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UNIVERSITY OF KANSAS PUBLICATIONS
MUSEUM OF NATURAL HISTORY
Volume 12, No. 3, pp. 181-216, 49 figs.
February 19, 1960
The Baculum in Microtine Rodents
BY
SYDNEY ANDERSON
UNIVERSITY OF KANSAS
LAWRENCE
1960
UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY
Editors: E. Raymond Hall, Chairman, Henry S. Fitch,
Robert W. Wilson
Volume 12, No. 3, pp. 181-216, 49 figs.
Published February 19, 1960
UNIVERSITY OF KANSAS
Lawrence, Kansas
PRINTED IN
THE STATE PRINTING PLANT
TOPEKA, KANSAS
1960
[Illustration]
28-774
The Baculum in Microtine Rodents
BY
SYDNEY ANDERSON
INTRODUCTION
Didier (1943, 1954) has described the bacula of several Old World
microtines, and other rodents. Argyropulo studied (1933a, 1933b) five
species of Cricetinae and _Microtus socialis_. Ognev (1950) illustrated
numerous species of Eurasian microtines. Hamilton (1946) figured and
described the baculum of 11 species of North American microtines. Hibbard
and Rinker (1942, 1943) figured the baculum of _Synaptomys cooperi
paludis_ and of _Microtus ochrogaster taylori_. Dearden (1958) studied
the baculum in two Asiatic species of _Lagurus_, in six subspecies of
_Lagurus curtatus_ of North America, and in six other species of
microtines of other genera.
The baculum can be preserved easily with standard study skins, and is
potentially useful in interpreting relationships on any taxonomic level,
and especially in determining the relationships of species within a
genus, if used together with other structures.
The anatomical orientation of the baculum needs comment because some
confusion exists in the literature, especially concerning the use of the
terms ventral and dorsal. The urethra lies on the anatomically ventral
side of the penis, and of the baculum. In the center of the penis lies a
single corpus cavernosum penis, shown in cross section proximal to the
baculum in Figure 1c. Dorsally an artery, thinner walled than the ventral
urethra, ends in a somewhat reticulate sinus surrounding primarily the
middle part of the baculum within the bulbous glans penis. The corpus
cavernosum penis (the structure has no median septum, at least distally)
terminates with the baculum and is closely knit to it. The site of this
bond is evident in the tuberosities and sculpturing of the base of the
baculum.
The part of the penis enclosing the baculum, when not erect, is folded
back as shown in Figures 1a and 1b. As a result the anatomically ventral
surface faces upwards, or at least posterodorsally. The use of the term
ventral in this account refers to the anatomically ventral side, that is
to say to the side of the baculum facing the urethra.
The baculum in microtines consists of an elongate stalk, having a
laterally, and to a lesser extent dorsoventrally, expanded base and an
attenuate distal shaft. Usually, three digitate processes of
cartilaginous material in which additional ossifications may occur arise
from the terminus of the shaft. The proportions and curvature of the
stalk vary as do the proportions of the terminal ossifications to each
other and to the stalk. In some species one or more of the digital
processes are frequently completely unossified.
[Illustration: FIGURE 1. The baculum in _Microtus
ochrogaster_--orientation and variation with age. _a._ Diagram of a
sagittal section of the posterior half of a vole, natural size. The
penis, containing the baculum (in black), extends ventrally from a point
posterior to the pubic symphysis (stippled), along the body wall, and
bends posteriorly at the distal end. _b._ Distal end of penis (x 2)
showing baculum (in black), the urethra (solid lines) adjacent to the
baculum, and the corpus cavernosum (broken lines) proximal to the
baculum. _c._ Oblique view of the cross section of penis (x 4) shown in
Figure 1 _b_. The thick-walled urethra lies ventral to the curved corpus
cavernosum. A thinner-walled blood-vessel lies dorsal to the corpus
cavernosum. The anatomically ventral side of the baculum, in the normal
non-erect penis shown, is seen to face dorsally. _d._ Graph showing the
relationship between size of baculum, size of animal, and development of
digital ossifications. Circles show presence of ossification in stalk
only; circles enclosing dots indicate presence of secondary ossification
in median process also; large dots indicate the addition of tertiary
ossification in one or both of the lateral digitate processes.]
