ and _Perodipus_. Grinnell (1919:203) showed that some of the four-toed _Dipodomys_ had five toes on one hind foot and four on the other and that _Perodipus_ must fall as a synonym of the earlier generic name _Dipodomys_ which was to be applied to all of the kangaroo rats.

_Dipodomys ordii_ was named by Woodhouse in 1853, from specimens from El Paso, Texas, but between that time and 1919 the name _ordii_ was used in combination with all of the generic names mentioned above (see synonymies under the accounts of the subspecies).

The nearest approach to a revision of the genus was Grinnell's (1922) "A Geographical Study of the Kangaroo Rats of California." Since that time, Hall and Dale (1939) revised the _D. microps_ group and Durrant and Setzer (1945) reported upon the kangaroo rats of Utah. The present paper is a review of the species _Dipodomys ordii_. Some of the objectives in this review have been to learn: (1) What kinds of kangaroo rats are subspecies of the species _Dipodomys ordii_; (2) the limits of geographic range of this full species; (3) the extremes of color, and of size and shape of the skull in this one species; (4) the significance of different sizes, shapes and colors; (5) the reasons for the existence, or formation, of selected subspecies; and (6) the relationships of _Dipodomys ordii_ to other species in the genus.

METHODS AND ACKNOWLEDGMENTS

Available specimens were arranged according to geographic origin. These were segregated as to sex and then under each sex by age. Individual variation was next measured in each of several samples in which individuals were of like geographic origin, sex, age and season. Finally, comparable materials were arranged geographically for detection of variations of systematic worth. Following preliminary studies of material thus arranged, additional specimens were collected from critical areas.

When fully adult animals (see next paragraph) were segregated as to sex, and then measured, the degree of secondary sexual variation was found to be less than the degree of individual variation; therefore in the tables of indices, no distinction as to sex has been made.

The only external measurements of the animals used were those recorded by the collectors on the labels attached to the skins. These measurements were total length, length of tail and length of hind foot. Measurements of the ear have not been used since they were not in all instances recorded by collectors and since measurements of dry ears proved to be unsatisfactory. Only measurements of fully adult specimens have been used. The term fully adult is applied only to those specimens in which the auditory bulla is shiny and translucent, the permanent P4 is fully erupted and worn, and the tail is fully striped and penicillate. No one of these characters alone was accepted as proof of adulthood but only the three in combination.

The following measurements of the skull have been used in the tables:

_Greatest length._--From the most anterior tip of the nasals to
the most posterior projection of the auditory bullae.

_Greatest breadth across bullae._--From the most lateral
projection of the auditory bulla on one side to the corresponding
position on the other bulla.

_Breadth across maxillary arches._--Greatest breadth across arches
in a plane perpendicular to the long axis of the skull.

_Width of rostrum._--Width of the premaxillae and the nasals taken
immediately anterior to the upper incisors (not greatest width of
nasals which is attained farther anteriorly).

_Length of nasals._--Maximum length of a nasal bone.

_Least interorbital breadth._--Least width between the orbits
immediately posterior to the lacrimal processes.

_Basilar length._--From the anterior margin of the foramen magnum
to the posterior border of the alveolus of one of the upper
incisors.

Capitalized color terms are from Ridgway, "Color Standards and Color Nomenclature," Washington, D. C., 1912. Color determinations were made by comparing a masked area of pure color on the side of the animal with a masked rectangle of named color on Ridgway's plates in natural light always from the same angle.

Abbreviations used for specimens examined from the various collections are as follows:

AMNH--American Museum of Natural History.
BYU--Brigham Young University.
CNHM--Chicago Natural History Museum.
CM--Carnegie Museum.
CMNH--Colorado Museum of Natural History.
DJC--Dixie Junior College.
DRD--Donald R. Dickey Collection.
KU--Museum of Natural History, University of Kansas.
LACM--Los Angeles County Museum.
MHS--Collection of Myron H. Swenk.
MVZ--Museum of Vertebrate Zoology, University of California.
OU--Museum of Zoology, University of Oklahoma.
RH--Collection of Ross Hardy.
UM--Museum of Zoology, University of Michigan.
UN--Museum of Natural History, University of Nebraska.
USAC--Utah State Agricultural College.
USBS--United States Biological Surveys Collection.
USNM--United States National Museum.
UU--Museum of Zoology, University of Utah.
TCWC--Texas Coöperative Wildlife Collection.

This study is based on 3,732 specimens which were assembled at the Museum of Natural History, University of Kansas, or studied at other institutions. For the loan of this material and for the opportunity afforded for its study, I am extremely grateful to the authorities of each of these institutions and to the owners of the private collections.

Acknowledgement is made to the Office of Research and Inventions of the United States Navy for assistance with the field work which permitted the acquisition of essential specimens from several of the critical geographic areas while the author was research assistant on a larger over-all project (N6 ori-164-T02) of which the determination of the geographic range of this rodent species, a potential host of tularemia, was one facet. Tularemia was not detected in this genus.

I extend my thanks also to Professor Stephen D. Durrant, of the University of Utah, for helpful corrections in the preparation of the manuscript; to Mrs. Virginia Cassell Unruh for the preparation of the drawings; to Professor E. Raymond Hall, of the University of Kansas, for guidance in the study and critical assistance with the manuscript; to Professors H. H. Lane and Worthie H. Horr for valued suggestions; to Mr. J. R. Alcorn for providing specimens for dissection when he was working under the University of Kansas endowment fund; and to the other friends and associates who have given of their time and criticism.

