Dipodomys ordii

“ 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. ”

“ 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. ”

“ In no cranial feature other than shortened nasals and narrowed rostrum, does Dipodomys ordii show 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 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. ”

“ 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. ”

“ In the degree of expansion of the auditory bullae, the width of the rostrum, the length of the nasals and interorbital width, animals from 5 miles northeast of Winona (2 specimens) are within the range of individual variation shown by D. o. longipes. The color is darker, being more as in Dipodomys ordii chapmani. Specimens examined [Pg 558] from 10 miles north of Angell, Springerville, Tanner Tank and Cedar Ranch Wash, Arizona (listed by Goldman as referable to D. o. cleomophila) , and those from Wupatki Ruins, Arizona, agree with D. o. ”

“ 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 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. ”