TABLE 2Visceral Measurements(in Millimeters) ofDipodomysheermannimerriamiagilisdesertiordiispectabilisvenustuspanamintinusingensLarge intestine432290464397237413374419430Small intestine165126220195131228207255274Percent of small tolarge intestine38.243.447.549.255.255.355.460.963.7From 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 whichHowellhas observed.The skeleton, particularly of the appendages, shows the mostmodification, ranging from a relatively generalized to a specialized condition. Skeletal indices, as established byHowell(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 Femorotarsal-metatarsal and Cranial indices are not fromHowell(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, asHowell(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.Phylogeny of the Dipodomyines.Figs. 2-10.Showing the compacting of the visceral mass; liver at the top, small intestine and caecum at the bottom. All figures approximately × 1.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 3Relative Specializations of the Species for each IndexHumeroradialIntermembralCruralTibioradialFemorotarsal-metatarsalCranialAverageordii5111152.33microps1245423.0panamintinus6333313.1agilis7456645.3heermanni3679535.5ingens47282105.5spectabilis211611787.5phillipsii1151021177.6merriami109941068.0nitratoides9887998.5deserti81011108119.6The 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 inTable 3in 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 isD. desertinor to brush as areD. agilisandD. venustus.D. merriamidoes, 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 inD. panamintinus mohavensiswhich has small ears and small bullae and inD. elephantinuswhich 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.Phylogeny of the Dipodomyines.Figs. 11-15.Ventral views of skulls showing the degree of development of the auditory bullae and the configuration of the pterygoid fossae. All figures approximately × 1.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.Phylogeny of the Dipodomyines.Figs. 16-20.Dorsal views of skulls showing the degrees of inflation of the auditory bullae and the correlation of large bullae with small interparietal. All figures approximately × 1.Fig.16.Dipodomys ordii compactus, for data seeFig. 11.Fig.17.Dipodomys ordii oklahomae, for data seeFig. 12.Fig.18.Dipodomys ordii richardsoni, for data seeFig. 13.Fig.19.Dipodomys ordii nexilis, for data seeFig. 14.Fig.20.Dipodomys deserti deserti, for data seeFig. 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(nowDipodomys ordii compactus),D. nitratoides,D. merriami,D. ordii,D. agilis,D. herrmanni,D. spectabilis, andD. deserti. This arrangement is at variance with that ofGrinnell(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. micropsandD. deserti. As noted, the only agreement between the two arrangements is the placing ofD. desertias the most specialized. Relying on skeletal indices alone, I would accord the same position toD. desertibut would not arrange the other species as have eitherWoodorGrinnell.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. nitratoidesandD. deserti.Grinnell(1922:95-96) arranged the Recent species ofDipodomysin nine groups.Davis(1942:332) also proposed an arrangement of nine groups in which he combined the Compactus and Ordii groups ofGrinnell, established a new Elator group by removingDipodomys elatorfromGrinnell'sPhillipsii group, and in linear arrangement,Davisshifted the Spectabilis and what remained of the Phillipsii groups to new positions.Burt(1936:152) arrangedGrinnell'sgroups into three (unnamed) groups solely on the basis of the structure of the baculum. In the arrangement proposed byGrinnell, 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 ofDipodomys ordii. To my mind neitherDavisnorBurtadded to or fundamentally changed the basic concepts as set forth in 1922 byGrinnell. Owingto the paucity of material at that time, especially from areas of intergradation,Grinnell'sgroupings and arrangement were as nearly natural as could be expected. With the accumulation of additional material and with the knowledge that certain kinds treated byGrinnellas 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 ofDipodomys ordiiandDipodomys microps.Grinnellplaced these two species in separate groups;Burton characters of the baculum alone placedD. micropswithDipodomys desertiandDipodomys spectabilis. Within the single speciesD. ordii, I find that the difference in shape and size of the baculum between the subspecies ofD. ordiiis as great as the difference whichBurt(1936:154-155) found between the full speciesD. agilisandD. 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 betweenD. desertiandD. micropsare so great that I doubt that the similarity in the baculum is significant, at least in this one instance. The chisel-shaped lower incisors ofD. micropsappear to be a specialization. They may enableD. micropsto utilize more woody types of vegetation than canD. 