Geographic Distribution and Taxonomy of the Chipmunks of Wyoming.

by John A. White.

PURPOSE

The purpose of the following account is to: (1) Show what kinds of chipmunks occur in Wyoming; (2) point out the interrelationships between these kinds; and (3) account, where possible, for the present distribution of these animals in Wyoming.

METHODS, MATERIALS, AND ACKNOWLEDGMENTS

Capitalized color terms in the following accounts are of Ridgway, "Color Standards and Color Nomenclature," Washington, D.C., 1912.

The measurements of the skull that were used in this study were made as shown in White (1953:566, fig. 1). These are: Greatest length of skull, zygomatic breadth, cranial breadth, length of nasals, length of lower tooth-row, condylo-alveolar length of mandible, and inner mandibular length.

Of the external measurements, only the total length and the length of the tail are recorded in table 1. Some field collectors measured the ear from the notch and others from the crown; most collectors measured the length of the hindfoot to the nearest millimeter rather than in tenths of a millimeter as would have been desirable.

Consequently, I decided against using the length of the ear and hindfoot in this report.

When the word "significantly" is used in comparisons, it is meant to show that there is a significant statistical difference between two or more samples. Whenever eight or more specimens from one locality were available, the mean, range, standard deviation, standard error of the mean, and coefficient of variability were calculated.

Only adult specimens were used in comparison. "Aging" of specimens is discussed on page 587 of this paper.

The geographic range of each species and subspecies is not described in writing, for, the localities are plotted on maps along with the geographic range of each subspecies, and under "specimens examined" the locality of each specimen or series of specimens is listed.

In the synonymy of each subspecies there appears, first the first usage of a name, second the first usage of the name combination now employed, and third, pure synonyms.

A total of 757 specimens of chipmunks are listed as examined in the course of preparing this report. Additional specimens were less carefully examined in the Biological Surveys Collection in Washington, D.C. Specimens used in my study, unless otherwise specified, are in the Museum of Natural History, University of Kansas. The symbols representing the collections containing specimens studied are as follows:

BS--United States Biological Surveys Collection.

FC--Collection of James S. Findley.

MM--Museum of Zoology, University of Michigan.

NM--United States National Museum.

KU--Museum of Natural History, University of Kansas.

I am grateful to Professor E. Raymond Hall for guidance in my study and thank Doctors Robert W. Wilson, E. Lendell c.o.c.krum, Keith R.

Kelson, A. Byron Leonard, Rollin H. Baker, and others at the Museum of Natural History and Department of Zoology, University of Kansas, for encouragement and advice. My wife, Alice M. White, made the ill.u.s.trations and helped me in many ways.

For permission to borrow and to study specimens, I thank Dr. W. H.

Burt of the Museum of Zoology, University of Michigan, Miss Viola S. Schantz of the United States Fish and Wildlife Service, Mr.

Colin C. Sanborn of the Chicago Natural History Museum, and Mr.

James S. Findley.

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.

VARIATION

Secondary s.e.xual variation in chipmunks is small; the females are slightly larger than the males. This difference in size is so slight that it can be ignored when making taxonomic comparisons, for, large samples of males and females of like age and from the same locality were compared and were found statistically not to be significantly different. This is in agreement with Johnson (1943:70) and Hall (1946:329).

Variations of taxonomic worth are treated in the accounts of species and subspecies.

Individual variation is slight, for, the a.n.a.lyses of measurements of the skulls of series of specimens of like age, reveal markedly low coefficients of variability resembling those published by Larrison (1949).

The age-categories here recognized are based primarily on the structure of the skull.

_Juveniles._--Nasals proportionally shorter and more pointed anteriorly than in other categories; zygomatic arches more appressed to cranium; suture separating basisphenoid and presphenoid noticeably "open"; deciduous P4 and p4 show no wear through enamel; M3 and m3 not yet erupted; peglike deciduous P3 strongly leaning posteriorly; molars show no wear through enamel; parietals paperlike or thin; skull convex dorsally; 1 to 1-1/2 months of age.

_Young._--Nasals of adult proportions; zygomatic arches still noticeably appressed anteriorly to cranium; suture between basisphenoid and presphenoid still "open"; nasals rounded, no longer so pointed as in juveniles; deciduous P4 and p4 show wear through enamel layer, and in some specimens, permanent P4 and p4 can be seen beneath; roots of deciduous P4 and p4 clearly show erosion beneath; M3 and m3 fully erupted; peglike deciduous P3 still present; parietals noticeably thicker and less paperlike; skull flattened (not so convex dorsally), but not so flattened as in adults; 1-1/2 to 4 months of age.