PALEONTOLOGY

The family Heteromyidae was defined by Wood (1935:81) essentially as follows: Cheek teeth brachydont to hypsodont and even rootless; usually six cusps per molar, three on each loph; enamel rarely divided into two plates, never reduced to one; skull light, thin and papery; mastoids inflated, mastoidal breadth often greatest, never appreciably less than zygomatic breadth; interorbital space wider than rostrum; palate nearly horizontal and little if any below level of zygomata; nasals extended beyond incisors; zygomata slender, with greatly reduced malar, almost, or quite, abutting against tympanic; frontals and parietals broad, with latter reaching, or nearly reaching, orbits; frontal trapezoidal; parietal quadrate, to pentagonal or triangular; interparietal primitively large, secondarily reduced; squamosal mostly or entirely confined to orbit; tympanic vesicular and inflated, in some forms highly inflated; mastoids inflated and bullous, reaching top of skull, and forming part of occipital surface; occipitals contracted and limited in area on occiput, but extending onto dorsum of skull; coronoid processes small, inclined caudad and lying below level of condyle; jaw small and weak with large, everted angle; tail as long as, or longer than, head and body; claws of manus elongate, fossorial, but forelimb slender; pelage usually coarse and frequently spinose; ears and eyes large; body murine in form; locomotion in many forms saltatorial.

This characterization of the family includes all of the members of the subfamilies Perognathinae, Heteromyinae and Dipodomyinae as well as the genus _Microdipodops_ which I am disinclined to place with any of the three subfamilies. Apparently it is more closely related to the subfamily Perognathinae.

The subfamily Dipodomyinae, which contains the genera _Dipodomys_, _Prodipodomys_ and _Cupidinimus_, might be characterized after Coues' (1875) original description of the subfamily as follows: Cheek teeth progressively hypsodont, in _Dipodomys_ becoming ever-growing; enamel progressively interrupted, eventually reduced to anterior and posterior plates; upper and lower third molars reduced in size; tooth pattern rapidly destroyed, leaving only an enamel oval; upper incisors smooth (some fossils) or grooved (living forms); progressive expansion of the auditory bullae and increase in saltatorial ability; pterygoid fossa double; calcaneal-navicular or even calcaneal-cuneiform articulation; tail tufted.

Owing to the fact that so little paleontological material is known and because even that is fragile and not easily accessible for study, knowledge of the fossil representatives has been drawn primarily from the literature, especially from Wood's (1935) account.

Heteromyids are known from the Chadron formation, of early Oligocene age, in which a single tooth was found. In the Orellan stage of the mid-Oligocene where the genus _Heliscomys_ occurs, it is notably generalized, in comparison with other members of the family, but it may not be ancestral at all. The lower premolar is tricuspidate and the first and second molars are quadritubercular with a broad cingulum. The teeth are bunodont and brachydont, with the cusps not uniting to form lophs. Wood (1935:78) shows _Mookomys formicorum_ (from the Arikeean) as the next heteromyid in the evolutionary sequence and postulates that this species arose from _Heliscomys gregoryi_. _Mookomys_ is judged by Wood to be the common ancestral form of the perognathines and the dipodomyines.

_Cupidinimus_, the genus next in line, is characterized by smooth upper incisors; lower molars with incipient H-pattern; cheek teeth progressively hypsodont and lophate (but always rooted); and calcaneal-navicular articulation.

The time range of this genus is from the late Miocene (Niobrara River, Local Fauna) of Nebraska to the medial Pliocene, Thousand Creek (Hemphillian) of Nevada.

Hibbard (1937:462) described _Dipodomys kansensis_ from the Ogallala formation (Hemphillian age) of Kansas. He redescribed his species, and made it the type of the new genus _Prodipodomys_ (Hibbard, 1939:458), differentiating it from _Dipodomys_ on the basis of the three-rooted p4, double-rooted m1 and m2 and the single rooted m3. It is shown to be closely allied to _Dipodomys_ by the form and position of a large foramen posterior and labial to m3, and by the development of the masseteric ridge.

The next youngest heteromyid fossils which have been described are of the genus _Prodipodomys_? from Arizona. Gidley (1922:123) described _Dipodomys minor_ from the Benson (Blancan) which Gazin (1942:486) refers to the genus _Prodipodomys_?. Wood (1935:156) described _Dipodomys gidleyi_ from the Curtis (Pleistocene). Both of these species are primitive as regards dentition; that is to say, the enamel ring of the tooth is complete and lacks any sign of a break. The limb bones of _D. gidleyi_ show lesser saltatorial ability, and therefore appear to be more primitive, than those of any living _Dipodomys_.

Several heteromyids which have not been assigned to any genus are known. Wilson (1939:36-37) recognized some from the Avawatz (Clarendonian) and the Ricardo (Clarendonian). Another, possibly of the genus _Diprionomys_?, from the Barstow (Barstovian) was described by Wood (1935:197) as follows: "The general shape of the tooth as figured strongly suggests either one of the most advanced species of _Dipodomys_ or else a Geomyid.... It is much more advanced than are any known contemporary heteromyids, and compares fairly well with such late Tertiary and Pleistocene geomyids as have been described. It certainly is not referable to any known heteromyid genus other than _Dipodomys_, and should probably be called a Geomyid." Wilson (_loc. cit._) refers to these specimens as Dipodomyine (?) n. gen. and sp. If these specimens referred to by Wood and Wilson are true heteromyids then a change in the phylogenetic scheme proposed by Wood (1935) would be necessary. Wilson (_loc. cit._) says, referring to the Avawatz specimen, "The cheek teeth are very hypsodont but are apparently not persistent in growth,... Wide enamel breaks are present in M/1 dividing the enamel into anterior and posterior bands. The enamel of P/4 is complete in the present stage of wear, but an examination of the tooth indicates that breaks would develop with additional attrition at the buccal and lingual margins of the metalophid, and at the buccal border of the protolophid. The incisor is of the slender heteromyid type."

Wood (1935:118) in referring to the ancestry of _Cupidinimus_ with regard to the grooving of the incisors says: "The philosophy of evolution which would prohibit its derivation from _Mookomys_, because of the grooved incisors in the latter genus, would require a separate line leading back at least to the Lower Miocene."

In view of the above statements, it is conceivable that additional material will be found carrying the dipodomyine line back into the early Miocene. Perhaps the line involving _Mookomys_ and _Cupidinimus_ which was regarded by Wood as the line of descent, is merely an aberrant side branch that parallels in its structures the main line of evolution of the dipodomyines (Figure 1).

As Wilson (1939:37) says: "Indeed it is hard to recognize such a form as _Cupidinimus nebraskensis_ as directly ancestral to _Dipodomys_ in view of the occurrence of the much more advanced Avawatz specimen in deposits that are at most only slightly later than those in which the former is found. The kangaroo rats were apparently much farther along in their development by lower Pliocene time than heretofore supposed."

