Henry W. Setzer

“ The indices used to determine the amount of specialization that each complex of subspecies has undergone are as follows:
The Body index (head and body/length of tail × 100) is the expression of the elongation of the tail as an organ of balance while the length of the head and body remain relatively constant. As the tail elongates the index decreases and as the tail becomes shorter the index increases. ”

“ The habitat at the type locality is such that a person would not expect it to be inhabited by kangaroo rats. The soil is a heavy clay with a generous admixture of stones but in isolated spots there are light sandy soils which should be suitable for kangaroo rats. Even so, 500 traps set in the area of the type locality over a period of two nights yielded no Dipodomys.
This subspecies of Dipodomys ordii, inhabiting west-central Colorado and southeastern Utah, is darker than any other subspecies with which its range comes in contact. ”

“ Animals from the Organ Mountains near Globe Springs, 20 miles north of Cliff, New Mexico, and those labeled with reference to Van Horn, Texas, on the average are darker, have a wider expanse across the auditory bullae, a wider interorbital region and, in most specimens, more distally flared nasals than the norm of other populations. This deviation from the normal is interpreted not as intergradation with any other subspecies but rather as individual variation in a given population. ”

“ 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 [Pg 501] 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. ”

“ 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. ”

“ The subspecies closest, geographically and morphologically, is Dipodomys ordii nexilis with which D. o. longipes intergrades, in size of body and color of pelage, north of the San Juan River, Utah. This intergradation is probably the result of a migration of D. o. longipes around the head waters of, or above the place in, the river where there is constantly flowing water. It is probably impossible for these animals to cross a constantly flowing stream as wide as the San Juan is in its lower reaches. ”

“ No specimens are available from the southeastern part of the range of D. o. chapmani. Animals from the northwestern part of the range of D. o. ordii, without exception, have dark-colored pelage which is characteristic of D. o. chapmani. In the shape of the nasals, vault of the braincase, interorbital width and auditory bullae, animals from Rice, Arizona, resemble D. o. chapmani, but in other characters resemble D. o. ordii to which subspecies they are referred. Animals from 20 miles northeast of Calva, Arizona, are dark-colored and are intermediate between the dark D. o. ”

“ 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. ”

“ Animals from 16 miles northwest of Moab, Utah, and from three localities in Colorado, even though intergrades with D. o. nexilis, are all referable to D. o. sanrafaeli. It appears that the Green River does not act as a complete barrier in this area since in the winter it occasionally freezes over, thus allowing the animals to cross. It is thought that kangaroo rats do not hibernate but remain more or less active throughout the winter. Man-made conveniences, such as bridges, might also serve as means of dispersal, permitting these animals to cross otherwise prohibitive barriers. ”

“ 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. ”

“ As the hind foot elongates the index will increase; elongation of the hind foot is interpreted as a specialization.
The Cranial index (breadth across bullae/length of the skull × 100) reflects the degree of development of the tympanic or mastoid region, or both, and is thought to be an adaptation for more acute hearing and possibly for more delicate balance. Inflation of the tympanic bullae is thought to be a specialization. As the auditory bullae become more inflated, the index increases toward 100. ”

“ 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. ”

“ Diagnosis.—Size small (see measurements) . Color dark, entire dorsal surface (a) between Cinnamon-Buff and Clay Color, purest on sides, flanks and cheeks, with heavy admixture of black in upper parts; arietiform markings, pinnae of ears, plantar surfaces of hind feet, dorsal and ventral stripes of tail blackish. Skull small; rostrum short and narrow; interorbital region moderately wide; interparietal large, spatulate anteriorly; lacrimal processes relatively large; braincase vaulted; zygomatic arches robust and straight; auditory bullae relatively, as well as actually, greatly inflated. ”

“ In a series of nine specimens from near Casas Grandes Viejo, Chihuahua, four individuals are typically D. o. ordii, four resemble an apparently undescribed form and one specimen is intermediate between D. o. ordii and the unnamed subspecies. This unnamed race differs from D. o. ordii in having darker pelage, darker dorsal stripe on tail, larger body, wider interorbital region, longer skull, greater breadth across the bullae, less vaulted braincase, more robust zygomatic arches and the foramen magnum more deeply notched on both the dorsal and [Pg 532] ventral rims. ”

“ Texline, Texas, the animals show intergradation in the length and shape of the nasals and degree of convexity of the cranium but are referable to D. o. richardsoni.
In fine, intergradation occurs at all points where the range of D. o. richardsoni touches that of any other geographic race. No one series of it is as uniform as are most series of specimens of other known races. Dipodomys ordii richardsoni shows a mixture of characters. ”

“ Size smaller; color darker; skull smaller; rostrum shorter and narrower; interorbital region narrower; auditory bullae less inflated laterally, posteriorly and ventrally; interparietal region larger; lacrimal processes smaller; foramen magnum with ventral notch as opposed to rounded ventrally; pterygoid fossae circular as opposed to subcircular; palate longer and narrower.
From Dipodomys ordii cinderensis, D. o. utahensis differs as follows: Size larger, except hind foot which is shorter; color somewhat lighter, more reddish; skull larger; rostrum longer; nasals longer; braincase more vaulted ”

“ 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 in Dipodomys ordii seems to have been influenced [Pg 504] by 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. ”

“ 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. ”

“ 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." ”

“ 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. ”

“ Six different complexes (groups) of subspecies of D. ordii have probably arisen as a result of geographical separation.
The Great Plains complex consisting of D. o. richardsoni, D. o. oklahomae, D. o. evexus, D. o. terrosus, D. o. luteolus, D. o. priscus [Pg 505] and D. o. medius are, with the exception of D. o. priscus, inhabitants of the high plains grassland habitat. D. o. priscus inhabits the red Desert of Wyoming.
The Gulf Coast complex, comprising D. o. sennetti and D. o. compactus are separable from all others by small auditory bullae and short tail. D. o. ”

“ 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. ”