Preserved specimens of _Microtus arvalis_, _Microtus agrestis_, _Microtus
orcadensis_, _Microtus nivalis_, _Microtus guentheri_, _Microtus
subterraneus_, _Clethrionomys glareolus_, and _Ellobius lutescens_ were
provided by Prof. Robert Matthey of Lausanne, Switzerland. J. Knox Jones,
Jr. carefully saved the bacula with specimens of _Microtus fortis_ and
_Clethrionomys rufocanus_ from Korea. Dr. W. B. Quay, Department of
Zoology, University of California, supplied specimens of _Synaptomys
cooperi_, _Phenacomys intermedius_, and _Microtus oregoni_. Dr. Franklin
Sturges and Mr. John W. Goertz, Museum of Natural History, Oregon State
College, Corvallis, have provided specimens including bacula of
_Clethrionomys occidentalis_, _Microtus oregoni_, and _Microtus
townsendii_. Dr. Randolph L. Peterson and Mr. Bristol Foster, Royal
Ontario Museum of Zoology, Toronto, Canada, provided specimens of
_Phenacomys intermedius_. Dr. J. N. Layne, University of Florida,
Gainsville, Florida, presented me with a baculum of _Microtus parvulus_.
I am indebted to all of these persons for their aid, and to various
collectors for the Museum of Natural History, who preserved bacula with
specimens. Many of these specimens were obtained through the assistance
of the University of Kansas Endowment Association and the National
Science Foundation.
METHODS
Bacula were obtained from fresh specimens, specimens preserved in alcohol
or formalin, and dried study skins. The processing of bacula has been
discussed by Hamilton (1946), Friley (1947), White (1951), and Dearden
(1958). The methods used to preserve bacula for my study differed some
from any of those reported. The terminal part of each penis including the
baculum imbedded in the glans penis was removed in its entirety and
placed in a vial. The catalogue number was kept with each specimen at all
times. A two per cent solution of potassium hydroxide was added. All
specimens were examined at least once a day. If tissues other than the
glans penis were present they were removed with forceps when softened
usually at the end of one day. Several drops of Alizarin red-S stain in a
saturated alcoholic solution were added to the 3 to 5 ccs. of KOH
solution in each vial. Solutions were replaced if they became turbid
enough to obstruct observation of the clearing penis. After one day the
solution containing stain was removed and replaced with two per cent KOH
solution without stain. When the glans became sufficiently cleared that
the stained baculum could be seen easily, the solution was replaced by
glycerin in which clearing was completed. The time required for the
entire process varied from one day to more than two weeks depending on
the size of the specimen and on its condition. Fresh specimens clear more
rapidly than dried specimens, and those that are dried more rapidly than
those that are preserved. A three or four per cent solution of hydroxide
will hasten the process, but more frequent observation is required to
prevent excessive maceration.
Specimens were then examined in a shallow dish containing glycerin under
a binocular microscope. The baculum can be viewed from any desired
direction. The method described above leaves the baculum intact within
the glans penis; therefore its orientation can be determined relative to
the thick walled urethra and the thin walled dorsal artery that extends
onto the dorsal side of the baculum. The ventral curvature of the penis
proximal to the baculum, and the distal extension, characteristic of most
species, of the dorsal border of the glans (both shown in Figure 1) are
other features aiding in correctly orienting cleared specimens. The
digitate processes are not so often injured, lost, or displaced when the
method described above is used as they are when the penis is dissected.