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 ofDipodomys panamintinusand the speciesDipodomys stephensi, if the latter is a full species. This group was included byGrinnellin 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 (seeTable 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 ofDipodomys heermanniandDipodomys agilis, the speciesDipodomys ingens,Dipodomys venustusandDipodomys elephantinus.D.ingenseven though larger in linear measurements than any of the other kinds included in this group, has almost the same degree of specialization as doesD. 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 speciesD. 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. venustusis somewhat more specialized than eitherD. heermanniorD. agilis, butD. venustusdoes show its affinities with this group. The speciesD. elephantinushas 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 ofDipodomys spectabilis. In two of the six indices,D. spectabilisshows 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) placedD. spectabiliswithD. desertion the basis of the baculum alone. I have not examinedD. nelsonibut place it with this group as did alsoGrinnellandDavis.Merriami Group.—Composed of the subspecies ofDipodomys merriami,Dipodomys nitratoidesandDipodomys phillipsii, and the speciesDipodomys platycephalus,Dipodomys margaritae,Dipodomys insularis,Dipodomys mitchelli,Dipodomys ornatusandDipodomys 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 ofDipodomys elatorbut from what I know of its morphology, I think thatGrinnellbetter indicated its relations in allying it withD. phillipsiithan didDavisin erecting a new group for it on the basis of linear measurements.Deserti Group.—Composed ofDipodomys desertiwhich has only two subspecies. In all morphological respects,D. desertiis the most specialized species in the genus as shown by thereduced 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 betweenGrinnell's(1922) arrangement and the one proposed in the present paper.Grinnell'sarrangementPresent arrangementHeermanni GroupHeermanni GroupDipodomys heermanniDipodomys heermanniDipodomys morroensisDipodomys agilisDipodomys mohavensisDipodomys ingensDipodomys leucogenysDipodomys venustusDipodomys panamintinusDipodomys elephantinusDipodomys stephensiDipodomys ingensSpectabilis GroupSpectabilis GroupDipodomys spectabilisDipodomys nelsoniDipodomys spectabilisDipodomys nelsoniPhillipsii GroupNow in Merriami Group BelowDipodomys phillipsiiDipodomys perotensisDipodomys ornatusDipodomys elatorMerriami GroupMerriami GroupDipodomys merriamiDipodomys merriamiDipodomys nitratoidesDipodomys nitratoidesDipodomys platycephalusDipodomys platycephalusDipodomys margaritaeDipodomys margaritaeDipodomys insularisDipodomys insularisDipodomys mitchelliDipodomys mitchelliDipodomys phillipsiiDipodomys ornatusDipodomys elatorOrdii GroupOrdii GroupDipodomys ordiiDipodomys ordiiDipodomys micropsCompactus GroupNow in Ordii Group AboveDipodomys compactusDipodomys sennettiAgilis GroupNow in Heermanni Group AboveDipodomys agilisDipodomys venustusDipodomys elephantinusMicrops GroupNow in Ordii Group AboveDipodomys micropsDipodomys levipesDeserti GroupDeserti GroupDipodomys desertiDipodomys desertiWere in Heermanii Group AbovePanamintinus GroupDipodomys panamintinusDipodomys stephensiNames of the subspecies are omitted from the groups named above and only the names of full species, as understood byGrinnelland as understood now, have been used. It will be noted that the phylogenetic order follows that ofGrinnellrather 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 throughCupidinimus; this is probably as correct as can be made. I cannot, however, share his view that the recent species ofDipodomyshave originated from a descendant ofCupidinimus nebraskensis; instead, I think that the Recent species originated from some unknown ancestor in the southwest.Phylogeny of the Dipodomyines.Fig.21. Diagrammatic representation of the relationships and history of the Recent speciesDipodomys.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 (seeFigure 21). Inasmuch as faunas of fossil mammals from the mid-Pleistocene contain few, if any, Recent species (seeHibbard, 1937:193), the living species ofDipodomyshave 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, onlyDipodomys agilisis known as a fossil; it was found in the latePleistocene 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 SPECIESIf 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 genusDipodomys.Dipodomys deserti,Dipodomys merriami,Dipodomys panamintinus,Dipodomys microps,Dipodomys phillipsiiandDipodomys ordiiare 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 genusDipodomys; 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 whereDipodomys ingens,Dipodomys heermanni,Dipodomys venustus,Dipodomys agilis,Dipodomys elephantinusandDipodomys nitratoidesare 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. nitratoideswas differentiated fromD. merriami, andD. heermanniunderwent an evolution of its own which resulted in animals having either four or five toes on the hind foot. At the same timeDipodomys ingensdeveloped 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 someextent 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. ingenshas not yet become quite so specialized asD. spectabilis, probably becauseD. ingenshas 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 asD. 