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Comments on pelage and molt of the spotted ground squirrel, Spermophilus spilosoma (Rodentia: Sciuridae).

ABSTRACT. -- Two seasonal molts are documented for the spotted ground squirrel, Spermophilus spilosoma. Autumnal molt begins on the top of the head and proceeds ventrally and caudally. In spring, the pattern of molt is reversed. The gray color phase described for the species represents a special type of dichromatism, associated only with winter pelage and of only regional occurrence in Texas. Key words: pelage dichromatism; molt; Spermophilus spilosoma.

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Little is known of pelage characteristics and molt in the spotted ground squirrel, Spermophilus spilosoma. Considerable color viariation occurs in some subspecies (Anderson, 1972), and Howell (1938) recognized two color extremes or phases--cinnamon and smoke gray (see Howell, 1938: pl. 5). There is also some confusion concerning the number of molts annually in this species. Both a spring and an autumnal molt usually have been reported (Struebal and Fitzgerald, 1978; Jones et al., 1983). However, Hoffmeister (1986) was unable to find evidence of a molt in spring among Arizonan specimens and speculated that a single autumnal molt produced the plush winter pelage, which later was reduced by wear and hair loss to the sparse coat of summer. Characterization of seasonally variable features often is difficult, given seasonal biases inherent in systematic collections (Stangl and Jones, 1987). These difficulties may be further compounded when dealing with such seasonally active species as the spotted ground squirrel.

We describe aspects of both a spring and autumnal molt in Spermophilus spilosoma, based on specimens from the collections of five institutions. Evidence is presented to indicate that the gray color phase reported for this squirrel is an individual feature of seasonal occurrence.

MATERIALS

A total of 97 study skins of Spermophilus spilosoma was examined from the collections of Midwestern State University (MWSU), Texas A & M University (TCWC), Texas Tech University (TTU), National Museum of Natural History (USNM), and The University of Kansas (KU). In an effort to minimize any geographic effects, only Texas specimens representing two subspecies were considered--S. s. canescens from the extreme western part of the state (N=32), and S. s. marginatus from elsewhere within the distribution of the species in Texas (N=65).

Specimens were assessed as to condition of pelage (summer or winter) and pattern of any obvious molt. Those exhibiting a grayish pelage were considered to be distinct from those of the various shades of red, brown, and tan. As a matter of convenience, shades other than gray are herein collectively referred to as cinnamon.

RESULTS AND DISCUSSION

Seasonal Molt

The cold-weather or winter pelage in Spermophilus spilosoma is relatively plush when compared to the sparse summer coat, the latter almost lacking an undercoat of wool hairs (Williams, 1938). Struebel and Fitzgerald (1978) surmised correctly that the species undergoes two seasonal molts. However, molt cannot be defined by casual inspection because color differences between the plush winter pelage and that of summer are often negligible, and a molt line may not be distinctive.

Important qualitative and quantitative differences are found between seasonal pelages of S. spilosoma, which only can be attributed to molt or to seasonal activity of two different generations of hair follicles (Stangl and Grimes, 1987). Seasonal molts produce summer and winter pelages similar to those figured for S. mexicanus by Stangl et al. (1986). Winter guard hairs are elongated and slender when compared to the stouter summer hairs. Numerous long and curly wool hairs comprise the winter underfur, whereas their counterparts in summer are short, sparsely distributed sprigs.

Jones et al. (1983) recorded spring molt in late May and an indistinct second molt in late summer and early autumn for S. spilosoma in Nebraska. These observations coincide closely with findings for Texas animals (Fig. 1). Spring molt in Texas most often commences in May (earliest 27 April, latest 17 June), and the autumnal molt in August (earliest 17 July, latest 14 October).

The spring molt pattern in S. spilosoma is similar to that detailed for S. mexicanus by Goetze and Stangl (1989:fig. 1); it begins on the head and proceeds dorsally and caudally. Also as in S. mexicanus, pelage replacement in S. spilosoma is actually accomplished by two separate but synchronized molts of guard hairs and wool hairs (underfur). Specimens exhibiting autumnal molt do not represent sufficiently different stages to fully detail this phenomenon. However, the pattern seems to closely follow the spring molt although in reverse, as hair on the head is the last to be replaced with winter pelage.