Specimens were stored in glycerin in glass shell vials having
polyethylene stoppers. A small card bearing the name, number, locality,
and other data was placed in each vial. A specimen thus enclosed can be
kept indefinitely, or removed and mounted in balsam as described by White
(1951:631) or in plastic as described by Dearden (1958:541) and thus
stored in the vial containing the skull of the specimen.
Drawings were made on millimeter ruled paper while the baculum was viewed
under a binocular microscope with a square ruled eyepiece.
Unless otherwise noted all specimens listed are in the University of
Kansas Museum of Natural History. Catalogue numbers are cited.
Measurements are accurate to within less than one-tenth of a millimeter.
Proportions as stated in the text are approximations, accurate to within
one-twelfth (8.33 per cent). The range of variation is unknown for some
species. Mention is made if maturity is known or suspected to differ in
specimens being compared.
The development of the baculum has been studied by Callery (1951) in
_Mesocricetus auratus_ and by Ruth (1934) in the laboratory rat. In the
rat (_Rattus norvegicus_) the bone is of endoblastemal origin being laid
down by a condensation of undifferentiated mesenchymal cells. At the
distal end of the bone dense fibrous tissue is then differentiated and at
the proximal end hyaline cartilage. Growth is by substitution at the
proximal end and by subperiosteal lamellation circumferentially. A marrow
cavity is formed by resorption. In the baculum of the hamster the primary
center of ossification is in the stalk, and is present at the age of
three days; the secondary centers are in lateral processes and are
present at 80 days and enlarge subsequently. A tertiary center, in each
median process, may or may not develop later. Maximum development of the
baculum is reached late in the reproductive life of the hamster.
The early ossification of the baculum noted in the rat and the hamster
occurs in _Microtus_ also. A specimen of _Microtus montanus fusus_
(76831, from 5 mi. N, 26 mi. W Saguache, 9600 ft., Saguache County,
Colorado) only 74 mm. in total length and weighing only 6.6 grams, had a
slender ossified baculum having enlarged ends. This vole was one-half of
the average length and less than one-fifth of the average weight of an
adult, and of approximately the size at which weaning takes place.
The development of the baculum in _Microtus ochrogaster_ was studied in
32 specimens of various ages. The specimens (between Nos. 74994 and
75074) were collected between August 15 and September 4, 1957, at
localities on the Great Plains. These specimens were from breeding
populations, as evidenced by pregnancy of females and by large size of
testes of males. The length and width of the stalk of the baculum, the
presence of digital ossifications, the total length of the animal, and
the size of the testes were noted. Variability in length of testes is
greatest when voles are from 140 to 150 mm. in total length. Sexual
maturity is reached rather abruptly when the total length of most
individuals is 140 to 150 millimeters. If the baculum likewise underwent
more rapid growth at the onset of sexual maturity, greater variability
should be evident in the length of the baculum of voles 140 to 150 mm. in
total length than in bacula of voles of other sizes. This was the case
(see Figure 1d). The baculum does not, however, suddenly reach its
maximum maturity.
The primary ossification is in the stalk. The secondary ossification is
in the median process except in _Lagurus_ (Dearden, 1958:551) and some
individuals of _Neofiber_ (see account on page 258). Tertiary centers of
ossification are in the lateral processes. The primary ossification is
present at an early age and subsequently increases in size and solidity.
The secondary and tertiary ossifications are progressively more common in
older voles. The increase in degree of ossification of all parts
continues after sexual maturity is reached. Individual variation and
variation with age in the baculum of _Microtus pennsylvanicus_ have been
illustrated by Hamilton (1946:380). Figures 14, 15, and 17 illustrate
variation with size, which is correlated with age, and also illustrate
individual variation. The three bacula are from adult voles having testes
that measured 15, 16 and 16 mm. in length, respectively. Each vole was
trapped in late June. The total lengths in millimeters of the three voles
are 172, 167, and 181; weights are 55, 52.4, and 65.5 grams. I judge that
the greater size of the stalk and the better developed base shown in
Figure 17 than in Figure 15 are illustrative of age variation; the
difference in the size of the lateral digitate processes is, in this
case, attributable to individual variation. Differences in the distal end
of the baculum in Figures 42 and 43, show individual variation also.