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 areDipodomys desertiandDipodomys 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, exceptingD. nitratoides, are all closely related one to another.Dipodomys venustusandDipodomys elephantinusare 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 ordiiandDipodomys 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 byDipodomys ordiiat that latitude.Dipodomys ordii,Dipodomys phillipsiiandDipodomys merriamioccupy 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 subspeciesD. o. palmeriwhich occurs in the area, is the most specialized of the speciesDipodomys ordii; andDipodomys phillipsiiandDipodomys merriamistand high in the scale of specialization with respect to the other species of the genus. The reason for this is not clear.SUBSPECIATIONDipodomys ordiiis, without question, a valid species if one acceptsMayr's(1942:120) definition that "Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups."D. ordiiis notknown 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; fromMayr, 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 inDipodomys ordiialmost certainly has been effected, by means of mutations, under the influence of natural selection. Natural selection enhanced by geographic andecologicisolation, probably has retained mutations of evolutionary significance, thus permitting the development of the many recognizable subspecies.In the subspecies ofDipodomys ordiithe color ranges from pale to dark. The difference in color is as pronounced as that between the full speciesD. desertiandD. heermanni. The lightest-colored subspecies areDipodomys ordii celeripes,D. o. extractusandD. o. compactus; the darkest areD. o. obscurusandD. o. palmeri.There is a marked tendency for intergrades between a light-colored subspecies, such asD. o. celeripes, and a dark-colored kind, such asD. 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 colorhave 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 ofD. o. sennettiso light as the darkest animals ofD. o. compactusfrom 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 asD. o. compactusto animals that approachD. o. ordiiin darkness of pelage.The subspecies ofD. ordiishow 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. InDipodomys ordii utahensis,D. o. cupidineus,D. o. obscurusandD. o. fuscusthese markings are pronounced. InD. o. celeripes,D. o. pallidus,D. o. compactusandD. o. attenuatusthese markings are either obliterated or nearly so.InDipodomys 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 byD. o. compactusof Padre and Mustang islands, Texas, which is pale-colored as is the sand on which it lives, andD. o. mediusfrom 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 speciesD. ordiiasGrinnell(1922:Fig. E, p. 14) recorded in the whole genus. InD. o. obscurus,D. o. fuscusandD. o. utahensisthe stripes are complete to the distal end of the tail and dark, whereas inD. o. pallidusandD. o. celeripesthe 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, andis 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 inD. o. ordiiandD. o. columbianuscolor is so variable that cranial features were relied on almost exclusively for the final diagnosis.Among the subspecies ofDipodomys ordiithere is relatively little variation in the length of the head and body. The smallest measurement is 95.5 mm. inD. o. idoneusand the largest is 118.3 mm. inD. o. richardsoni. The shortest tail is found to be 112.0 mm. inD. o. celeripesand the longest is 154.7 mm. inD. o. terrosus. The length of the hind foot varies from 35.0 mm. inD. o. idoneusto 44.5 mm. inD. o. nexilis.Allen's Rule is not operative in the speciesD. 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 subspeciesD. o. terrosusranges farthest north and has the longest tail, whereasD. 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 inD. o. idoneuswhich is at the extreme south of the range of the species, whereas the longest hind foot is found inD. o. nexiliswhich 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 inD. 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. compactushas a short tail (117.0 mm.) but a medium-sized hind foot.D. o. nexilisandD. o. terrosushave 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. inD. o. attenuatusto 41.3 mm. inD. 