Wood (1935:78) suggested that _Dipodomys gidleyi_ gave rise to _Dipodomys spectabilis_ and _Dipodomys ordii_, and _Dipodomys minor_ gave rise to _Dipodomys compactus_. However, my own study indicates that _Dipodomys compactus_ is conspecific with _Dipodomys ordii_ and should stand as _Dipodomys ordii compactus_. Consequently a different phyletic arrangement than that proposed by Wood (_loc._ _cit._) is required. Since _D. compactus_ is more closely allied to _Prodipodomys? minor_ than _D. ordii_ is to _D. gidleyi_, it is possible that _P.? minor_ gave rise to _D. ordii_ and that _D. spectabilis_ is the end product of the phyletic trend of _D. gidleyi_ (Figure 1).

The trend of phyletic development in the dipodomyines has been toward the saltatorial habit. To acquire this habit from a scampering ancestor, certain morphological modifications were necessary. Among these modifications were a lengthening of the tail, a lengthening of the hind legs, the development of a calcaneal-navicular-ectocuneiform contact instead of a calcaneal-navicular contact for additional strength in leaping, a shortening of the forelimb, an increase in size and inflation of the mastoid and tympanic portions of the skull with a consequent reduction in size of the interparietal region and the fusion of certain of the cervical vertebrae. Late Miocene (_Cupidinimus_) and Pliocene (Avawatz specimen and _Prodipodomys_) forms had acquired certain of these morphological modifications that are present in the modern genus _Dipodomys_.

PHYLOGENY OF THE SPECIES OF THE GENUS

Representatives of nine species of _Dipodomys_ were dissected in an attempt to determine the degree of specialization and the relative systematic position of each species.

The myology was found to agree in detail as to origin, insertion and innervation with that of _Dipodomys spectabilis_ as reported by Howell (1932). The only variation noted in the muscular system was the size of the individual muscles in those animals of widely divergent body size.

_Dipodomys ordii_ is the most generalized and _Dipodomys deserti_ is the most specialized of the kangaroo rats (see Table 1), as judged by the osteology. Information gained by the study of the viscera of the various species supports this judgment. The visceral mass is relatively loose in _D. ordii_, but is markedly compact in _D. deserti_. This compactness appears to be brought about by the foreshortening of the mesenteries which support the entire gut and by the closer apposition of the large intestine to the caecum; both the intestine and caecum occupy a ventral position in the abdominal cavity. In _Dipodomys ordii_ the entire visceral mass is loosely interconnected and the caecum is relatively small as compared to the tightly compact viscera and the large caecum in _Dipodomys deserti_. Another striking feature is the size, proportion and position of the liver. In all animals dissected, even in _D. deserti_, the right lobe of the liver descends and forms a capsule around the anterior end of the right kidney. In the Ord kangaroo rat, the bulk of the liver lies on the right side of the body cavity. That is to say, there is a greater bulk of the liver on the right side and it is situated more dorsad than in any of the other species examined. In the most specialized condition, as in _Dipodomys deserti_, the bulk of the liver is almost equal on the right and left sides, and instead of having the greater bulk situated dorsally as in _D. ordii_ it is cup-shaped, with the dorsal and ventral parts of approximately equal size and situated on almost the same transverse plane. The entire mass of the liver is concave posteriorly.

TABLE 1

SKELETAL INDICES OF DIPODOMYS

microps 138.5 56.17 132.3 57.27 90.95 60.8

panamintinus 146.1 55.3 132.0 57.5 90.5 60.8

agilis 147.0 55.05 133.65 57.25 94.55 62.65

heermanni 142.9 54.2 135.9 55.35 92.2 60.93

ingens 142.9 54.1 130.6 56.2 89.65 66.2

spectabilis 140.9 53.05 133.9 54.2 95.6 64.6

phillipsii 163.4 55.05 137.85 58.97 101.5 64.5

merriami 160.75 53.85 137.5 57.35 99.75 63.9

nitratoides 155.0 54.1 137.4 57.0 98.25 65.5

deserti 149.5 53.4 139.4 54.9 96.6 67.6
------------------------------------------------------

The right kidney is variable in position in reference to the left. In all species the right kidney lies anterior to the left but in some, _D. deserti_ and _D. ingens_, it is markedly anterior.

In _Dipodomys agilis_, _D. merriami_ and _D. deserti_ there are small to large patches of lymphoid tissue on the caecum. These patches were not noted in any of the other species examined and I do not know their function. In the three above mentioned species, however, the large intestine is shorter in proportion to the small intestine than in any other species except _D. heermanni_ (see Table 2) and with the exception of _D. heermanni_, _D. venustus_ and _D. ordii_ the actual measurements are less.

Inasmuch as little is known of the food habits of the various species of kangaroo rats, any ascription of adaptive significance to the varying proportions of the digestive system would be only speculative.

Midgley (1938) describes the visceral anatomy of _D. ordii_ and _D. microps_. Except for the differences here noted the description of the viscera as given by Midgley (_loc. cit._) applies to the rest of the species studied.

TABLE 2

VISCERAL MEASUREMENTS (in Millimeters) of DIPODOMYS
=================================================================
Column Headings:

H: heermanni
M: merriami
A: agilis
D: deserti
O: ordii
S: spectabilis
V: venustus
P: panamintinus
I: ingens

H M A D O S V P I
-----------------------------------------------------------------
Large intestine 432 290 464 397 237 413 374 419 430

Small intestine 165 126 220 195 131 228 207 255 274

Percent of small to
large intestine 38.2 43.4 47.5 49.2 55.2 55.3 55.4 60.9 63.7
-----------------------------------------------------------------

From the differences noted in the skeleton, in the entire visceral mass, and in the shape and position of the liver it appears that as a saltator becomes more specialized skeletally, there is a concurrent compacting and aligning of the viscera into a more or less bilaterally balanced mass. It seems that this alignment is for a stabilization in leaping. It seems reasonable that the individual that has a loose and unconsolidated visceral mass, or in which the viscera or at least the heaviest part of the viscera is relatively unilateral, would be thrown slightly off balance at the end of the jump. This would place the animal at a slight disadvantage before being able to make the next jump. Howell (1944:40) comments on the fact that kangaroo rats often land off balance, "owing apparently to clumsy use of the tail." Possibly the unilaterality of the visceral mass plus a shorter tail and a more clumsy use of that organ accounts for the off balance landings which Howell has observed.