Figures 35 and 36 represent two different subspecies; different
individuals of _M. mexicanus mogollonensis_, however, exhibit individual
variation of the same degree.
Hall and Cockrum (1953) list 44 species of microtines in North America.
At least twelve of these are insular or local forms perhaps derived from
some other species; for example _Microtus coronarius_, an insular form
derived from _Microtus longicaudus_; _Microtus provectus_, considered by
Chamberlain (1954:587) and by Wheeler (1956:176) as a subspecies of
_Microtus pennsylvanicus_; and _Microtus ludovicianus_, a close relative
of _Microtus ochrogaster_.
All North American genera have been studied. Of the genus _Microtus_ in
North America, all subgenera but _Orthriomys_ and all species but the
following nine, have been studied: _M. (Orthriomys) umbrosus_, the
insular _M. (Stenocranius) abbreviatus_, _M. (Microtus) breweri_, _M.
(Microtus) nesophilus_, _M._
[Illustration: FIGURES 2-13. Bacula of microtines. Unless indicated
otherwise views are (_a_) of the dorsum, (_b_) the right side, and (_c_)
the proximal end with the dorsal surface upward. Exact localities are
given in accounts of species concerned.
2. _Lemmus trimucronatus_, 50678, Point Barrow, Alaska.
3. _Dicrostonyx groenlandicus_, 50539, Porcupine Lake, Brooks Range,
Alaska.
4. _Dicrostonyx groenlandicus_, 52524, Point Barrow, Alaska.
5. _Synaptomys cooperi saturatus_, WBQ 3-C-454, 3 mi. S Demotte, Indiana.
6. _Synaptomys cooperi paludis_, 13716, Meade County State Park, Kansas.
7. _Phenacomys intermedius celatus_, SA 2044, Quebec.
8. _Phenacomys intermedius intermedius_, WBQ 3-C-309, 5.4 mi. S Moran,
Teton Co., Wyoming.
9. _Clethrionomys rufocanus_, 60438, 1 mi. NW Oho-ri, Korea, (_d_)
ventral view.
10. _Clethrionomys gapperi_, 42108, 31 mi. N Pinedale, Wyoming.
11. _Clethrionomys rutilus_, 42865, 5 mi. NNE Gulkana, Alaska.
12. _Clethrionomys occidentalis_, FWS 30, Mary's Peak, Benton Co.,
Oregon.
13. _Clethrionomys glareolus_, 67100, Zermatt, Valais, Switzerland.]
[Illustration: FIGURES 14-25. Bacula of _Microtus_. Unless indicated
otherwise views are (_a_) of the dorsum, (_b_) the right side, and (_c_)
the proximal end with dorsal surface upward.
14. _M. pennsylvanicus_, 42439, 1 mi. S, 2 mi. E Eagle Nest, Colfax Co.,
New Mexico; abnormality perhaps owing to injury; dorsal view.
15. _M. pennsylvanicus_, 42306, 5 mi. N, 26 mi. W Saguache, Colorado;
dorsal view.
16. _M. pennsylvanicus_, 43043, 20 mi. NE Anchorage, Alaska, ventral
view.
17. _M. pennsylvanicus_, 42430, 1 mi. S, 2 mi. E Eagle Nest, New Mexico.
18. _M. agrestis_, 67102, Gryon, Switzerland.
19. _M. montanus amosus_, 62241, 1/2 mi. E Soldier Summit, Wasatch Co.,
Utah.
20. _M. montanus nanus_, 57470, 2 mi. N, 2 mi. W Pocatello, Idaho.
21. _M. montanus fusus_, 42307, 5 mi. N, 26 mi. W Saguache, Colorado.
22. _M. arvalis_, 67101, Vidy, Switzerland, possibly not mature.
23. _M. guentheri_, 67104, Palestine.
