The following paragraphs treat the characters listed by Howell,
Ellerman, and Bryant, and such additional characters as I have found
useful in characterizing the genera and subgenera of chipmunks. Some of
the findings, I think, illustrate how study of such mammalian structures
as the baculum, malleus, and hyoid apparatus--structures that seem to
be little influenced by the changing external environment--clarifies
relationships, if these previously were estimated only from other parts
of the anatomy of Recent specimens.
The structural features and characters to be discussed, or listed, below
may be arranged in three categories as follows: 1) Characters in which
the subgenera _Eutamias_ and _Neotamias_ agree but are different from
the genus _Tamias_; 2) Characters in which the subgenus _Eutamias_
and the genus _Tamias_ agree but are different from the subgenus
_Neotamias_; 3) Structural features that are too weakly expressed to
be of taxonomic use.
CHARACTERS IN WHICH THE SUBGENERA EUTAMIAS AND NEOTAMIAS AGREE, BUT
DIFFER FROM THE GENUS TAMIAS
_Structure of the Malleus._--The malleus in chipmunks is composed of
a head and neck, a manubrium which has a spatulate process at the end
opposite the head, and a muscular process situated about halfway between
the spatulate process and the head of the malleus. An articular facet
begins on the manubrium near the neck and spirals halfway around the
head of the malleus. A lamina extends from the anterior edge of the head
and neck, tapers to a point and joins the tympanic bulla anteriorly
where there is a suture between the lamina and bulla. The lamina is one
half as long as the rest of the malleus (see figs. 1-3).
The head of the malleus in _Tamias_ is clearly more elongated than in
_Eutamias_. The plane formed by the lamina in _Eutamias_ makes an angle
of approximately 90 degrees with the plane formed by the manubrium; in
_Tamias_ the two planes make an angle of approximately 60 degrees.
Examination of series of mallei of _Eutamias_ and _Tamias_ indicate that
there is slight individual variation, slight variation with age, and no
secondary sexual variation. Intraspecific variation in the subgenus
_Neotamias_ is slight, consisting of differences in size. Specimens
of the subgenus _Eutamias_ from Manchuria have mallei which are
morphologically close to the mallei of the subgenus _Neotamias_.
[Illustration: FIGS. 1-3. Dorsomedial views of left malleus.
FIG. 1. _Tamias striatus lysteri_, No. 11920 sex?;
from Carroll Co., New Hampshire.
FIG. 2. _Eutamias sibiricus asiaticus_, No. 199637 male NM;
from I-mien-po, N. Kirin, Manchuria.
FIG. 3. _Eutamias townsendii senex_, No. 165 male;
from Lake Tahoe, California.]
_Structure of the Baculum._--In discussing the baculum in _Eutamias_ and
_Tamias_, it seems desirable to do so in the light of the structure of
the baculum in other sciurids.
The bacula of North American sciurids are divisible into six distinct
types represented by those of the genera _Spermophilus_, _Marmota_,
_Sciurus_, _Tamiasciurus_, _Eutamias_, and _Glaucomys_.
The type of baculum in _Spermophilus_ is spoonshaped with a ventral
process that is spinelike or keellike. Also, spines usually are present
along the margin of the "spoon." The base (proximal end) of the baculum
is broad, and some species have a winglike process extending dorsally
and partly covering a longitudinal groove. The shaft is more or less
curved downward in the middle (see figs. 7, 10).
In _Marmota_ the baculum is greatly enlarged at the posterior end and
forms a shieldlike surface. The ventral surface of the base is flattened
and the ventral surface of the shaft curves slightly ventrally then
dorsally to the tip. The dorsal region of the base culminates in a
point, from which there is a ridge that extends anteriorly and that
tapers rapidly into the shaft near the tip. The tip, dorsally, has a
slight depression surrounded by knobs, which are more or less well
defined, and which resemble, topographically, the spines described for
_Spermophilus_ (see fig. 8).
In _Sciurus_ the baculum is semispoonshaped and asymmetrical. There
is a winglike process on one side and a spine, which projects
lateroventrally, on the other side of the tip. The base of the baculum
is broad but not so broad as in most species of _Spermophilus_.
Extending posteriorly from the region of the tip, at which point a
spine projects lateroventrally, there is a ridge, which is often
partly ossified and that extends to a point near the base (see fig. 4).
In _Tamiasciurus_ the baculum is absent or vestigial (Layne,
1952:457-459).
In _Eutamias_ the baculum is broad at the base and the shaft tapers
distally to the junction of the shaft and tip, or the base is only
slightly wider than any part of the shaft. The tip often forms an abrupt
angle with the shaft and there is a keel on the dorsal surface of the
tip (see figs. 5, 6).