Pelage Color

Except for S. s. annectens from the south of Texas, which is uniformly represented by gray pelage, a continuum of reds, cinnamon, buffs, and tans characterizes most subspecies of S. spilosoma, with gray pelage described as a color phase (Howell, 1938). Color differences between summer and winter pelages in molting individuals usually was negligible and, given poorly defined molt lines, molt often was inconspicuous. Coupled with the observation that only a few specimens evinced a worn pelage of frayed guard hairs, environmental factors (bleaching or wear, for example) seem to play an unimportant role in seasonal color variation.

[FIGURE 1 OMITTED]

In Texas, gray or gray-washed pelage was found to be of seasonal and of regional (Trans-Pecos) occurrence in both S. s. canescens and S. s. marginatus (Table 1). Evidence of gray pelage was restricted to specimens in winter pelage, although three individuals in process of autumnal molt possessed remnants of both gray winter and cinnamon summer pelages.

Given the evident seasonality of the gray pelage, it seems to us that the gray color phase of the spotted ground squirrel often referred to in the literature results from individual cases of seasonal dichromatism, with some (but not all) cinnamon-colored animals molting into a gray winter pelage. The separate and distinctive nature of two molts, which permitted seasonal divergence of hair morphology, similarly accounts for seasonal divergence of color.
TABLE 1. -- Seasonal incidence of gray or gray-washed pelage among two
subspecies of Spermophilus spilosoma from Trans-Pecos Texas (sample
sizes in parentheses).

 Summer Winter pelage
Taxon pelage Complete Molting Total

S. s. canescens (40) 0.00 (20) 0.00 (4) 0.33 (6) 0.00 (10)
S. s. marginatus (43) 0.00 (25) 0.38 (8) 1.00 (1) 0.44 (9)


ACKNOWLEDGMENTS

Curators and collection managers of Texas A & M University, Texas Tech University, the National Museum of Natural History, and the University of Kansas kindly loaned specimens or permitted the examination of specimens under their care.

LITERATURE CITED

Anderson, S. 1972. Mammals of Chihuahua: taxonomy and distribution. Bull. Amer. Mus. Nat. Hist., 148:149-410.

Goetze, J. R., and F. B. Stangl, Jr. 1989. Spring molt in the Mexican ground squirrel, Spermophilus mexicanus (Rodentia: Sciuridae). Texas J. Sci., 41:430-431.

Hoffmeister, D. F. 1986. Mammals of Arizona. Univ. Arizona Press, Tucson, xix + 602 pp.

Howell, A. H. 1938. Revision of the North American ground squirrels with a classification of the North American Sciuridae. N. Amer. Fauna, 56:1-256.

Jones, J. K., Jr., D. M. Armstrong, R. S. Hoffmann, and C. Jones. 1983. Mammals of the Northern Great Plains. Univ. Nebraska Press, Lincoln, xii + 379 pp.

Stangl, F. B., Jr., and J. V. Grimes. 1987. Phylogenetic implications of comparative pelage morphology in Aplodontidae and the Nearctic Sciuridae, with observations on seasonal pelage variation. Occas. Papers Mus., Texas Tech Univ., 112:1-21.

Stangl, F. B., Jr., and E. M. Jones. 1987. An assessment of geographic and seasonal biases in systematic mammal collections from two Texas universities. Texas J. Sci., 39:129-137.

Stangl, F. B., Jr., J. V. Grimes, and J. R. Goetze. 1986. Characterization of seasonal pelage extremes in Spermophilus mexicanus (Rodentia: Sciuridae). Texas J. Sci., 38:147-152.

Streubel, D. P., and J. P. Fitzgerald. 1978. Spermophilus spilosoma. Mamm. Species, 101:1-4.

Williams, C. S. 1938. Aids to the identification of mole and shrew hairs with general comments on hair structure and hair determination. J. Wildlife Manag., 2:239-250.

FREDERICK B. STANGL, JR., AND JIM R. GOETZE

Department of Biology, Midwestern State University, Wichita Falls, Texas 76308, and Department of Biological Sciences and The Museum, Texas Tech University, Lubbock, Texas 79409
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Author:Stangl, Frederick B., Jr.; Goetze, Jim R.
Publication:The Texas Journal of Science
Geographic Code:1U7TX
Date:Aug 1, 1991
Words:1334
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