In both juveniles and young the P4 and p4 are deciduous and differ in occlusal pattern from the permanent P4 and p4. In the deciduous P4 the anterior cingulum is projected strongly anteriorly forming the apex of the sharpest angle of a triangle, whereas the permanent P4 is trapezoidal in occlusal pattern. In the deciduous p4 the protoconid and metaconid are close together giving this tooth a triangular appearance in occlusal pattern, whereas this pattern in permanent p4 is trapezoidal (see Hall 1926:390).

_Subadults._--Adult configuration of skull reached; suture between basisphenoid and presphenoid completely closed; nasals rounded anteriorly; permanent P4 and p4 show no wear through enamel layer; wear through enamel layer of molars noticeable, especially through protocones; peglike permanent P3 slanting only slightly posteriorly; skull only slightly convex dorsally; parietals solid and resistant to pressure; lambdoidal crest weakly developed; 4 to 10 months of age.

_Adults._--Lambdoidal crest well developed; supraorbital ridges p.r.o.nounced; P4 and p4 show wear through enamel layer and frequently as worn as molars; noticeable wear on lophs and lophids of molars; occlusal pattern always visible; ten months to 2 years of age.

_Old adults._--Ridges and crests extremely well developed; occlusal pattern of molariform teeth obliterated or nearly so; P3 noticeably worn; 2 to 4 years or older.

The hypohyal and ceratohyal bones of the hyoid apparatus are distinct from one another in juveniles and young, but are fused in subadults, adults, and old adults.

Lack of suitable material prevented me from studying chipmunks younger than juveniles. The patterns of growth of these younger chipmunks probably closely follow the changes described by Hall (1926) for _Citellus beecheyi_.

The tip of the baculum in juveniles and young is proportionally longer, in relation to the shaft, than in subadults, adults, and old adults.

Juvenal (juveniles and young) pelage in chipmunks is characterized by silkiness and spa.r.s.eness, especially on the venter. The coloration of this juvenal pelage resembles that of adults in winter pelage which is duller than adult summer pelage. Adult pelage (subadults, adults, and old adults) is not so silky as juvenal pelage, but there are more hairs, especially on the venter. The color pattern is the same in both juvenal and adult pelages.

Chipmunks are born naked and blind and in about two weeks the "body is covered with silken hair clearly demonstrating the color pattern so characteristic of chipmunks...." (Shaw 1944:282). This "silken hair" is replaced by adult summer pelage, and juvenal chipmunks which are molting into adult summer pelage closely resemble the adult males, and later on in the summer, the adult females. Adult females molt later, as a rule, than adult males probably because of lactation. Summer molt begins, on chipmunks in Wyoming and South Dakota, in the latter part of June and is completed by the latter part of August or the first part of September.

Summer molt begins, topographically, in the region of the head and progresses posteriorly to the base of the tail, for, the tail does not molt into summer pelage. The winter molt starts at the same time at the tip of the tail and at the base of the tail, and from each place proceeds anteriorly. The sequence described above is the rule; exceptionally, there are some specimens which molted in patches. In most skins, molts are easily detected because distinct molt-lines were formed. The above description of molting is based on study of a large series of specimens of _Eutamias minimus silvaticus_ taken in several seasons of the year.

The summer pelage is bright, more especially on the sides. In late summer the pelage on the tail is markedly worn, and the hairs around its outer margin are broken. In texture, the summer pelage is not so soft as winter pelage, and this is probably owing to the presence of large amounts of "kinky" underfur in the winter pelage.

The winter pelage is soft, dull in color, and gives the specimen a grayish or an umbrous appearance. The guard hairs are longer than in the summer pelage.

KEY TO THE SPECIES OF THE CHIPMUNKS OF WYOMING

1. Dorsal stripes faint; general tone of upper parts grayish.

_Eutamias dorsalis_, p. 603

1'. Dorsal stripes distinct; general tone of upper parts tawny (not grayish).

2. Venter yellowish or buff; tip of baculum more than 30 per cent of length of shaft; shaft of baculum not widened at base.

_Eutamias amoenus_, p. 602

2'. Venter white; tip of baculum less than 29 per cent of length of shaft--if more than 29 per cent, shaft widened at base.

3. Size small to medium; greatest length of skull less than 34 mm.; shaft of baculum not widened at base; outermost dorsal dark stripe never obsolete _Eutamias minimus_, p. 590