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.Cranial indices (breadth across bullae/length of skull × 100) as established for random samples of the different species of the genus(exclusive ofD. ordii) ranged from 60.8 to 67.6. In the subspecies ofD. ordiithe same index varies from 59.7 to 65.2 with an average of 63.4. In other words, the degree of specialization indicated by this one index, in a few subspecies ofD. ordii, is almost as great as that inDipodomys deserti, which on the basis of total morphology appears to be the most specialized species in the genus. Also, on the basis of this same index, some subspecies ofD. ordiiare more generalized than is any other species in the genus.There is a general tendency for the nasals to decrease in length and the rostrum to decrease in width as the southern limits of the range ofD. ordiiare approached. In ascertaining the decrease in length of the nasals an index was obtained as follows: nasals/interorbital width × 100 (seeTable 4). The width of the rostrum, however, does not decrease in the same degree, nor at the same rate, as does the length of the nasals. This decrease in length of the nasals and in width of the rostrum may be correlated with the mean annual relative humidity of the environment. It is known (Howelland Gersh, 1936:8) that desert rodents, more exactly kangaroo rats, have a water retention mechanism in the kidneys and walls of the urinary bladder which enables them more efficiently to conserve metabolic water. The significance of the decrease of the area of the nasal mucosa, which seems to be related to relative aridity, is not yet properly understood.In no cranial feature other than shortened nasals and narrowed rostrum, doesDipodomys ordiishow a gradation such that it might be termed a cline. Other parts of the skull that were measured do not vary greatly.Perhaps the greatest amount of variation in the skull is in features which are not readily measurable by the usual physical means. The shape and size of the pterygoid fossae vary from almost round to rather ovoid in a given series of animals from one locality; the size and configuration of the zygomatic arches vary from slender to robust and from straight to curved laterally; the size of the lacrimal processes varies much in any given series, as do also the degree of expansion of the distal end of the nasals, the convexity of the braincase and the curvature of the upper incisors. In all instances where these features varied much, one size or shape was more pronounced in the series than any other size or shape. Thus, when comparisons were made, the size and certain shapes were the criteria used in assigning the animals under consideration to a given subspecies.Subspeciation inDipodomys ordiiseems to have been influencedby water barriers. It is known (Grinnell, 1922:28) that kangaroo rats lack the ability to swim. Large stable rivers such as the Colorado, Snake and Columbia serve as effective barriers to further dispersal of kangaroo rats. Streams that freeze over in the winter months, however, are not efficient barriers. This is indicated by the "blending" of morphological characters ofD. o. nexilisandD. o. sanrafaelialong the Green River which freezes over.Any mountain which has vegetational belts above the Transition Life-zone would serve as a barrier to the dispersal of these animals. The Uinta Mountains, lying in an east-west direction, are interposed between the ranges ofD. o. priscusandD. o. uintensis. The high Wasatch Mountains and their associated outliers, lying in a north-south direction in Utah, serve as an efficient barrier to the east-west movement of kangaroo rats and as a result, the subspecies east of the mountain mass are remarkably different from those to the west.
Visceral Measurements(in Millimeters) ofDipodomys
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 whichHowellhas observed.
The skeleton, particularly of the appendages, shows the mostmodification, ranging from a relatively generalized to a specialized condition. Skeletal indices, as established byHowell(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 Femorotarsal-metatarsal and Cranial indices are not fromHowell(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, asHowell(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.
Phylogeny of the Dipodomyines.
Figs. 2-10.Showing the compacting of the visceral mass; liver at the top, small intestine and caecum at the bottom. All figures approximately × 1.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.
Figs. 2-10.Showing the compacting of the visceral mass; liver at the top, small intestine and caecum at the bottom. All figures approximately × 1.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.
Relative Specializations of the Species for each Index
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 inTable 3in 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 isD. desertinor to brush as areD. agilisandD. venustus.D. merriamidoes, 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 inD. panamintinus mohavensiswhich has small ears and small bullae and inD. elephantinuswhich 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.
Phylogeny of the Dipodomyines.
Figs. 11-15.Ventral views of skulls showing the degree of development of the auditory bullae and the configuration of the pterygoid fossae. All figures approximately × 1.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.
Figs. 11-15.Ventral views of skulls showing the degree of development of the auditory bullae and the configuration of the pterygoid fossae. All figures approximately × 1.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.
Phylogeny of the Dipodomyines.
