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Width Length Least Nasals
of of interorbital ------------
rostrum nasals orbital Interorbital
width × 100
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terrosus 4.1 14.75 13.5 91.6
luteolus 4.35 13.9 12.95 93.1
evexus 4.3 14.35 13.75 94.8
montanus 4.1 13.5 12.65 93.8
ordii 3.5 13.3 13.0 97.7
idoneus 3.7 13.2 13.75 103.5
palmeri 3.3 12.8 13.0 101.1
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The first three columns represent the actual measurements of the
various elements; the fourth column is the index established.
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_ 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. compactus_ probably has differentiated from _D. o.
sennetti_ since the cutting off, by wave action, from the mainland, of
the islands on which _D. o. compactus_ lives.
The Mexican complex consisting of _D. o. obscurus_, _D. o. fuscus_, _D.
o. idoneus_ and _D. o. palmeri_ have probably differentiated by natural
selection acting on fortuitous variations, but I lack first hand
knowledge of the region concerned.
The southwestern complex consists of _D. o. chapmani_, _D. o.
extractus_, _D. o. attenuatus_ and _D. o. ordii_. _D. o. attenuatus_ and
_D. o. chapmani_ are subspecifically distinct owing to geographic
isolation, although both kinds show intergradation where their ranges
approach that of _D. o. ordii_.
The western desert complex, composed of _D. o. monoensis_, _utahensis_,
_cineraceus_, _columbianus_, _cinderensis_, _fetosus_, _celeripes_,
_marshalli_, _inaquosus_, _pallidus_, _panguitchensis_ and _fremonti_
was isolated from the other complexes of _D. ordii_ by the Quaternary
uplift of the Wasatch Mountain mass, consisting of the Wasatch, Fish
Lake and San Pitch mountains and the Wasatch, Aquarius, Paunsaugunt and
Kaiparowits plateaus, and the concurrent reëstablishment of drainage
systems. The drainages are those of the Colorado and Columbia rivers and
that of the Snake River from Blackfoot, Idaho, to the junction with the
Columbia. _D. o. fremonti_ has been isolated on the upper reaches of the
Fremont River which arises from the eastern side of the Wasatch Divide.
_D. o. panguitchensis_ has been isolated in Panguitch Valley as a result
of the canyons formed by the Sevier River in Utah. _D. o. cineraceus_,
although its subspecific and insular status are in doubt, appears to
have been isolated on Dolphin Island, Great Salt Lake, Utah.
The intermontane complex consisting of _D. o. montanus_, _longipes_,
_cupidineus_, _nexilis_, _sanrafaeli_ and _uintensis_, like the western
desert complex, has become separated from the remainder of the
subspecies of the species _D. ordii_ by Quaternary geologic events. _D.
o. cupidineus_ has been cut off by the gorges of the Colorado River to
the south and the Virgin River to the north. _D. o. sanrafaeli_ is
separated from _D. o. uintensis_ by the Tavaputs Plateau and by the Roan
and Book cliffs, and is separated from the range of _D. o. nexilis_ by
the Colorado River although there is intergradation between _D. o.
nexilis_ and _D. o. sanrafaeli_. _D. o. longipes_ has been separated
from the rest of this intermontane complex by the San Juan and Colorado
rivers, but to the east it intergrades freely with adjacent subspecies.
_D. o. montanus_ has been relatively isolated in the San Luis Valley of
Colorado and New Mexico, but in the southern part of its range it does
show intergradation with other subspecies.
[Illustration: FIG. 22. An arrangement, according to morphological
indices, of the subspecies of _Dipodomys ordii_.]
The complexes mentioned above are represented graphically in Figure 22,
in a way that expresses some of my ideas as to their genetic
relationships.
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 Pedal index (hind foot/head and body × 100) is the expression of the
development of the hind foot as an element essential for the saltatorial
habit. 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.
The Bullar index (width of maxillary arches/breadth across bullae × 100)
also expresses the degree of inflation of the auditory bullae. In a
generalized mammal, at least in the heteromyids, the index would be 100,
but as the auditory bullae become larger the index will decrease from
100.
In attempting to arrange the subspecies of _D. ordii_ according to
degree of specialization, the geographic positions of the subspecies
have been considered along with the information provided by the
above-mentioned indices. These indices were used in the same way as were
the indices for the species of the genus. In Tables 5 and 6 and in the
accounts and maps the subspecies are arranged from the least specialized
to the most specialized.