The skeleton, particularly of the appendages, shows the most modification, ranging from a relatively generalized to a specialized condition. Skeletal indices, as established by Howell (1944:199) have been used in estimating the amount of such specialization.

These indices are obtained by dividing the length of one segment of a limb by the length of another segment and are expressed in percentages. The Femorotarsalmetatarsal and Cranial indices are not from Howell (_loc. cit._).

The Humeroradial index (radius/humerus ×100) in the generalized animal is theoretically 100 because the humerus and radius are of the same length. In kangaroo rats, which are saltators, the index rises to more than 100 owing to the lengthening of the radial component.

The Intermembral index (humerus and radius/femur and tibia ×100) in a generalized animal is theoretically 100, but, as Howell (1944:205) points out, the index in generalized mammals is probably nearer 75. If the hind leg elongates at the expense of the forelimb the animal will be a better saltator and the skeletal elements will yield a lower intermembral index.

The Femorotibial or Crural index (tibia/femur ×100) expresses the development of the tibia as an adaptation to the saltatorial habit and in generalized animals would be expected to be 100. As an adaptation to saltation the tibia would elongate at the expense of the femur and the index would be more than 100. The degree of divergence from 100 would be an expression of the degree of saltatorial ability.

The Tibioradial index (radius/tibia ×100) in the generalized animal also would be expected to approximate 100 but it is doubtful if any living mammals, except brachiating kinds, yield an index of more than 75. In saltators, the index is low because of the elongation of the hind appendages, whereas the forelimbs do not change their length or are shortened.

The Femorotarsalmetatarsal index (tarsometatarus/femur ×100) in the generalized condition would be less than 50 and an index approaching 100 would indicate a specialization for saltation owing to the elongation of the tarsometatarsal elements.

The Cranial index (breadth across bullae/length of skull ×100) reflects the development of the auditory or mastoid region of the skull as an adaptation for more acute hearing and possibly for more delicate balance. In heteromyids, the generalized condition would be represented by an index of 50 or less, and as the width across the bullae increases, the index rises toward 100.

FIG. 2. _Dipodomys ordii inaquosus_, [M], adult, no. 23365, KU; 7
mi. W Fallon, Churchill County, Nevada; trapped 27 October, 1945.

FIG. 3. _Dipodomys panamintinus mohavensis_, [M], adult, no.
22094, KU; 1-1/2 mi. N Mojave, Kern County, California; 3
February, 1948.

FIG. 4. _Dipodomys heermanni morroensis_, [M], adult, no. 22082,
KU; S side Morro Bay, 4 mi. S Morro, San Luis Obispo County,
California; 25 January, 1948.

FIG. 5. _Dipodomys ingens_, [F], adult, no. 22069, KU; 25 mi. SW
Mendota, San Benito County, California; 2 February, 1948.

FIG. 6. _Dipodomys agilis perplexus_, [F], adult, no. 22091, KU;
1-3/10 mi. N Monolith, Kern County, California; 3 February, 1948.

FIG. 7. _Dipodomys venustus sanctiluciae_, [M], adult, no. 22071,
KU; 1-1/2 mi. S Jolon, Monterey County, California; 26 January,
1948.

FIG. 8. _Dipodomys spectabilis spectabilis_, [F], adult, no.
22110, KU; 5 mi. NE Willcox, Cochise County, Arizona; 19 January,
1948.

FIG. 9. _Dipodomys merriami merriami_, [F], adult, no. 23366, KU;
E side Carson Lake, Churchill County, Nevada; 2 October, 1945.

FIG. 10. _Dipodomys deserti deserti_, [M], adult, no. 23364, KU;
15 mi. WSW Fallon, Churchill County, Nevada; 3 November, 1945.]

TABLE 3

RELATIVE SPECIALIZATIONS OF THE SPECIES FOR EACH INDEX

Column headings:

A: Humeroradial
B: Intermembral
C: Crural
D: Tibioradial
E: Femorotarsalmetatarsal
F: Cranial
G: Average

==============================================
A B C D E F G
----------------------------------------------
ordii 5 1 1 1 1 5 2.33
microps 1 2 4 5 4 2 3.0
panamintinus 6 3 3 3 3 1 3.1
agilis 7 4 5 6 6 4 5.3
heermanni 3 6 7 9 5 3 5.5
ingens 4 7 2 8 2 10 5.5
spectabilis 2 11 6 11 7 8 7.5
phillipsii 11 5 10 2 11 7 7.6
merriami 10 9 9 4 10 6 8.0
nitratoides 9 8 8 7 9 9 8.5
deserti 8 10 11 10 8 11 9.6
----------------------------------------------

The figure 1 represents the least specialized condition for the
index, while the figure 11 represents the most specialized
condition. The remainder of the numbers indicate the relative
degree of specialization of each species for each index.

The species that have been examined are listed in Table 3 in increasing order of specialization from top to bottom.