24. _M. orcadensis_, 67106, Orkney Islands, orientation uncertain.
25. _M. fortis_, 63841, Chipo-ri, Korea, (_d_) ventral view.]
[Illustration: FIGURES 26-39. Bacula of microtines. Unless indicated
otherwise views are (_a_) of the dorsum, (_b_) the right side, and (_c_)
the proximal end with the dorsal surface upward.
26. _M. (Pitymys) fatioi_, 67103, Zermatt, Switzerland, immature.
27. _M. (Pitymys) pinetorum_, 76834, 2 mi. N Baldwin, Douglas Co.,
Kansas.
28. _M. (Pitymys) pinetorum_, 68545, 1 mi. NE Pleasant Grove, Kansas.
29. _M. (Pitymys) quasiator_, 30709, Teocelo, Veracruz, (_d_) ventral
view.
30. _M. (Pitymys) quasiator_, 19878, 5 km. N Jalapa, Veracruz.
31. _M. (Pedomys) ochrogaster_, 75036, 1 mi. N, 2 mi. E Oberlin, Kansas.
32. _M. (Stenocranius) miurus_, 51152, Lake Schrader, Brooks Range,
Alaska.
33. _M. (Stenocranius) miurus_, 51169, Lake Schrader, Brooks Range,
Alaska.
34. _M. (Stenocranius) gregalis_, 8059, "Eastern Europe."
35. _M. mexicanus mexicanus_, 63094, Valle de Bravo, Estado de Mexico,
Mexico.
36. _M. mexicanus mogollonensis_, 63298, Mt. Taylor, Valencia Co., New
Mexico.
37. _M. californicus_, 76828, 1 mi. NE Berkeley, California; (_d_)
ventral view.
38. _M. (Arvicola) richardsoni_, 42454, 31 mi. N Pinedale, Sublette Co.,
Wyoming.
39. _M. richardsoni_, 37903, 23-1/2 mi. S, 5 mi. W Lander, Wyoming;
distal end.]
[Illustration: FIGURES 40-49. Bacula of microtines. Unless indicated
otherwise views are (_a_) of the dorsum, (_b_) the right side, and (_c_)
the proximal end with the dorsal surface upward.
40. _Microtus (Pitymys) parvulus_, UF 1508, 1 mi. W Micanopy, Florida.
41. _Microtus townsendii_, 79186, Sec. 33, T. 11S, R. 5W, Benton Co.,
Oregon.
42. _Microtus (Herpetomys) guatemalensis_, 65895, 2 mi. S San Juan Ixcoy,
Guatemala.
43. _M. guatemalensis_, 65921, 10 mi. E, 4 mi. S Totonicapan, Guatemala,
dorsal view of tip.
44. _Microtus oeconomus_, 43048, Kelsall Lake, British Columbia.
45. _Microtus (Chilotus) oregoni_, WBQ 3-C-248, 5 mi. N Orick,
California.
46. _Lagurus (Lemmiscus) curtatus_, 26053, 9 mi. S Robertson, Uinta Co.,
Wyoming.
47. _Microtus (Chionomys) nivalis_, 65127, Wetterstein, Germany,
orientation uncertain.
48. _Microtus (Chionomys) longicaudus_, 50253, Crane Flat, Mariposa Co.,
California.
49. _Neofiber alleni_, 27268, 2 mi. S Gainesville, Florida, orientation
uncertain.]
(_Microtus_) _provectus_ (the last three are probably insular derivatives
of _M. pennsylvanicus_), _M._ (_Microtus_) _fulviventer_ (perhaps derived
from the same stock as _Microtus mexicanus_), _M._ (_Microtus_)
_xanthognathus_ (perhaps related to _Microtus chrotorrhinus_), _M._
(_Microtus_) _coronarius_, and _M._ (_Pedomys_) _ludovicianus_.
