Genera and Subgenera of Chipmunks Part 1

Genera and Subgenera of Chipmunks.

by John A. White.

INTRODUCTION

The supraspecific categories of the chipmunks, as in most other groups of squirrels, have been a source of controversy for many years. Before presenting new evidence and a review of older evidence bearing on the problem, it seems desirable to review briefly in chronological order, the taxonomic history of the genera and subgenera of the chipmunks.

HISTORICAL

Linnaeus (1758:64) described the eastern North American chipmunks under the name _Sciurus striatus_ and based his description on that of Catesby (1743:75). The Asiatic chipmunk was first described, under the name _Sciurus sibiricus_, by Laxmann (1769:69). Schreber (1785, 4:790) separated the Asiatic and North American chipmunks into the Asiatic and American varieties. Gmelin (1788:50) followed Schreber and, employing trinomials, used the names _Sciurus striatus asiaticus_ and _S. s.

america.n.u.s_. Illiger (1811:83) proposed _Tamias_ as the generic name of the chipmunk of eastern North America. Say (1823:45) described _Sciurus quadrivittatus_, the first species of chipmunk known from western North America.

Trouessart (1880:86-87) proposed _Eutamias_ as the subgeneric name to include the western North American and Asiatic chipmunks.

Merriam (1897:189-190) raised _Eutamias_ to full generic rank. In so doing he neither listed nor described any characters but wrote that "it will be observed that the name _Eutamias_, proposed by Trouessart in 1880 as a subgenus of _Tamias_ is here adopted as a full genus. This is because of the conviction that the superficial resemblance between the two groups is accidental parallelism, in no way indicative of affinity.

In fact the two groups, if my notion of their relationship is correct, had different ancestors, _Tamias_ being an offshoot of the ground-squirrels of the subgenus _Ictidomys_ of Allen, and _Eutamias_ of the subgenus _Ammospermophilus_, Merriam."

Howell (1929:23) proposed _Neotamias_ as the subgeneric name for the chipmunks of western North America, of the genus _Eutamias_.

Ellerman (1940, 1:426) gave _Eutamias_ and _Neotamias_ equal subgeneric rank with _Tamias_ under the genus _Tamias_; on pages 427-428 he quoted Merriam, as I have done above, and later, after quoting the key to the genera and subgenera of chipmunks of Howell (1929:11), Ellerman wrote (_op. cit._: 428-429), "This key convinces me that all these forms must be referred to one genus only. The characters given to separate '_Eutamias_' from _Tamias_ are based only on the absence or presence of the functionless premolar, and on the colour pattern. If colour pattern is to be used as a generic character, it seems _Citellus suslicus_ will require a new name when compared with _C. citellus_, etc." And again, "The Asiatic chipmunk is intermediate between typical _Tamias_ and the small American forms in many characters." To substantiate this, Ellerman (_loc. cit._) quotes Howell (_loc. cit._), in comparing the subgenera _Eutamias_ and _Neotamias_, as follows: "'the ears [of subgenus _Eutamias_] are broad, rounded, of medium height, much as in _Tamias_; pos...o...b..tal broad at base, tapering to a point, much as in _Tamias_; interorbital constriction slight, as in _Tamias_; upper molariform tooth rows slightly convergent posteriorly, as in _Tamias_.'" Ellerman (_loc.

cit._) again quotes Howell (_loc. cit._), "'_Eutamias_ of Asia resembles _Tamias_ of North America and differs from American _Eutamias_ in a number of characters, notably the shape of the anteorbital foramen, the pos...o...b..tal process, the breadth of the interorbital region, the development of the lambdoidal crest, and the shape of the external ears.

On the other hand, American _Eutamias_ agrees with the Asiatic members of the genus in the shape of the rostrum, the well-defined striations of the upper incisors, the presence of the extra peg-like premolar, and in the pattern of the dorsal stripes.'"

Bryant (1945:372) wrote, "I am convinced that Ellerman's interpretation of the relationships of the chipmunks is correct." After commenting that the presence or absence of P3, "is of significance only in distinguishing between species of squirrels," Bryant adds that "The other differences between the eastern and the western chipmunks do not appear to be of sufficient phylogenetic importance to warrant the retention of the two groups as genera."

METHODS, MATERIALS, AND ACKNOWLEDGMENTS

Characters previously mentioned in the literature as having taxonomic worth for supraspecific categories of chipmunks were checked by me on specimens old enough to have worn permanent premolars. Some structural features not previously used were found to have taxonomic significance. The baculum in each of the supraspecific categories of sciurids of North America was examined; the bacula were processed by the method described by White (1951:125) to obviate "variation" caused by shriveling of the smaller bacula or breaking of the more delicate parts of the larger bacula. Mallei and hyoid bones of the genera and subgenera of the chipmunks were mostly studied in the dry state. Part of the hyoid musculature in these same groups of chipmunks was dissected.

In all, I studied more than 1,000 skulls and skins of the subgenus _Neotamias_, approximately 50 skulls and skins of _Tamias striatus_, and 15 skulls and skins of the subgenus _Eutamias_ (_Eutamias sibiricus asiaticus_ from Manchuria).

Numerous other specimens were examined but not in such detail.

I am grateful to Professor E. Raymond Hall for guidance in the study. For encouragement and advice I am grateful also to Doctors Robert W. Wilson, Cecil G. Lalicker, Edwin C.

Galbreath, Keith R. Kelson, E. Lendell c.o.c.krum, Olin L. Webb, and others at the Museum of Natural History, and in the Department of Zoology of the University of Kansas. My wife, Alice M. White, made the drawings and helped me in many other ways. For lending specimens I thank Dr. David H. Johnson of the United States National Museum, and Dr. George C. Rinker of the Department of Anatomy, University of Michigan.

a.s.sistance with field work is acknowledged from the Kansas University Endowment a.s.sociation, the National Science Foundation, and the United States Navy, Office of Naval Research, through contract No. NR161 791.

EVALUATION OF CHARACTERS

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, ill.u.s.trate 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 s.e.xual 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_.

[Ill.u.s.tration: FIGS. 1-3. Dorsomedial views of left malleus.

FIG. 1. _Tamias striatus lysteri_, No. 11920 s.e.x?; 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 k.n.o.bs, 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 Poc.o.c.k (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 k.n.o.blike 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 cla.s.sified 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).

[Ill.u.s.tration: 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.