Usually animals of extreme morphological specialization are much restricted environmentally. Attempts to correlate the relative evolutionary position of the various species, as indicated by the degree of specialization interpreted from the indices with that of habitus has proven unsuccessful. For example, _Dipodomys merriami_, which is third from the top in the list as arranged above, is neither restricted to loose sandy soil as is _D. deserti_ nor to brush as are _D. agilis_ and _D. venustus_. _D. merriami_ does, nevertheless, inhabit a variety of habitats from loose sandy soils to rather hard rocky ground. Throughout the genus there is, however, a general trend toward increased specialization as the animals adopt the more open desert environment, as is indicated by the elongation of the tail and hind appendage and increase in size of the auditory region of the skull. A marked difference is noted in the size of the pinna of the ear in the various species. Generally, those species having small pinnae inhabit open desert country while those with large pinnae inhabit brushy country. This is in direct contradistinction to the hares and rabbits in which the small-eared kinds are brush dwellers whereas the large-eared kinds are inhabitants of open country. Three possible explanations for hares and rabbits having this specialization of the pinnae are: (1) To enable the open-dwelling animals with the larger pinnae to hear more readily the approach of an enemy when it is yet far away, while the brush-living forms, which rely for escape on a short dash into cover, do not need so large a "funnel"; (2) large pinnae have been developed by those animals which live in the open desert as an aid in dissipating the body heat; (3) large pinnae in brush-dwelling animals would be a decided disadvantage in rapid movement through the brush. Grinnell (1922:20) points out that animals with large pinnae usually have small auditory bullae and conversely, animals with small pinnae have large bullae. This compensatory factor, implying an auditory function, appears to be inoperative in _D. panamintinus mohavensis_ which has small ears and small bullae and in _D. elephantinus_ which has large ears and large auditory bullae. Grinnell (_loc. cit._) suggests that several additional factors enter into the problem, such as the amount of digging each species must do to gain safety, the texture of the soil for burrowing, the extent of forage area and the type of cover in connection with the mode of attack of predators. Of these factors, perhaps the most important are the two first mentioned.

FIG. 11. _Dipodomys ordii compactus_, [M], adult, no. 646, TCWC;
19 mi. S Port Aransas, Mustang Island, Nueces County, Texas;
24 April, 1939.

FIG. 12. _Dipodomys ordii oklahomae_, [F], adult, no. 265456,
USBS; 2-1/4 mi. S Norman, Cleveland County, Oklahoma; 21 March,
1934.

FIG. 13. _Dipodomys ordii richardsoni_, [F], adult, no. 15995, KU;
1 mi. S Lamar, Prowers County, Colorado; 8 September, 1945.

FIG. 14. _Dipodomys ordii nexilis_, [F], adult, no. 149941, USBS;
5 mi. W. Naturita, Montrose County, Colorado; 20 July, 1907.

FIG. 15. _Dipodomys deserti deserti_, [F], adult, no. 18670, KU;
14 mi. WSW Fallon, Churchill County, Nevada; 3 November, 1945.]

FIG. 16. _Dipodomys ordii compactus_, for data see Fig. 11.

FIG. 17. _Dipodomys ordii oklahomae_, for data see Fig. 12.

FIG. 18. _Dipodomys ordii richardsoni_, for data see Fig. 13.

FIG. 19. _Dipodomys ordii nexilis_, for data see Fig. 14.

FIG. 20. _Dipodomys deserti deserti_, for data see Fig. 15.]

Wood (1935:155), on the basis of structure of the teeth, listed the species which he examined in the following increasing order of specialization: _Dipodomys compactus_ (now _Dipodomys ordii compactus_), _D. nitratoides_, _D. merriami_, _D. ordii_, _D. agilis_, _D. herrmanni_, _D. spectabilis_, and _D. deserti_. This arrangement is at variance with that of Grinnell (1922) who listed the species in order of increasing specialization as: _Dipodomys herrmanni_, _D. panamintinus_, _D. ingens_, _D. spectabilis_, _D. merriami_, _D. nitratoides_, _D. ordii_, _D. agilis_, _D. venustus_, _D. microps_ and _D. deserti_. As noted, the only agreement between the two arrangements is the placing of _D. deserti_ as the most specialized. Relying on skeletal indices alone, I would accord the same position to _D. deserti_ but would not arrange the other species as have either Wood or Grinnell.

In this study, the amount of specialization of each species, as indicated by the skeleton, was determined by assigning consecutive numbers from 1 to 11 to each species in its place in each index, and then totaling and averaging these arbitrary numbers (Table 3). It will be noted that there is a tendency for each species to occupy the same relative position in each of the indices.

It is felt, however, that a more nearly correct arrangement, according to degree of specialization, is obtained by using the six skeletal indices plus the information obtained from the study of the viscera. On this basis the species may be arranged from least to most specialized as follows: _Dipodomys ordii_, _D. microps_, _D. panamintinus_, _D. agilis_, _D. herrmanni_, _D. ingens_, _D. spectabilis_, _D. phillipsii_, _D. merriami_, _D. nitratoides_ and _D. deserti_.

Grinnell (1922:95-96) arranged the Recent species of _Dipodomys_ in nine groups. Davis (1942:332) also proposed an arrangement of nine groups in which he combined the Compactus and Ordii groups of Grinnell, established a new Elator group by removing _Dipodomys elator_ from Grinnell's Phillipsii group, and in linear arrangement, Davis shifted the Spectabilis and what remained of the Phillipsii groups to new positions. Burt (1936:152) arranged Grinnell's groups into three (unnamed) groups solely on the basis of the structure of the baculum. In the arrangement proposed by Grinnell, two of his nine groups contained only one species each, one other, the Microps group, has since been shown to contain only one species and another, the Compactus group, contained only kinds which are, by me, regarded only as subspecies of _Dipodomys ordii_. To my mind neither Davis nor Burt added to or fundamentally changed the basic concepts as set forth in 1922 by Grinnell. Owing to the paucity of material at that time, especially from areas of intergradation, Grinnell's groupings and arrangement were as nearly natural as could be expected. With the accumulation of additional material and with the knowledge that certain kinds treated by Grinnell as full species are in actuality subspecies, it is felt that the several species of kangaroo rats can best be arranged in six groups which, from the least to the most specialized, are as follows:

ORDII GROUP.--Composed of the subspecies of _Dipodomys ordii_ and
_Dipodomys microps_. Grinnell placed these two species in separate
groups; Burt on characters of the baculum alone placed _D. microps_
with _Dipodomys deserti_ and _Dipodomys spectabilis_. Within the
single species _D. ordii_, I find that the difference in shape and
size of the baculum between the subspecies of _D. ordii_ is as
great as the difference which Burt (1936:154-155) found between the
full species _D. agilis_ and _D. microps_. The characters of the
baculum are an aid, but not in and of themselves an adequate basis,
for determining the natural relationships of the groups of species.
Certainly the remainder of the morphological differences between
_D. deserti_ and _D. microps_ are so great that I doubt that the
similarity in the baculum is significant, at least in this one
instance. The chisel-shaped lower incisors of _D. microps_ appear
to be a specialization. They may enable _D. microps_ to utilize
more woody types of vegetation than can _D. ordii_. Both species
occupy the same territory over much of their geographic range,
probably because they eat different kinds of food.

