The Baculum In The Chipmunks Of Western North America Part 1

The Baculum in the Chipmunks of Western North America.

by John A. White.

INTRODUCTION

The baculum is the bony part of the p.e.n.i.s. 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 cla.s.sification based on the structure of the baculum with a cla.s.sification based on the structure and appearance of the skull and skin.

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 a.s.sociates 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 ill.u.s.trations 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.

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. 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.

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