_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.
Cranial indices (breadth across bullae/length of skull × 100) as
established for random samples of the different species of the genus
(exclusive of _D. ordii_) ranged from 60.8 to 67.6. In the subspecies of
_D. ordii_ the 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 of _D. ordii_, is almost as great as that in
_Dipodomys 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 of _D. ordii_ are 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 of _D.
ordii_ are approached. In ascertaining the decrease in length of the
nasals an index was obtained as follows: nasals/interorbital width × 100
(see Table 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 (Howell and 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,
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 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 in _Dipodomys ordii_ seems to have been influenced 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. This is indicated by the "blending" of morphological
characters of _D. o. nexilis_ and _D. o. sanrafaeli_ along 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 of _D. o. priscus_ and _D. 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.