PANAMINTINUS GROUP.--Composed of all the now known subspecies of
_Dipodomys panamintinus_ and the species _Dipodomys stephensi_, if
the latter is a full species. This group was included by Grinnell
in the Heermanni group, with which it agrees in broadness of the
maxillary arches and the configuration of the penis bone, but on
the basis of the degree of specialization, as indicated by the
indices (see Table 1), I feel that the Panamintinus group is more
properly placed after the Ordii group and should be separated from
the Heermanni group. Actually, animals in the Panamintinus group
are intermediate between those of the Ordii and Heermanni groups.

HEERMANNI GROUP.--Composed of the subspecies of _Dipodomys
heermanni_ and _Dipodomys agilis_, the species _Dipodomys ingens_,
_Dipodomys venustus_ and _Dipodomys elephantinus_. _D._ _ingens_
even though larger in linear measurements than any of the other
kinds included in this group, has almost the same degree of
specialization as does _D. heermanni_. _D. agilis_, even though
somewhat less specialized than the other kinds placed in this
group, by the general nature of the indices, by the form of the
visceral mass and to some degree by the shape of the baculum, shows
itself properly to belong with this group. The species _D.
venustus_, judged by characters of the visceral anatomy, also
belongs here rather than with some other group or as a separate
group. From the appearance of the visceral mass, _D. venustus_ is
somewhat more specialized than either _D. heermanni_ or _D.
agilis_, but _D. venustus_ does show its affinities with this
group. The species _D. elephantinus_ has not been examined as
thoroughly as have the other species but the external morphology
and the configuration of the cranium place it with this group.

SPECTABILIS GROUP.--Composed of the subspecies of _Dipodomys
spectabilis_. In two of the six indices, _D. spectabilis_ shows a
high degree of specialization toward saltation, but in the other
four indices it shows a relatively low degree of specialization or
is average for the genus. Burt (1936:155) placed _D. spectabilis_
with _D. deserti_ on the basis of the baculum alone. I have not
examined _D. nelsoni_ but place it with this group as did also
Grinnell and Davis.

MERRIAMI GROUP.--Composed of the subspecies of _Dipodomys
merriami_, _Dipodomys nitratoides_ and _Dipodomys phillipsii_, and
the species _Dipodomys platycephalus_, _Dipodomys margaritae_,
_Dipodomys insularis_, _Dipodomys mitchelli_, _Dipodomys ornatus_
and _Dipodomys elator_. I have not examined five of these species.
However, the indices and characters of the viscera indicate that
the three species first mentioned are closely allied. Owing to the
lack of known intergradation between the three, I judge that they
should be retained as full species, but the difference in degree
of morphological specialization is no more than would be expected
between subspecies. I have examined no specimens of _Dipodomys
elator_ but from what I know of its morphology, I think that
Grinnell better indicated its relations in allying it with _D.
phillipsii_ than did Davis in erecting a new group for it on the
basis of linear measurements.

DESERTI GROUP.--Composed of _Dipodomys deserti_ which has only two
subspecies. In all morphological respects, _D. deserti_ is the
most specialized species in the genus as shown by the reduced
number (4) of toes on the hind foot, the bilateral arrangement of
the viscera, the extreme development of the auditory region of the
skull and by developing, early in life, the hiatus in the enamel
wall of each molariform tooth.

The parallel arrangement below emphasizes the differences and similarities between Grinnell's (1922) arrangement and the one proposed in the present paper.

Grinnell's arrangement Present arrangement

HEERMANNI GROUP HEERMANNI GROUP
_Dipodomys heermanni_ _Dipodomys heermanni_
_Dipodomys morroensis_ _Dipodomys agilis_
_Dipodomys mohavensis_ _Dipodomys ingens_
_Dipodomys leucogenys_ _Dipodomys venustus_
_Dipodomys panamintinus_ _Dipodomys elephantinus_
_Dipodomys stephensi_
_Dipodomys ingens_

SPECTABILIS GROUP SPECTABILIS GROUP
_Dipodomys spectabilis_ _Dipodomys spectabilis_
_Dipodomys nelsoni_ _Dipodomys nelsoni_

PHILLIPSII GROUP NOW IN MERRIAMI GROUP BELOW
_Dipodomys phillipsii_
_Dipodomys perotensis_
_Dipodomys ornatus_
_Dipodomys elator_

MERRIAMI GROUP MERRIAMI GROUP
_Dipodomys merriami_ _Dipodomys merriami_
_Dipodomys nitratoides_ _Dipodomys nitratoides_
_Dipodomys platycephalus_ _Dipodomys platycephalus_
_Dipodomys margaritae_ _Dipodomys margaritae_
_Dipodomys insularis_ _Dipodomys insularis_
_Dipodomys mitchelli_ _Dipodomys mitchelli_
_Dipodomys phillipsii_
_Dipodomys ornatus_
_Dipodomys elator_

ORDII GROUP ORDII GROUP
_Dipodomys ordii_ _Dipodomys ordii_
_Dipodomys microps_

COMPACTUS GROUP NOW IN ORDII GROUP ABOVE
_Dipodomys compactus_
_Dipodomys sennetti_

AGILIS GROUP NOW IN HEERMANNI GROUP ABOVE
_Dipodomys agilis_
_Dipodomys venustus_
_Dipodomys elephantinus_

MICROPS GROUP NOW IN ORDII GROUP ABOVE
_Dipodomys microps_
_Dipodomys levipes_

DESERTI GROUP DESERTI GROUP
_Dipodomys deserti_ _Dipodomys deserti_

WERE IN HEERMANI GROUP ABOVE PANAMINTINUS GROUP
_Dipodomys panamintinus_
_Dipodomys stephensi_

Names of the subspecies are omitted from the groups named above and only the names of full species, as understood by Grinnell and as understood now, have been used. It will be noted that the phylogenetic order follows that of Grinnell rather than the one proposed herein.

