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 masseteric knob, 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 assumed 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. Pocock (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.” Pocock (op. cit.), correctly in my opinion, then established a supraspecific classification of the sciurids based almost exclusively on the structure of the baculum and glans penis. 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 Pocock (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 Pocock (1923:239-240) primarily on differences in the structure of the baculum. I assign Tamias to the tribe Marmotini. I assign 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.