SPECIES OF WHICH BACULA WERE EXAMINED
Subfamily: Microtinae Number of Specimens
Tribe: Lemmi
_Dicrostonyx groenlandicus_ (Traill) 4
_Lemmus trimucronatus_ (Richardson) 6
_Synaptomys cooperi_ Baird 5
Tribe: Microti
Genus: _Clethrionomys_ Tilesius, 1850
_Clethrionomys rutilus_ Pallas 4
_Clethrionomys gapperi_ (Vigors) 9
_Clethrionomys occidentalis_ (Merriam) 1
_Clethrionomys glareolus_ Schreber 1
_Clethrionomys rufocanus_ Sundevall 1
Genus: _Phenacomys_ Merriam, 1897
_Phenacomys intermedius_ Merriam 5
Genus: _Ondatra_ Link, 1795
_Ondatra zibethicus_ (Linnaeus) 1
Genus: _Microtus_ Schrank, 1798
(_Herpetomys_) _guatemalensis_ Merriam 3
(_Arvicola_) _richardsoni_ (DeKay) 2
(_Chilotus_) _oregoni_ (Bachman) 3
(_Stenocranius_) _gregalis_ (Pallas) 1
(_Stenocranius_) _miurus_ Osgood 9
(_Chionomys_) _longicaudus_ (Merriam) 6
(_Chionomys_) _nivalis_ Martins 2
(_Microtus_) _arvalis_ (Pallas) 1
(_Microtus_) _orcadensis_ Millais 1
(_Microtus_) _guentheri_ Danford and Alston 1
(_Microtus_) _fortis_ Buechner 2
(_Microtus_) _montanus_ (Peale) 15
(_Microtus_) _townsendii_ (Bachman) 3
(_Microtus_) _oeconomus_ (Pallas) 10
(_Microtus_) _mexicanus_ (Saussure) 13
(_Microtus_) _californicus_ (Peale) 2
(_Microtus_) _pennsylvanicus_ (Ord) 13
(_Microtus_) _agrestis_ (Linnaeus) 1
(_Pedomys_) _ochrogaster_ (Wagner) 41
(_Pitymys_) _pinetorum_ (LeConte) 2
(_Pitymys_) _parvulus_ (Howell) 1
(_Pitymys_) _quasiater_ (Coues) 5
(_Pitymys_) _fatioi_ Mottaz 1
Genus: _Neofiber_ True, 1884
_Neofiber alleni_ True 2
Genus: _Lagurus_ Gloger, 1841
_Lagurus curtatus_ (Cope) 7
Total number examined 184
ACCOUNTS OF SPECIES
Dicrostonyx groenlandicus (Traill)
Figs. 3 and 4
Baculum: stalk elongate, greatest length (3.1 mm.) 2-1/5 to 2-1/2 times
greatest breadth, and 4-1/2 times greatest depth; digitate processes
usually cartilaginous, occasionally lateral processes partly ossified;
basal tuberosities weakly to moderately developed, medially confluent;
posterior profile in dorsal view rounded with rounded posterior apex or
shallow notch; dorsal concavity in end-view shallower and not so wide as
ventral concavity; median constriction approximately 2/3 greatest depth;
ventral part of base in end-view wider than dorsal part; shaft straight
or slightly curved; base of stalk placed dorsally relative to axis of
shaft; stalk spatulate, sometimes with distal enlargement; at mid-point
stalk wider than high; lateral profile in dorsal view sloping gradually
without abrupt curvature anterior to point of greatest width.
The baculum of _Dicrostonyx torquatus_ figured by Ognev (1948:476) agrees
with that of _D. groenlandicus_ in shape of stalk, and in lateral
digitate processes that are small relative to size of median process; but
differs in more elongate, terminally enlarged, bulbar shape of median
process. None of my specimens showed ossification in the lateral
processes, observed by Hamilton (1946:381) in _Dicrostonyx rubricatus
richardsoni_ [ = _D. groenlandicus richardsoni_]. In all of my specimens
the cartilaginous median process was larger than that figured by
Hamilton, or by Dearden (1958:542).
