The Baculum in the Chipmunks of Western North America

Part 1

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The Baculum in the Chipmunks of Western North America

JOHN A. WHITE

University of Kansas Publications Museum of Natural History

Volume 5, No. 35, pp. 611-631, 19 figures in text December 1, 1953

University of Kansas LAWRENCE 1953

UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY

Editors: E. Raymond Hall, Chairman, A. Byron Leonard, and Robert W. Wilson

Volume 5, No. 35, pp. 611-631, 19 figures in text

December 1, 1953

UNIVERSITY OF KANSAS Lawrence, Kansas

PRINTED BY FERD VOILAND, JR., STATE PRINTER TOPEKA, KANSAS 1953

24-8968

The Baculum in the Chipmunks of Western North America

JOHN A. WHITE

CONTENTS

PAGE

Introduction 613

Methods, Materials, and Acknowledgments 614

Variation 614

Key to the Bacula of the Species of Eutamias of Western North America 615

Accounts by Species 616 Eutamias alpinus 616 E. minimus 617 E. townsendii 618 E. sonomae 619 E. amoenus 619 E. dorsalis 620 E. merriami 621 E. quadrivittatus 621 E. ruficaudus 622 E. cinereicollis 624 E. quadrimaculatus 624 E. speciosus 625 E. panamintinus 625 E. umbrinus 626 E. palmeri 627 E. bulleri 627

Figures 1-19. Lateral views of bacula 629

Discussion 630

Literature Cited 631

INTRODUCTION

The baculum is the bony part of the penis. In the species of the subgenus Neotamias the proximal part of the baculum is termed the shaft, and the distal upturned part is termed the tip. On the dorsal side of the tip there is a longitudinal ridge termed the keel. The proximal end of the shaft is termed the base (see fig. 19). Depending on the species, the shaft varies from 2.11 to 5.28 mm. in length, and the base may or may not be widened or deepened.

The purpose of this report is to: (1) Show the usefulness of the structure of the baculum as a taxonomic character in chipmunks; and (2) compare a classification based on the structure of the baculum with a classification based on the structure and appearance of the skull and skin.

Part 2

METHODS, MATERIALS, AND ACKNOWLEDGMENTS

The bacula which were borrowed from the University of Michigan, Museum of Zoology, were processed according to the method described by Friley (1947:395-397), whereas all others were processed according to the method described by White (1951:125). Thus the bacula that were borrowed from the University of Michigan, are maintained there in a separate collection, whereas the bacula borrowed from other museums and those that are at the University of Kansas, Museum of Natural History, are housed with the skulls of the corresponding specimens.

All measurements of the bacula were made by means of an eyepiece micrometer.

A total of 194 bacula were seen. All of these are in the Museum of Natural History of the University of Kansas, unless otherwise indicated by the following symbols:

BS United States Biological Surveys Collection. CN Chicago Natural History Museum. LA Los Angeles County Museum. MM University of Michigan, Museum of Zoology. NM United States National Museum. UU University of Utah, Museum of Zoology.

I am grateful to Professor E. Raymond Hall for guidance in my study and thank Drs. Robert W. Wilson, Keith R. Kelson, and Edwin C. Galbraith, as well as other friends and associates of the Museum of Natural History, University of Kansas, for encouragement and valuable suggestions. Dr. William L. Jellison, United States Public Health Service, aided me in part of my field work and kindly sent me some specimens of chipmunks. My wife, Alice M. White, made the illustrations and helped me in many ways.

For the loan of bacula I thank Dr. William H. Burt, University of Michigan, Museum of Zoology. For permission to search for bacula on study skins, and to process those that were found, I thank Miss Viola S. Schantz, United States Fish and Wildlife Service, Mr. Colin C. Sanborn, Chicago Natural History Museum, Mr. Kenneth E. Stager, Los Angeles County Museum, Dr. David H. Johnson, United States National Museum, and Dr. Stephen D. Durrant, Museum of Zoology, University of Utah.

Assistance with field work is acknowledged from the Kansas University Endowment Association, the National Science Foundation and the United States Navy, Office of Naval Research, through contract No. NR 161 791.

VARIATION

Individual variation.--Individual variation is small. This is shown by a coefficient of variability of only 3.85 in the length of the shaft in a series of 12 specimens of E. umbrinus umbrinus from Paradise Park, 21 mi. W and 15 mi. N Vernal, 10,050 ft., Uintah County, Utah.