The baculum in _Glaucomys_ is the most distinctive of that of any
American sciurid. According to Pocock (1923:243-244), "The baculum [of
_G. volans_] is exceedingly long and slender, slightly sinuous in its
proximal third, and inclined slightly upwards distally. The extreme apex
is bifid, the lower process being rounded, the upper more pointed. On
the left side there is a long crest running from the summit of the
upper terminal process and ending abruptly behind the left side about
one-third of the distance from the proximal end of the bone. It lies
over a well-marked groove, and there is a second shallower groove on the
right side of the bone." The baculum of _G. sabrinus_ is markedly wider,
more flattened and shorter than in _G. volans_. The crest, which is also
present in _G. volans_, starts from the upper terminal process and
extends to the base of the baculum on the left side. There is a knoblike
process on the crest at a point three fourths the length of the baculum
from its base. The distal one third of the baculum curves sharply but
smoothly upwards (see fig. 9).
Keeping in mind that the baculum in the North American sciurids can be
classified into six structural groups, as given above, the baculum in
each of the subgenera _Eutamias_ and _Neotamias_ and in the genus
_Tamias_ is briefly described.
In the subgenus _Neotamias_ the baculum resembles a leg and foot of man,
with a narrow ridge (keel) in the center of the "instep" of the foot
(Howell 1929:27). The tip (=foot) curves dorsally at the distal end
(see figs. 5, 6).
[Illustration: FIGS. 4-10. Lateral views of right side
(except left-lateral view in fig. 9) of baculum.
FIG. 4. _Sciurus aureogaster aureogaster_,
No. 37000; from 70 km. S C. Victoria (by highway), and 6 km. W of
highway, Tamaulipas.
FIG. 5. _Eutamias quadrimaculatus_, No. 95780 BS;
from Mountains near Quincy, Plumas Co., California.
FIG. 6. _Eutamias sibiricus asiaticus_, No. 199632 NM;
from 120 mi. up the Yalu River, Korea.
FIG. 7. _Tamias striatus lysteri_, No. 193493 NM;
from Locust Grove, New York.
FIG. 8. _Marmota flaviventer dacota_, No. 41641;
from 1-1/2 mi. E Buckhorn, 6,150 ft., Weston Co., Wyoming.
FIG. 9. _Glaucomys sabrinus bangsi_, No. 15079;
from 10 mi. NE Pinedale, 8,000 ft., Sublette Co., Wyoming.
FIG. 10. _Spermophilus armatus_, No. 14888;
from W end Half Moon Lake, 7,900 ft., Sublette Co., Wyoming.]
In the subgenus _Eutamias_, the baculum "tapers gradually from base
to tip, the distal portion upturned in an even curve and slightly
flattened ..." (_op. cit._:26). Microscopic examination reveals that
there is a faint keel on the dorsal surface of the tip.
_Eutamias_, like _Callosciurus_, _Menetes_, _Dremomys_, _Lariscus_,
_Rhinosciurus_, and _Nannosciurus_, has a keel on the dorsal surface of
the tip of the baculum (compare figures 5 and 6 with the descriptions
and figures in Pocock, 1923:217-225).
In _Tamias_ the baculum is "a slender bone 4.5-5 millimeters in length,
nearly straight, upturned at the tip and slightly expanded into the
shape of a narrow spoon or scoop, with a slight median ridge on the
under surface." (Howell _op. cit._:13.) The "median ridge" is a keel on
the ventral surface. In having a keel on the ventral surface of the tip,
the baculum of _Tamias_ is comparable to that of _Spermophilus_.
Examination of series of bacula of the subgenus _Neotamias_ and the
genus _Tamias_ indicates, as in the case of the mallei, that there is
slight individual variation and slight variation with age. In the
subgenus _Neotamias_ interspecific variation in the baculum is
considerable, but the general plan of structure remains constant. From
this study of variation of the baculum in American chipmunks, it can be
extrapolated that the baculum in the Asiatic _Eutamias_ would show
little individual variation in structure. I have seen only two bacula
of the Asiatic _Eutamias_.
[Illustration:
FIGS. 11-12. Ventral views of the hyoid apparatus in
_Tamias_ and _Eutamias_.
FIG. 11. _Tamias striatus venustus_, No. 11072 female;
from Winslow, Washington Co., Arkansas.
FIG. 12. _Eutamias minimus operarius_, No. 5376 male;
from 14 mi. N El Rito, Rio Arriba Co., New Mexico.]
_Structure of the Hyoid Apparatus._--The hyoid apparatus in the
chipmunks is made up of an arched basihyal with a thyrohyal attached to
each limb of this "arch." To each junction between the "arch" and the
thyrohyals, a hypohyal is attached by ligaments to a flat articular
surface. A ceratohyal then is attached posteriorly to the hypohyal and
a stylohyal ligament is attached to each ceratohyal posteriorly. The
stylohyal is loosely attached along its sides to the tympanic bulla and
finally attached, at the posterior end, to the bulla at a point slightly
ventral and posterior to the auditory meatus.
In the genus _Eutamias_ the hypohyal and ceratohyal are completely fused
in adults, the suture between these two bones being visible in juvenal
specimens (see fig. 12).
In the genus _Tamias_ the hypohyal and ceratohyal remain distinct
throughout life. The hypohyal may frequently be divided into two parts,
a variation which is also present in _Marmota_.