The fossil record of the kangaroo rats is so scanty that one can but speculate on the evolutionary sequence. Wood (1935) presented a diagnosis of the early phyletic history up to and through _Cupidinimus_; this is probably as correct as can be made. I cannot, however, share his view that the recent species of _Dipodomys_ have originated from a descendant of _Cupidinimus nebraskensis_; instead, I think that the Recent species originated from some unknown ancestor in the southwest.

In view of the foregoing evidence it seems best to estimate the relationships and history of the various species and groups of species only as far back as the early Pleistocene (see Figure 21). Inasmuch as faunas of fossil mammals from the mid-Pleistocene contain few, if any, Recent species (see Hibbard, 1937:193), the living species of _Dipodomys_ have probably had a geologic history no longer than the period of time which has elapsed since the middle Pleistocene, or at the earliest the early Pleistocene. Of the Recent species, only _Dipodomys agilis_ is known as a fossil; it was found in the late Pleistocene tar pits of California. Under the heading "_Dipodomys near ingens_," however, Schultz (1938:206) recorded remains of kangaroo rats from the tar seeps of McKittrick in the San Joaquin Valley of California.

DISPERSAL OF THE SEVERAL SPECIES

If we assume the region of origin and center of dispersal of a group of animals to be the one in which the greatest numbers of the most specialized species of a given genus are found, then the northern Tableland of Mexico and the adjoining region of the United States in southeastern California and southwestern Nevada is the region of origin and the center of dispersal for the genus _Dipodomys_. _Dipodomys deserti_, _Dipodomys merriami_, _Dipodomys panamintinus_, _Dipodomys microps_, _Dipodomys phillipsii_ and _Dipodomys ordii_ are found in the region mentioned. That the aforementioned region may be the center of differentiation for this genus is further indicated by: First, the finding, in this region, of saline deposits of Cenozoic (Miocene) age, indicating aridity, which is thought to have been one of the essential stimuli for the development of the saltatorial habit in the genus _Dipodomys_; second, the recovery of the advanced heteromyids from the Avawatz and Ricardo of the Clarendonian (Pliocene) of this same region; and third, the present abundance and diversification of kangaroo rats in this same geographic region which has been more or less arid since Miocene time.

A secondary center of evolution has been the low, hot interior valleys and adjacent foothills of central California where _Dipodomys ingens_, _Dipodomys heermanni_, _Dipodomys venustus_, _Dipodomys agilis_, _Dipodomys elephantinus_ and _Dipodomys nitratoides_ are now found. Although there are as many species as in the principal center of origin, the amount of specialization and adaptive radiation in California is not so great. Probably during the Quaternary, when the process of mountain building was actively under way the animals that had reached central California from the parental center became isolated by the emergence of the Tehachapi Mountains. This mountain range separated the California animals from populations farther south and east. As a result, _D. nitratoides_ was differentiated from _D. merriami_, and _D. heermanni_ underwent an evolution of its own which resulted in animals having either four or five toes on the hind foot. At the same time _Dipodomys ingens_ developed there and has since been undergoing an evolution parallel to that of the large-sized species, _Dipodomys spectabilis_. The two species have paralleled each other not only in large size but to some extent in habits such as building large mounds that are kept free of vegetation and in occupying areas of rather hard clayey soil. Structurally, however, _D. ingens_ has not yet become quite so specialized as _D. spectabilis_, probably because _D. ingens_ has had less time in which to become so. A second species, if it is a full species, _Dipodomys elephantinus_, has also been isolated in central California but has not attained so high a degree of specialization as _D. ingens_. It is interesting to note that in each of the two stocks, two large-sized species have been evolved. In the parental stock the two species are _Dipodomys deserti_ and _Dipodomys spectabilis_; the former is the most specialized species in the genus. In the stock isolated in California, however, even though two large species have been formed they are still below the average in degree of specialization for the genus. As noted elsewhere in this paper, the species from these low, hot valleys, excepting _D. nitratoides_, are all closely related one to another. _Dipodomys venustus_ and _Dipodomys elephantinus_ are either closely related species or possibly only subspecies of one species, _Dipodomys agilis_.

It is worthy of note that as the distance away from the center of differentiation increases, the number of species decreases. For example, in the northern Great Basin there are only two species (_Dipodomys ordii_ and _Dipodomys microps_) and farther eastward, on the eastern side of the Rocky Mountains, there is only the one species, _Dipodomys ordii_. In north-central Texas, _Dipodomys elator_, perhaps a relict species, is found occupying an area farther east than that occupied by _Dipodomys ordii_ at that latitude.

_Dipodomys ordii_, _Dipodomys phillipsii_ and _Dipodomys merriami_ occupy the southern portion of the range of the genus. Instead of being generalized at this southern part of the periphery of the range as are the kinds found on the other parts of the periphery of the range of the genus, these three southern kinds are notably specialized there in the south. The subspecies _D. o. palmeri_ which occurs in the area, is the most specialized of the species _Dipodomys ordii_; and _Dipodomys phillipsii_ and _Dipodomys merriami_ stand high in the scale of specialization with respect to the other species of the genus. The reason for this is not clear.

SUBSPECIATION

_Dipodomys ordii_ is, without question, a valid species if one accepts Mayr's (1942:120) definition that "Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups." _D. ordii_ is not known to hybridize with other species where their geographic ranges are adjacent or overlap. The first part of the definition "actually or potentially interbreeding populations" is substantiated by the 35 recognizable subspecies which can be defined as "a complex of interbreeding and completely fertile individuals which are morphologically identical or vary only within the limits of individual, ecological and seasonal variability. The typical characters of this group of individuals are genetically fixed and no other geographic race of the same species occurs within the same range" (after Rensch, 1934; from Mayr, 1942:106). Thus we find that certain populations of individuals differ from others and that in geographic areas between two of these populations, individuals (intergrades) are found which resemble those of both populations. In another instance, a population may be geographically isolated yet in its characters it may be recognizable as a subspecies without actual intergradation because of slight degrees of difference, or a group may be different from another without being geographically separated and may or may not show intergradation.

