Genus #Eutamias# Trouessart
_Eutamias_ Trouessart, E. L. Catal. Mamm. viv. et foss., Rodentia, in Bull. Soc. d"Etudes Sci. d"Angers, 10:86-87, 1880. Type _Sciurus striatus asiaticus_ Gmelin.
_Eutamias_, Merriam, C. H., Proc. Biol. Soc. Washington, 11:189-190, July 1, 1897.
_Eutamias_, Howell, A. H., N. Amer. Fauna, 52:26, November 30, 1929.
_Tamias_, Ellerman, J. R., The families and genera of living rodents.
British Mus. (Nat. Hist.), 1:426, June 8, 1940.
_Tamias_, Bryant, M. D., Amer. Midland Nat., 33:732, March 1945.
_Diagnosis._--Skull lightly built, narrow; pos...o...b..tal process light and weak; lacrimal not elongated; infraorbital foramen lacks ca.n.a.l, relatively larger than in most sciurids; P3 present; head of malleus not elongated; plane of manubrium of malleus 90 degrees to plane of lamina; hypohyal and ceratohyal bones of hyoid apparatus fused in adults; conjoining tendon between anterior and posterior sets of digastric muscles ribbonlike; keel on dorsal side of tip of baculum; tail more than 40 per cent of total length; five longitudinal dark stripes evenly s.p.a.ced and subequal in width; two lateral dark stripes short.
Subgenus #Eutamias# Trouessart
_Eutamias_ Trouessart, E. L. Catal. Mamm. viv. et foss., Rodentia, in Bull. Soc. d"Etudes Sci. d"Angers 10:86-87, 1880. Type _Sciurus striatus asiaticus_ Gmelin.
_Eutamias_, Howell, A. H., N. Amer. Fauna, 52:26, November 30, 1929.
_Eutamias_, Ellerman, J. R., The families and genera of living rodents.
British Mus. (Nat. Hist.), 1:426, June 8, 1940.
_Eutamias_, Bryant, M. D., Amer. Midland Nat. 33:732, March 1945.
_Diagnosis._--Size large; lambdoidal crest moderately developed; supraorbital notches distinctly anterior to posterior notch of zygomatic plate; baculum with faint keel on dorsal surface of tip which curves upward; pelage coa.r.s.e; ears broad, rounded, of medium height.
_Geographic range._--Palearctic. West to Dvina and Kama rivers, Vologda, and Kazan, in European Russia. South to southern Ural Mountains, Altai Mountains; Kansu, Szechwan, Shensi, Shansi, and Chihli provinces of China; Manchuria and Korea. East to Hokkaido Island, j.a.pan; Kunashiri Island, southern Kurile Islands; Sakhalin Island, and Yakutsk, Siberia.
North nearly to Arctic Coast in Siberia and European Russia (Ellerman and Morrison-Scott 1951:503).
Subgenus #Neotamias# Howell
_Neotamias_ Howell, A. H., N. Amer. Fauna, 52:26, November 30, 1929.
Type, _Eutamias merriami_ J. A. Allen [=_Tamias asiaticus merriami_ J. A. Allen].
_Neotamias_, Ellerman, J. R., The families and genera of living rodents.
British Mus. (Nat. Hist.), 1:426, June 8, 1940.
_Neotamias_, Bryant, M. D., Amer. Midland Nat., 33:372, March, 1945.
_Diagnosis._--Size small to medium; lambdoidal crest barely discernible; supraorbital notches even with, or posterior to, posterior notch of zygomatic plate; baculum with distinct keel on dorsal surface of tip which curves upward; pelage silky; ears long and pointed.
_Geographic range._--Western Nearctic. West to Pacific Coast. South to Lat. 2030" in Baja California and to northwestern Durango and southeastern Coahuila, Mexico. East to eastern New Mexico, westernmost Oklahoma, eastern Colorado, Wyoming, northwestern Nebraska, western and northwestern South Dakota, western and northwestern North Dakota, northeastern Minnesota, northern Wisconsin and Upper Peninsula of Michigan, and eastern Ontario. North to southwestern sh.o.r.e of Hudson Bay, southern sh.o.r.e of Great Slave Lake and Yukon River, Yukon.
Genus #Tamias# Illiger
_Tamias_ Illiger, J. K. W., Prodromus Syst. Mam. Avium, pp. 83, 1811.
Type, _Sciurus striatus_ Linnaeus.
_Tamias_, Howell, A. H., N. Amer. Fauna, 52:26, November 30, 1929.
_Tamias_, Ellerman, J. R., The families and genera of living rodents.
British Mus. (Nat. Hist.), 1:426, June 8, 1940.
_Tamias_, Bryant, M. D., Amer. Midland Nat. 33:372, March, 1945.