The musculature associated with the hyoid apparatus in _Eutamias_ and
_Tamias_ is as described by Bryant (1945:310, 316) for the Nearctic
squirrels. However, the conjoining tendon of the anterior and posterior
pairs of digastric muscles is ribbonlike in _Eutamias_ and rodlike
(rounded in cross section) in _Tamias_.
_The presence or absence of P3 and the projection of the anterior root
of P4 in relation to the masseteric knob._--Only rarely is P3 absent
in _Eutamias_ or present in _Tamias_. P3 in specimens of old adult
_Eutamias_, shows wear, thus suggesting that P3 is functional in older
chipmunks. In _Eutamias_, which normally has a P3, the anterior root of
P4 projects to the outside of the masseteric knob, whereas in _Tamias_,
which normally lacks a P3, the anterior root of P4 projects directly
to the masseteric knob or to the lingual side of this structure. The
projection of the anterior root of P4 seems to be correlated with the
presence or absence of P3. However, in a specimen of _Tamias striatus
rufescens_ (No. 11117 KU), the left P3 is present, yet the anterior
root of P4 still projects to the lingual side of the masseteric knob.
Ellerman (1940:48) and Bryant (1945:368-369, 372) think that the
presence or absence of P3 is not of generic significance in chipmunks,
since P3 is vestigial and probably is in the process of being lost, and
since this character is rarely used as a generic character in other
sciurids. I think that the presence or absence of P3, together with the
projection of the anterior root of P4 in relation to the masseteric
knob, is of generic significance, for, squirrels in general have
retained the dentition and dental formula of a primitive rodent, and
any change in the pattern of the teeth or in dental formula is, in my
opinion, of a fundamental nature.
_Length of tail in relation to total length._--The tail in _Eutamias_ is
more than 40 per cent of the total length, whereas in _Tamias_ the tail
is less than 38 per cent of the total length. In this respect _Tamias_
resembles most ground squirrels of the genus _Spermophilus_.
_Color pattern._--The chipmunks vary but little in color pattern, for,
even in _Eutamias dorsalis_, which is one of the most aberrant of the
chipmunks in color pattern, the pattern is characteristic of _Eutamias_.
The width of the longitudinal stripes is uniform in _Eutamias_ whereas
in _Tamias_ the dorsal, longitudinal light stripes are more than twice
as wide as the other stripes.
In _Eutamias_, only the two lateralmost dark stripes are short, whereas
in _Tamias_ all four of the lateral dark stripes are short; none extends
to the rump or to the shoulder.
The dark median stripe is present in both _Eutamias_ and _Tamias_ as
well as in other genera such as _Callosciurus_ and _Menetes_ (Ellerman
1940:390).
CHARACTERS IN WHICH THE SUBGENUS EUTAMIAS AND THE GENUS TAMIAS AGREE,
BUT DIFFER FROM THE SUBGENUS NEOTAMIAS
_Shape of the infraorbital foramen._--In the subgenus _Eutamias_ and
in the genus _Tamias_ the infraorbital foramen is rounded, whereas in
most species of the subgenus _Neotamias_ the foramen is slitlike. In
_Eutamias townsendii_, however, the infraorbital foramen is rounded
as much as in the subgenus _Eutamias_ and in the genus _Tamias_.
_Width of the postorbital process at base._--The postorbital process is
broader at the base in the subgenus _Eutamias_ and in the genus _Tamias_
than in most species of the subgenus _Neotamias_. In _E. townsendii_,
however, this process is relatively as broad as in the subgenus
_Eutamias_ and in the genus _Tamias_.
_Position of the supraorbital notch in relation to the posterior notch
of the zygomatic plate._--In the subgenus _Eutamias_ and in the genus
_Tamias_ the supraorbital notch is distinctly anterior to the posterior
notch of the zygomatic plate, whereas in the subgenus _Neotamias_, the
supraorbital notch is only slightly anterior to the posterior notch of
the zygomatic plate. This difference may be correlated with differences
in size, since specimens of the subgenus _Eutamias_ and the genus
_Tamias_ are larger than specimens of the subgenus _Neotamias_.
_Degree of convergence of the upper tooth-rows._--The rows of upper
cheek-teeth converge posteriorly in the subgenus _Eutamias_ and in
the genus _Tamias_, except that in some specimens of _E. sibiricus
asiaticus_ the rows of upper cheek-teeth are nearly parallel to each
other. In most species of the subgenus _Neotamias_ the rows of upper
cheek-teeth are nearly parallel to each other, although in the specimens
that I have seen of _E. townsendii_, the upper rows of cheek-teeth
converge posteriorly.
_Degree of constriction of the interorbital region._--The interorbital
region is more constricted in most species of the subgenus _Neotamias_
than in the subgenus _Eutamias_ and the genus _Tamias_. In specimens of
_E. t. townsendii_ of the subgenus _Neotamias_, however, the degree of
constriction of the interorbital region is approximately the same as in
the subgenus _Eutamias_ and the genus _Tamias_.
_Shape of the pinna._--The pinna is narrower and more pointed in the
subgenus _Neotamias_ than in the subgenus _Eutamias_ and the genus
_Tamias_.