Variation with age.--In the chipmunks the baculum varies but little with age. In the youngest specimens that I have taken, the M3 and m3 have not yet erupted and there is no wear on P4 and p4; nevertheless, the baculum in these specimens has nearly an adult configuration and size. In juvenal Eutamias minimus the tip of the baculum is longer in relation to the length of the shaft than it is in adults; the tip is 18 to 28 per cent of the length of the shaft in adults, as opposed to 29 to 32 per cent in juveniles.

Aberrations.--In a small percentage of specimens of E. minimus and E. umbrinus the baculum is small and S-shaped, even in adults.

Variations of taxonomic worth.--Variations in this category are described in the section immediately following the key.

KEY TO THE BACULA IN EUTAMIAS OF WESTERN NORTH AMERICA

1. Distal 1/2 to 2/3 of shaft markedly compressed laterally; base markedly widened.

2. Distal 1/2 of shaft laterally compressed and curved downward to base of tip.

3. Height of keel 1/2 of length of tip; keel markedly enlarged. Eutamias bulleri, p. 627

3'. Height of keel 1/4 of length of tip; keel not markedly enlarged. Eutamias umbrinus and E. palmeri, pp. 626, 627

2'. Distal 2/3 of shaft laterally compressed and curved downward to base of tip.

4. Base of keel 1/3 of length of tip; angle formed by tip and shaft less than 100° Eutamias speciosus, p. 625

4'. Base of keel 1/2 of length of tip; angle formed by tip and shaft more than 102° Eutamias panamintinus, p. 625

1'. Distal 1/12 to 2/5 of shaft slightly compressed laterally; base not markedly widened.

5. Shaft thin; shaft less than .20 mm. in diameter at widest point.

6. Ridges on either side of keel enlarged, partially obscuring lateral view of keel; height of keel 1/10 of length of tip. Eutamias sonomae, p. 619

6'. Ridges on either side of tip not enlarged, not partially obscuring lateral view of keel; height of keel at least 1/7 of length of tip.

7. Base not widened or dorsoventrally thickened.

8. Shaft more than 4.5 mm. in length; tip 16 per cent of length of shaft; shaft strongly incised on dorsal side of base Eutamias merriami, p. 621

8'. Shaft less than 4.4 mm. in length; tip more than 25 per cent of length of shaft; shaft not incised on dorsal side of base.

Part 3

9. Height of keel 1/7 of length of tip; angle formed by tip and shaft distinct Eutamias alpinus, p. 616

9'. Height of keel at least 1/5 of length of tip; angle formed by tip and shaft poorly defined.

10. Height of keel 1/3 of length of tip; angle formed by tip and shaft 140° Eutamias dorsalis, p. 620

10'. Height of keel 1/5 of length of tip; angle formed by tip and shaft 130° or less.

11. Tip more than 29 per cent of length of shaft. Eutamias amoenus, p. 619

11'. Tip less than 28 per cent of length of shaft. Eutamias minimus, p. 617

7'. Base widened and dorsoventrally thickened. Eutamias townsendii, p. 618

5'. Shaft thick; shaft more than .25 mm. in diameter at widest point.

12. Length of shaft less than 3.00 mm.; length of tip less than 1.10 mm. Eutamias quadrivittatus hopiensis, p. 622

12'. Length of shaft more than 3.10 mm.; length of tip more than 1.15 mm.

13. Tip less than 28 per cent of length of shaft. Eutamias quadrimaculatus, p. 624

13'. Tip more than 29 per cent of length of shaft.

14. Angle formed by tip and shaft more than 140°; ridges on either side of tip indistinct. Eutamias cinereicollis, p. 624

14'. Angle formed by tip and shaft less than 135°; ridges on either side of tip distinct.

15. Shaft less than 3.65 mm. in length, and .55 mm. or less in diameter at widest point. Eutamias quadrivittatus quadrivittatus, p. 621

15'. Shaft usually more than 3.65 mm. in length, but when shorter, diameter is .60 mm. or more at widest point.

16. Diameter of shaft at widest point less than .58 mm.; tip less than 35 per cent of length of shaft. Eutamias ruficaudus ruficaudus, p. 622

16'. Diameter of shaft at widest point more than .65 mm.; tip more than 40 per cent of length of shaft. Eutamias ruficaudus simulans, p. 623

ACCOUNTS BY SPECIES

#Eutamias alpinus# (Merriam)

Figure 1

Pelage silky; tail bright orange beneath; markings relatively obscure; size small; skull broad, flattened, and large in proportion to body.

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