Wood (1935:155), on the basis of structure of the teeth, listed the
species which he examined in the following increasing order of
specialization: _Dipodomys compactus_ (now _Dipodomys ordii compactus_),
_D. nitratoides_, _D. merriami_, _D. ordii_, _D. agilis_, _D.
herrmanni_, _D. spectabilis_, and _D. deserti_. This arrangement is at
variance with that of Grinnell (1922) who listed the species in order of
increasing specialization as: _Dipodomys herrmanni_, _D. panamintinus_,
_D. ingens_, _D. spectabilis_, _D. merriami_, _D. nitratoides_, _D.
ordii_, _D. agilis_, _D. venustus_, _D. microps_ and _D. deserti_. As
noted, the only agreement between the two arrangements is the placing of
_D. deserti_ as the most specialized. Relying on skeletal indices alone,
I would accord the same position to _D. deserti_ but would not arrange
the other species as have either Wood or Grinnell.
In this study, the amount of specialization of each species, as
indicated by the skeleton, was determined by assigning consecutive
numbers from 1 to 11 to each species in its place in each index, and
then totaling and averaging these arbitrary numbers (Table 3). It will
be noted that there is a tendency for each species to occupy the same
relative position in each of the indices.
It is felt, however, that a more nearly correct arrangement, according
to degree of specialization, is obtained by using the six skeletal
indices plus the information obtained from the study of the viscera. On
this basis the species may be arranged from least to most specialized as
follows: _Dipodomys ordii_, _D. microps_, _D. panamintinus_, _D.
agilis_, _D. herrmanni_, _D. ingens_, _D. spectabilis_, _D. phillipsii_,
_D. merriami_, _D. nitratoides_ and _D. deserti_.
Grinnell (1922:95-96) arranged the Recent species of _Dipodomys_ in nine
groups. Davis (1942:332) also proposed an arrangement of nine groups in
which he combined the Compactus and Ordii groups of Grinnell,
established a new Elator group by removing _Dipodomys elator_ from
Grinnell's Phillipsii group, and in linear arrangement, Davis shifted
the Spectabilis and what remained of the Phillipsii groups to new
positions. Burt (1936:152) arranged Grinnell's groups into three
(unnamed) groups solely on the basis of the structure of the baculum. In
the arrangement proposed by Grinnell, two of his nine groups contained
only one species each, one other, the Microps group, has since been
shown to contain only one species and another, the Compactus group,
contained only kinds which are, by me, regarded only as subspecies of
_Dipodomys ordii_. To my mind neither Davis nor Burt added to or
fundamentally changed the basic concepts as set forth in 1922 by
Grinnell. Owing to the paucity of material at that time, especially
from areas of intergradation, Grinnell's groupings and arrangement were
as nearly natural as could be expected. With the accumulation of
additional material and with the knowledge that certain kinds treated by
Grinnell as full species are in actuality subspecies, it is felt that
the several species of kangaroo rats can best be arranged in six groups
which, from the least to the most specialized, are as follows:
ORDII GROUP.--Composed of the subspecies of _Dipodomys ordii_ and
_Dipodomys microps_. Grinnell placed these two species in separate
groups; Burt on characters of the baculum alone placed _D. microps_
with _Dipodomys deserti_ and _Dipodomys spectabilis_. Within the
single species _D. ordii_, I find that the difference in shape and
size of the baculum between the subspecies of _D. ordii_ is as
great as the difference which Burt (1936:154-155) found between the
full species _D. agilis_ and _D. microps_. The characters of the
baculum are an aid, but not in and of themselves an adequate basis,
for determining the natural relationships of the groups of species.
Certainly the remainder of the morphological differences between
_D. deserti_ and _D. microps_ are so great that I doubt that the
similarity in the baculum is significant, at least in this one
instance. The chisel-shaped lower incisors of _D. microps_ appear
to be a specialization. They may enable _D. microps_ to utilize
more woody types of vegetation than can _D. ordii_. Both species
occupy the same territory over much of their geographic range,
probably because they eat different kinds of food.
PANAMINTINUS GROUP.--Composed of all the now known subspecies of
_Dipodomys panamintinus_ and the species _Dipodomys stephensi_, if
the latter is a full species. This group was included by Grinnell
in the Heermanni group, with which it agrees in broadness of the
maxillary arches and the configuration of the penis bone, but on
the basis of the degree of specialization, as indicated by the
indices (see Table 1), I feel that the Panamintinus group is more
properly placed after the Ordii group and should be separated from
the Heermanni group. Actually, animals in the Panamintinus group
are intermediate between those of the Ordii and Heermanni groups.
HEERMANNI GROUP.--Composed of the subspecies of _Dipodomys
heermanni_ and _Dipodomys agilis_, the species _Dipodomys ingens_,
_Dipodomys venustus_ and _Dipodomys elephantinus_. _D._ _ingens_
even though larger in linear measurements than any of the other
kinds included in this group, has almost the same degree of
specialization as does _D. heermanni_. _D. agilis_, even though
somewhat less specialized than the other kinds placed in this
group, by the general nature of the indices, by the form of the
visceral mass and to some degree by the shape of the baculum, shows
itself properly to belong with this group. The species _D.
venustus_, judged by characters of the visceral anatomy, also
belongs here rather than with some other group or as a separate
group. From the appearance of the visceral mass, _D. venustus_ is
somewhat more specialized than either _D. heermanni_ or _D.
agilis_, but _D. venustus_ does show its affinities with this
group. The species _D. elephantinus_ has not been examined as
thoroughly as have the other species but the external morphology
and the configuration of the cranium place it with this group.
SPECTABILIS GROUP.--Composed of the subspecies of _Dipodomys
spectabilis_. In two of the six indices, _D. spectabilis_ shows a
high degree of specialization toward saltation, but in the other
four indices it shows a relatively low degree of specialization or
is average for the genus. Burt (1936:155) placed _D. spectabilis_
with _D. deserti_ on the basis of the baculum alone. I have not
examined _D. nelsoni_ but place it with this group as did also
Grinnell and Davis.
MERRIAMI GROUP.--Composed of the subspecies of _Dipodomys
merriami_, _Dipodomys nitratoides_ and _Dipodomys phillipsii_, and
the species _Dipodomys platycephalus_, _Dipodomys margaritae_,
_Dipodomys insularis_, _Dipodomys mitchelli_, _Dipodomys ornatus_
and _Dipodomys elator_. I have not examined five of these species.
However, the indices and characters of the viscera indicate that
the three species first mentioned are closely allied. 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.
DESERTI GROUP.--Composed of _Dipodomys deserti_ which has only two
subspecies. In all morphological respects, _D. deserti_ is the
most specialized species in the genus as shown by the reduced
number (4) of toes on the hind foot, the bilateral arrangement of
the viscera, the extreme development of the auditory region of the
skull and by developing, early in life, the hiatus in the enamel
wall of each molariform tooth.
The parallel arrangement below emphasizes the differences and
similarities between Grinnell's (1922) arrangement and the one proposed
in the present paper.