_Diagnosis._--Skull lightly built, narrow; pos...o...b..tal process small and weak; lacrimal not elongated; infraorbital foramen lacks ca.n.a.l, relatively larger than in most sciurids; P3 absent; head of malleus elongated; plane of manubrium of malleus forms 60 degree angle with plane of lamina; hypohyal and ceratohyal bones of hyoid apparatus fused in adults; conjoining tendon of anterior and posterior digastric muscles rounded in cross section; keel on ventral surface of tip which curves upward in baculum; tail less than 38 per cent of total length; five longitudinal dark and four longitudinal light stripes present but two dorsal light stripes at least twice as broad as other stripes; four lateral dark stripes short.
_Geographic range._--Eastern Nearctic. West to Turtle Mountains, North Dakota; eastern North Dakota, eastern South Dakota, Nebraska, Kansas, and Oklahoma. South to southern Louisiana, Mississippi, Alabama, northwestern Georgia. East to Atlantic Coast from South Carolina to Nova Scotia. North to northeastern Quebec and southern tip of Hudson Bay.
DISCUSSION
Chipmunks are small striped squirrels that inhabit the Holarctic Realm and that are found in similar niches in each of the three regions: Palearctic, western Nearctic, and eastern Nearctic. Ellerman (1940) and Bryant (1945) placed the chipmunks in three subgenera, corresponding to the regions mentioned above, under the one genus _Tamias_. Critical examination of new and old evidence reveals, nevertheless, that the subgenera _Eutamias_ and _Neotamias_ of the genus _Eutamias_ are more closely related to one another than either is to the genus _Tamias_.
This relationship can be seen clearly in the structure of the malleus, baculum, hyoid apparatus, hyoid musculature, the presence or absence of P3, the projection of the anterior root of P4 in relation to the ma.s.seteric k.n.o.b, and in the color pattern.
Because the genera _Eutamias_ and _Tamias_ occupy similar ecological niches, the structural similarities that permit these animals to be called chipmunks, show convergence, and thus can be a.s.sumed to be adaptive. These similarities are in the molars, in shape of the skull, in color pattern and in other features which have been used by many systematists to interpret the phylogenetic relationships of the squirrels. Poc.o.c.k (1923:211), however, reviewed the taxonomic literature on sciurids and wrote: "The conclusion very forcibly suggested by the literature of the subject is the untrustworthiness of such characters."
Poc.o.c.k (_op. cit._), correctly in my opinion, then established a supraspecific cla.s.sification of the sciurids based almost exclusively on the structure of the baculum and glans p.e.n.i.s. I have studied the baculum in chipmunks and in all the major supraspecific groups of Nearctic squirrels. The bacula of the Nearctic squirrels and those of the Palearctic and Indian squirrels, other than the chipmunks, are described and figured by Poc.o.c.k (_op. cit._).
The baculum in _Eutamias_, in general plan of structure, resembles the baculum in the genera _Callosciurus_, _Menetes_, _Rhinosciurus_, _Lariscus_, _Dremomys_, and _Nannosciurus_, of the tribe Callosciurini Simpson. The baculum in _Tamias_, in general plan of structure, resembles that in _Spermophilus_ (=_Citellus_) and _Cynomys_ of the tribe Marmotini Simpson. These tribes, designated by Simpson (1945:79), are based on the corresponding subfamilies defined by Poc.o.c.k (1923:239-240) primarily on differences in the structure of the baculum.
I a.s.sign _Tamias_ to the tribe Marmotini. I a.s.sign _Eutamias_ to the tribe Callosciurini, but do so only tentatively because I have not, at first hand, studied the bacula of most of the Callosciurini. The fossil record is too incomplete to reveal the time when the two tribes diverged. The subgenera _Eutamias_ and _Neotamias_ are closely related.
Indications are that the divergence of the two subgenera occurred, geologically, but a short time ago, possibly in Pleistocene time.
CONCLUSIONS
1. _Eutamias_ and _Tamias_ are distinct genera of chipmunks.
2. The subgenera _Eutamias_ and _Neotamias_ are valid, for, _Eutamias sibiricus_ differs from all the species of the subgenus _Neotamias_ to a greater degree than these species differ from one another.
3. The genera _Eutamias_ and _Tamias_ probably evolved from two distinct lines of sciurids; one line (_Eutamias_) is represented by the tribe Callosciurini, and the other (_Tamias_) by the tribe Marmotini.
LITERATURE CITED
CATESBY, M.
1743. The natural history of Carolina, Florida, and the Bahama Islands, etc., 2:i-xliv, 1-20, 1-100.
ELLERMAN, J. R.
1940. The families and genera of living rodents. British Mus. (Nat.
Hist.), Vol. 1, pp. xxvi + 689, 189 figs., June 8.