Part 3

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 sparseness, 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

3'. Size large; greatest length of skull rarely less than 34 mm.; shaft of baculum widened at base; outermost dorsal dark stripe often obsolete, never strongly evident. Eutamias umbrinus, p. 606

ACCOUNTS OF SPECIES AND SUBSPECIES

Eutamias minimus (Bachman)

Diagnosis.--Size small; tip of baculum of adults less than 28 per cent of length of shaft; outermost dorsal dark stripes distinct; skull small to medium; when skull medium, zygomatic breadth not proportionally narrower.

Comparisons.--From Eutamias amoenus luteiventris, the only subspecies of that species in Wyoming, E. minimus differs in: Size smaller; tip of baculum in adults less than 28 per cent of length of shaft; zygomatic arches proportionally wider; underparts white or with less yellow or tawny.

From E. umbrinus, E. minimus differs in: Size smaller; general tone of upper parts lighter; base of baculum not widened but almost as narrow as least diameter of shaft.

1. E. m. minimus 2. E. m. consobrinus 3. E. m. pallidus 4. E. m. confinis 5. E. m. silvaticus 6. E. m. operarius

From E. dorsalis utahensis, the only subspecies of this species in Wyoming, E. minimus differs in: Dorsal dark stripes distinct and usually blackish; skull smaller; tip of baculum of adults less than 28 per cent of length of shaft.]

Remarks.--This is the smallest of the species of chipmunks in Wyoming, and in the state can be readily distinguished from the other species by the smaller size and by the characteristic proportions of the baculum.

E. minimus occurs in all the Life-zones of Wyoming, and inhabits open country, such as in the great expanses where sagebrush (Artemesia sp.) is predominant, or inhabits the edges of forests, never occurring in the forest proper.

Analyses of measurements of the skull indicate that of the six subspecies of E. minimus that are found in Wyoming, two are small (E. m. minimus and E. m. consobrinus) and the other four are large (E. m. pallidus, E. m. confinis, E. m. silvaticus, and E. m. operarius). Within these size-groups the subspecies can be distinguished by differences in color pattern.

Eutamias minimus minimus (Bachman)

Tamias minimus Bachman, Jour. Acad. Nat. Sci. Philadelphia, 8(pt. 1):71, 1839.

Eutamias minimus, Miller and Rehn, Proc. Boston Soc. Nat. Hist., 30:42, December 27, 1901.