In California, Johnson (1943) recognized ten species of chipmunks and
assigned these to the five main groups of species which were proposed by
Howell (1929). In characterizing each species, Johnson (_op. cit._) not
only made a careful study of skins and skulls, but also employed many
ecological data.
Study of the bacula of the Californian chipmunks supports Johnson's
(_op. cit._) conclusion that there are ten species, but suggests that
there are three (not five) groups of species in California--as well as
elsewhere within the geographic range of the subgenus _Neotamias_. The
three groups are (see figs. 1-19): 1. _minimus_-group (_E. alpinus_, _E.
minimus_, _E. townsendii_, _E. sonomae_, _E. amoenus_, _E. dorsalis_,
and _E. merriami_); 2. _quadrivittatus_-group (_E. quadrivittatus_, _E.
ruficaudus_, _E. cinereicollis_, and _E. quadrimaculatus_); and 3.
_speciosus_-group (_E. speciosus_, _E. panamintinus_, _E. umbrinus_, _E.
palmeri_, and _E. bulleri_).
_Eutamias panamintinus_, according to Howell (_op. cit._:78) and Johnson
(_op. cit._:83), is a near relative of _E. amoenus_. But, the baculum in
_E. panamintinus_ more closely resembles that in _E. speciosus_ than
that in _E. amoenus_ (compare figs. 5, 15, and 16). Consequently I have
placed _E. panamintinus_ in the _speciosus_-group.
In north-central Colorado, specimens that really are _E. umbrinus_
(subspecies _montanus_) have, in the past (Howell _op. cit._:82), been
referred to _E. quadrivittatus quadrivittatus_, but the bacula of the
two species differ markedly from each other (compare figs. 8-9 with
17-18) and permit the specimens readily to be correctly identified to
species. Further, Howell (_op. cit._:95) placed _E. umbrinus_
(subspecies _umbrinus_ and _fremonti_ of current usage) in the
_quadrivittatus_-group, whereas the structure of the baculum leads me to
place _E. umbrinus_ in the _speciosus_-group.
Thus, groups of species established on the basis of only skulls and
skins, in a few instances differ from those established on a broader
basis which includes the bacula.
Johnson (_op. cit._:63) writes, "Each species [of Eutamias] has a
characteristic habitat which differs from those of other species. Where
two or more species occur together in a general locality they are
usually mutually exclusive in their choice of foraging and nesting sites
and in the time of breeding." Thus he classified the species of
Californian chipmunks not only by morphologic characteristics but by
habits and habitats as well. The characteristics of the skulls and skins
of chipmunks probably reflect the habitats in which these animals live.
The characteristics of the bacula, on the other hand, may also reflect
the habitats in which the animals live, but to a lesser degree. Because
the structures of the bacula are probably less affected by the action of
the external environment they probably indicate relationships between
groups of species of chipmunks more clearly than do characteristics of
the skulls and skins.
If the structures of the bacula indicate relationships between groups of
species of chipmunks more clearly than do the characteristics of the
skulls and skins, the close resemblance of the skulls of _E.
quadrivittatus_ and _E. umbrinus_ may be thought of as convergence. The
same can be said of _E. amoenus_ and _E. panamintinus_.