Figs. 16-20.Dorsal views of skulls showing the degrees of inflation of the auditory bullae and the correlation of large bullae with small interparietal. All figures approximately × 1.Fig.16.Dipodomys ordii compactus, for data seeFig. 11.Fig.17.Dipodomys ordii oklahomae, for data seeFig. 12.Fig.18.Dipodomys ordii richardsoni, for data seeFig. 13.Fig.19.Dipodomys ordii nexilis, for data seeFig. 14.Fig.20.Dipodomys deserti deserti, for data seeFig. 15.
Figs. 16-20.Dorsal views of skulls showing the degrees of inflation of the auditory bullae and the correlation of large bullae with small interparietal. All figures approximately × 1.Fig.16.Dipodomys ordii compactus, for data seeFig. 11.Fig.17.Dipodomys ordii oklahomae, for data seeFig. 12.Fig.18.Dipodomys ordii richardsoni, for data seeFig. 13.Fig.19.Dipodomys ordii nexilis, for data seeFig. 14.Fig.20.Dipodomys deserti deserti, for data seeFig. 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(nowDipodomys ordii compactus),D. nitratoides,D. merriami,D. ordii,D. agilis,D. herrmanni,D. spectabilis, andD. deserti. This arrangement is at variance with that ofGrinnell(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. micropsandD. deserti. As noted, the only agreement between the two arrangements is the placing ofD. desertias the most specialized. Relying on skeletal indices alone, I would accord the same position toD. desertibut would not arrange the other species as have eitherWoodorGrinnell.
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. nitratoidesandD. deserti.
Grinnell(1922:95-96) arranged the Recent species ofDipodomysin nine groups.Davis(1942:332) also proposed an arrangement of nine groups in which he combined the Compactus and Ordii groups ofGrinnell, established a new Elator group by removingDipodomys elatorfromGrinnell'sPhillipsii group, and in linear arrangement,Davisshifted the Spectabilis and what remained of the Phillipsii groups to new positions.Burt(1936:152) arrangedGrinnell'sgroups into three (unnamed) groups solely on the basis of the structure of the baculum. In the arrangement proposed byGrinnell, 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 ofDipodomys ordii. To my mind neitherDavisnorBurtadded to or fundamentally changed the basic concepts as set forth in 1922 byGrinnell. Owingto the paucity of material at that time, especially from areas of intergradation,Grinnell'sgroupings and arrangement were as nearly natural as could be expected. With the accumulation of additional material and with the knowledge that certain kinds treated byGrinnellas 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 ofDipodomys ordiiandDipodomys microps.Grinnellplaced these two species in separate groups;Burton characters of the baculum alone placedD. micropswithDipodomys desertiandDipodomys spectabilis. Within the single speciesD. ordii, I find that the difference in shape and size of the baculum between the subspecies ofD. ordiiis as great as the difference whichBurt(1936:154-155) found between the full speciesD. agilisandD. 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 betweenD. desertiandD. micropsare so great that I doubt that the similarity in the baculum is significant, at least in this one instance. The chisel-shaped lower incisors ofD. micropsappear to be a specialization. They may enableD. micropsto utilize more woody types of vegetation than canD. 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 ofDipodomys panamintinusand the speciesDipodomys stephensi, if the latter is a full species. This group was included byGrinnellin 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 (seeTable 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 ofDipodomys heermanniandDipodomys agilis, the speciesDipodomys ingens,Dipodomys venustusandDipodomys elephantinus.D.ingenseven though larger in linear measurements than any of the other kinds included in this group, has almost the same degree of specialization as doesD. 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 speciesD. 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. venustusis somewhat more specialized than eitherD. heermanniorD. agilis, butD. venustusdoes show its affinities with this group. The speciesD. elephantinushas 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 ofDipodomys spectabilis. In two of the six indices,D. spectabilisshows 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) placedD. spectabiliswithD. desertion the basis of the baculum alone. I have not examinedD. nelsonibut place it with this group as did alsoGrinnellandDavis.
Merriami Group.—Composed of the subspecies ofDipodomys merriami,Dipodomys nitratoidesandDipodomys phillipsii, and the speciesDipodomys platycephalus,Dipodomys margaritae,Dipodomys insularis,Dipodomys mitchelli,Dipodomys ornatusandDipodomys 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 ofDipodomys elatorbut from what I know of its morphology, I think thatGrinnellbetter indicated its relations in allying it withD. phillipsiithan didDavisin erecting a new group for it on the basis of linear measurements.
