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.