Subspeciation in _Dipodomys ordii_ almost certainly has been effected, by means of mutations, under the influence of natural selection. Natural selection enhanced by geographic and ~ecologic~ isolation, probably has retained mutations of evolutionary significance, thus permitting the development of the many recognizable subspecies.

In the subspecies of _Dipodomys ordii_ the color ranges from pale to dark. The difference in color is as pronounced as that between the full species _D. deserti_ and _D. heermanni_. The lightest-colored subspecies are _Dipodomys ordii celeripes_, _D. o. extractus_ and _D. o. compactus_; the darkest are _D. o. obscurus_ and _D. o. palmeri_.

There is a marked tendency for intergrades between a light-colored subspecies, such as _D. o. celeripes_, and a dark-colored kind, such as _D. o. utahensis_, to show varying degrees of blending in color. The insular population, _D. o. compactus_, has, however, two distinct color phases, a light phase and a slightly darker phase, and shows no tendency toward blending. In other kinds of mammals, blending of color is known to be the result of the action of multiple alleles, but in the insular kangaroo rat (_D. o. compactus_) the color appears to be the result of either a reduced multiple allelomorphic complex or even a unit factor. The two color phases of this insular subspecies, which might be an expression of a unit factor, more probably are specializations in which the multiple alleles for color have been reduced. The probability that there is either a unit factor or a reduced number of alleles at work is suggested by the taking of more dark-colored than light-colored animals and by the absence of blending of color. This insular population has undoubtedly been derived from the mainland kangaroo rat, _D. o. sennetti_, which has the usual range of variation but, to my knowledge, there are no individuals of _D. o. sennetti_ so light as the darkest animals of _D. o. compactus_ from the islands.

Populations from a given locality are remarkably stable in color except the animals from Samalayuca, Chihuahua, which vary in color from individuals almost as light as _D. o. compactus_ to animals that approach _D. o. ordii_ in darkness of pelage.

The subspecies of _D. ordii_ show no noticeable variation in the extent of the hip stripe, supraorbital and postauricular spots, basal white ring of the tail, lateral stripes of the tail or the extent of white on the venter and feet. There is, however, variation in the degree and extent of the arietiform facial markings. In _Dipodomys ordii utahensis_, _D. o. cupidineus_, _D. o. obscurus_ and _D. o. fuscus_ these markings are pronounced. In _D. o. celeripes_, _D. o. pallidus_, _D. o. compactus_ and _D. o. attenuatus_ these markings are either obliterated or nearly so.

In _Dipodomys ordii_, color does not seem to be correlated with amount of moisture or geography, but rather with color of soil. For example, all animals from the Bonneville Basin of western Utah, are light colored as are the soils; animals from the San Rafael Desert of eastern Utah are reddish, as is the soil. More striking extremes of this are shown by _D. o. compactus_ of Padre and Mustang islands, Texas, which is pale-colored as is the sand on which it lives, and _D. o. medius_ from east-central New Mexico and western Texas, which is reddish as is the soil there, which is derived from Permian rocks. In localities where alkaline soils are present, kangaroo rats may be found with a roseaceous cast to the pelage as a result of the action of the alkaline salts on the pigment of the hair. The roseaceous color is lost when the animal sheds the old pelage.

In the dorsal and ventral stripes of the tail, I find as much variation in the species _D. ordii_ as Grinnell (1922:Fig. E, p. 14) recorded in the whole genus. In _D. o. obscurus_, _D. o. fuscus_ and _D. o. utahensis_ the stripes are complete to the distal end of the tail and dark, whereas in _D. o. pallidus_ and _D. o. celeripes_ the ventral stripe is either absent or nearly so and the dorsal stripe is pale.

Color as a taxonomic character is valuable in a broad sense, and is useful in placing an individual or a group of individuals in the subspecies to which they pertain. In most subspecies studied, color was quite uniform throughout the range of the animals, but in _D. o. ordii_ and _D. o. columbianus_ color is so variable that cranial features were relied on almost exclusively for the final diagnosis.

Among the subspecies of _Dipodomys ordii_ there is relatively little variation in the length of the head and body. The smallest measurement is 95.5 mm. in _D. o. idoneus_ and the largest is 118.3 mm. in _D. o. richardsoni_. The shortest tail is found to be 112.0 mm. in _D. o. celeripes_ and the longest is 154.7 mm. in _D. o. terrosus_. The length of the hind foot varies from 35.0 mm. in _D. o. idoneus_ to 44.5 mm. in _D. o. nexilis_.

Allen's Rule is not operative in the species _D. ordii_. According to this rule, shorter tails and smaller feet in conjunction with a large body would be expected as the more northerly limits of the species are approached, and conversely, smaller body and larger appendages would be expected as the southerly limits of the species are approached. This is not the case, however, since the subspecies _D. o. terrosus_ ranges farthest north and has the longest tail, whereas _D. o. celeripes_, found in the central part of the range of the species, has the shortest tail. Again, in regard to the hind foot, the shortest is found in _D. o. idoneus_ which is at the extreme south of the range of the species, whereas the longest hind foot is found in _D. o. nexilis_ which occupies a nearly central position in the range.

Long tail and long hind foot would seem to be specializations for saltation and the two would be expected to be correlated. Actually there is no significant correlation in _D. ordii_. _D. o. celeripes_, in which the hind foot is near the mean for the species (39.8 as opposed to the mean of 40.7), has the shortest tail. _D. o. compactus_ has a short tail (117.0 mm.) but a medium-sized hind foot. _D. o. nexilis_ and _D. o. terrosus_ have both a long hind foot and long tail.

Cranial measurements vary less, probably because one person can measure a series with a uniformly subjective error. External measurements, however, are liable to a greater degree of subjective error. The total length of the skull varies from 35.4 mm. in _D. o. attenuatus_ to 41.3 mm. in _D. o. terrosus_. In no one series of adults from one locality, however, is the variation so marked as it is for the species as a whole. The usual range of variation in length of skull in any given series is not, as a rule, more than 2.5 mm.

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Subspeciation in the Kangaroo Rat, Dipodomys ordiiChapter I: Part 1

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