Deserti Group.—Composed ofDipodomys desertiwhich has only two subspecies. In all morphological respects,D. desertiis the most specialized species in the genus as shown by thereduced 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 betweenGrinnell's(1922) arrangement and the one proposed in the present paper.
Names of the subspecies are omitted from the groups named above and only the names of full species, as understood byGrinnelland as understood now, have been used. It will be noted that the phylogenetic order follows that ofGrinnellrather 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 throughCupidinimus; this is probably as correct as can be made. I cannot, however, share his view that the recent species ofDipodomyshave originated from a descendant ofCupidinimus nebraskensis; instead, I think that the Recent species originated from some unknown ancestor in the southwest.
Phylogeny of the Dipodomyines.Fig.21. Diagrammatic representation of the relationships and history of the Recent speciesDipodomys.
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 (seeFigure 21). Inasmuch as faunas of fossil mammals from the mid-Pleistocene contain few, if any, Recent species (seeHibbard, 1937:193), the living species ofDipodomyshave 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, onlyDipodomys agilisis known as a fossil; it was found in the latePleistocene 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.
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 genusDipodomys.Dipodomys deserti,Dipodomys merriami,Dipodomys panamintinus,Dipodomys microps,Dipodomys phillipsiiandDipodomys ordiiare 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 genusDipodomys; 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 whereDipodomys ingens,Dipodomys heermanni,Dipodomys venustus,Dipodomys agilis,Dipodomys elephantinusandDipodomys nitratoidesare 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. nitratoideswas differentiated fromD. merriami, andD. heermanniunderwent an evolution of its own which resulted in animals having either four or five toes on the hind foot. At the same timeDipodomys ingensdeveloped 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 someextent 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. ingenshas not yet become quite so specialized asD. spectabilis, probably becauseD. ingenshas 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 asD. 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 areDipodomys desertiandDipodomys 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, exceptingD. nitratoides, are all closely related one to another.Dipodomys venustusandDipodomys elephantinusare 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 ordiiandDipodomys 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 byDipodomys ordiiat that latitude.
Dipodomys ordii,Dipodomys phillipsiiandDipodomys merriamioccupy 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 subspeciesD. o. palmeriwhich occurs in the area, is the most specialized of the speciesDipodomys ordii; andDipodomys phillipsiiandDipodomys merriamistand high in the scale of specialization with respect to the other species of the genus. The reason for this is not clear.
Dipodomys ordiiis, without question, a valid species if one acceptsMayr's(1942:120) definition that "Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups."D. ordiiis notknown 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; fromMayr, 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 inDipodomys ordiialmost certainly has been effected, by means of mutations, under the influence of natural selection. Natural selection enhanced by geographic andecologicisolation, probably has retained mutations of evolutionary significance, thus permitting the development of the many recognizable subspecies.
In the subspecies ofDipodomys ordiithe color ranges from pale to dark. The difference in color is as pronounced as that between the full speciesD. desertiandD. heermanni. The lightest-colored subspecies areDipodomys ordii celeripes,D. o. extractusandD. o. compactus; the darkest areD. o. obscurusandD. o. palmeri.
There is a marked tendency for intergrades between a light-colored subspecies, such asD. o. celeripes, and a dark-colored kind, such asD. 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 colorhave 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 ofD. o. sennettiso light as the darkest animals ofD. o. compactusfrom 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 asD. o. compactusto animals that approachD. o. ordiiin darkness of pelage.
The subspecies ofD. ordiishow 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. InDipodomys ordii utahensis,D. o. cupidineus,D. o. obscurusandD. o. fuscusthese markings are pronounced. InD. o. celeripes,D. o. pallidus,D. o. compactusandD. o. attenuatusthese markings are either obliterated or nearly so.
InDipodomys 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 byD. o. compactusof Padre and Mustang islands, Texas, which is pale-colored as is the sand on which it lives, andD. o. mediusfrom 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 speciesD. ordiiasGrinnell(1922:Fig. E, p. 14) recorded in the whole genus. InD. o. obscurus,D. o. fuscusandD. o. utahensisthe stripes are complete to the distal end of the tail and dark, whereas inD. o. pallidusandD. o. celeripesthe 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, andis 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 inD. o. ordiiandD. o. columbianuscolor is so variable that cranial features were relied on almost exclusively for the final diagnosis.
