Column headings:
A: Humeroradial
B: Intermembral
C: Crural
D: Tibioradial
E: Femorotarsalmetatarsal
F: Cranial
G: Average
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A B C D E F G
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ordii 5 1 1 1 1 5 2.33
microps 1 2 4 5 4 2 3.0
panamintinus 6 3 3 3 3 1 3.1
agilis 7 4 5 6 6 4 5.3
heermanni 3 6 7 9 5 3 5.5
ingens 4 7 2 8 2 10 5.5
spectabilis 2 11 6 11 7 8 7.5
phillipsii 11 5 10 2 11 7 7.6
merriami 10 9 9 4 10 6 8.0
nitratoides 9 8 8 7 9 9 8.5
deserti 8 10 11 10 8 11 9.6
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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 in Table 3 in 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 is _D. deserti_ nor to brush as are _D. agilis_ and
_D. venustus_. _D. merriami_ does, 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 in _D. panamintinus mohavensis_ which has small ears and
small bullae and in _D. elephantinus_ which 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.
[Illustration: 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.]
[Illustration: 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 see Fig. 11.
FIG. 17. _Dipodomys ordii oklahomae_, for data see Fig. 12.
FIG. 18. _Dipodomys ordii richardsoni_, for data see Fig. 13.
FIG. 19. _Dipodomys ordii nexilis_, for data see Fig. 14.