Among the subspecies ofDipodomys ordiithere is relatively little variation in the length of the head and body. The smallest measurement is 95.5 mm. inD. o. idoneusand the largest is 118.3 mm. inD. o. richardsoni. The shortest tail is found to be 112.0 mm. inD. o. celeripesand the longest is 154.7 mm. inD. o. terrosus. The length of the hind foot varies from 35.0 mm. inD. o. idoneusto 44.5 mm. inD. o. nexilis.
Allen's Rule is not operative in the speciesD. 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 subspeciesD. o. terrosusranges farthest north and has the longest tail, whereasD. 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 inD. o. idoneuswhich is at the extreme south of the range of the species, whereas the longest hind foot is found inD. o. nexiliswhich 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 inD. 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. compactushas a short tail (117.0 mm.) but a medium-sized hind foot.D. o. nexilisandD. o. terrosushave 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. inD. o. attenuatusto 41.3 mm. inD. 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.
Cranial indices (breadth across bullae/length of skull × 100) as established for random samples of the different species of the genus(exclusive ofD. ordii) ranged from 60.8 to 67.6. In the subspecies ofD. ordiithe same index varies from 59.7 to 65.2 with an average of 63.4. In other words, the degree of specialization indicated by this one index, in a few subspecies ofD. ordii, is almost as great as that inDipodomys deserti, which on the basis of total morphology appears to be the most specialized species in the genus. Also, on the basis of this same index, some subspecies ofD. ordiiare more generalized than is any other species in the genus.
There is a general tendency for the nasals to decrease in length and the rostrum to decrease in width as the southern limits of the range ofD. ordiiare approached. In ascertaining the decrease in length of the nasals an index was obtained as follows: nasals/interorbital width × 100 (seeTable 4). The width of the rostrum, however, does not decrease in the same degree, nor at the same rate, as does the length of the nasals. This decrease in length of the nasals and in width of the rostrum may be correlated with the mean annual relative humidity of the environment. It is known (Howelland Gersh, 1936:8) that desert rodents, more exactly kangaroo rats, have a water retention mechanism in the kidneys and walls of the urinary bladder which enables them more efficiently to conserve metabolic water. The significance of the decrease of the area of the nasal mucosa, which seems to be related to relative aridity, is not yet properly understood.
In no cranial feature other than shortened nasals and narrowed rostrum, doesDipodomys ordiishow a gradation such that it might be termed a cline. Other parts of the skull that were measured do not vary greatly.
Perhaps the greatest amount of variation in the skull is in features which are not readily measurable by the usual physical means. The shape and size of the pterygoid fossae vary from almost round to rather ovoid in a given series of animals from one locality; the size and configuration of the zygomatic arches vary from slender to robust and from straight to curved laterally; the size of the lacrimal processes varies much in any given series, as do also the degree of expansion of the distal end of the nasals, the convexity of the braincase and the curvature of the upper incisors. In all instances where these features varied much, one size or shape was more pronounced in the series than any other size or shape. Thus, when comparisons were made, the size and certain shapes were the criteria used in assigning the animals under consideration to a given subspecies.
Subspeciation inDipodomys ordiiseems to have been influencedby water barriers. It is known (Grinnell, 1922:28) that kangaroo rats lack the ability to swim. Large stable rivers such as the Colorado, Snake and Columbia serve as effective barriers to further dispersal of kangaroo rats. Streams that freeze over in the winter months, however, are not efficient barriers. This is indicated by the "blending" of morphological characters ofD. o. nexilisandD. o. sanrafaelialong the Green River which freezes over.
Any mountain which has vegetational belts above the Transition Life-zone would serve as a barrier to the dispersal of these animals. The Uinta Mountains, lying in an east-west direction, are interposed between the ranges ofD. o. priscusandD. o. uintensis. The high Wasatch Mountains and their associated outliers, lying in a north-south direction in Utah, serve as an efficient barrier to the east-west movement of kangaroo rats and as a result, the subspecies east of the mountain mass are remarkably different from those to the west.