_Specimens examined_: Four from; Point Barrow, Alaska, 52524 (Barrow
Village), 67264 (died in captivity); Brooks Range, Alaska, 50536 (Wahoo
Lake, 69 deg.08', 146 deg.58'), 50539 (Porcupine Lake, 68 deg.51'57", 146 deg.29'50",
3140 ft.).
Lemmus trimucronatus (Richardson)
Fig. 2
Baculum: Stalk heavy, broad, greatest length (2.8 mm.) in mature
individuals (Fig. 2) as little as 1-1/3 times greatest breadth, greatest
length no less than 2-2/3 times greatest depth of base; three ossified
processes, median one from as long as to 1/2 longer than the lateral
processes, and approximately 2/3 wider and twice as deep as lateral
processes; length of median process almost 3-1/2 times its breadth,
approximately 1/2 length of stalk; basal fossae broadly confluent;
posterior profile in dorsal view evenly rounded; in end-view ventral
concavity deeper than dorsal concavity, constriction as little as 1/2
greatest depth in mature specimens; shaft straight, bluntly rounded, or
slightly decurved and laterally inflated terminally; lateral profile in
dorsal view a gradual slope from widest point of stalk anteriorly onto
shaft; in younger individuals stalk slenderer, otherwise as described
above.
Five specimens examined by me differ from one figured and described by
Hamilton (1946:379) in that stalk is better developed, larger relative to
size of processes, length of stalk in my specimen (Fig. 2) 2.8 as opposed
to 2.1 mm. in Hamilton's specimen; median process shorter, 1.5 as opposed
to 1.8 mm., proximal end rounded rather than concave, not partially
enclosing tip of shaft; proportion of and relative sizes of median and
lateral processes approximately same as in Hamilton's _Lemmus helvolus_
[ = _Lemmus trimucronatus helvolus_]. A specimen figured by Dearden
(1958:542) has a basally trilobed median process.
The baculum of the Asiatic _Lemmus lemmus_ figured by Ognev (1948:413)
agrees with my specimens in the ossification of three processes, the
relative sizes of these processes to each other and to the stalk, the
well-developed base of the stalk and heavy bluntly rounded shaft; the
baculum of _Lemmus lemmus_ differs in greater anterolateral extent of
basal tuberosities, in proximal notch seemingly separating these
tuberosities, and in median process being slenderer.
_Specimens examined_: Five, of two subspecies; _Lemmus trimucronatus
alascensis_, Point Barrow, Alaska, numbers 50591, 50678, 50731, 50758;
_Lemmus trimucronatus subarcticus_, Wahoo Lake, 69 deg.08', 146 deg.58', 2350
ft., Brooks Range, Alaska, 50948.
Synaptomys cooperi Baird
Figs. 5 and 6
Baculum: Stalk elongate, greatest length (2.7 to 2.8 mm.) 2-1/3 to 2-1/2
times greatest breadth, 4 to 5 times greatest depth; three processes
ossified or lateral processes unossified, ossifications relatively small
(in 78380, median ossification less than 1/4 as large as lateral
ossifications although median cartilaginous process is larger), length of
median process 1/5 to 1/6 of length of stalk, cartilaginous part of
median process larger; posterior profile in dorsal view convex throughout
or bilobate; tuberosities moderately developed, deflected dorsal to axis
of shaft; in end-view medial construction 3/5 greatest depth of
tuberosities; shaft tapered from point of greatest width, slightly
inflated terminally.
The specimen (KU 13716) figured by Hibbard and Rinker (1942:29) has been
restudied. It was first cleared and stained to soften the dry cartilage
binding the digital processes together and to differentiate bone and
cartilage. The lateral processes are small and cartilaginous (Fig. 6) and
seem intact. The differences between this specimen and others examined by
Hamilton (1946:381), Dearden (1958:542), and myself, namely the
relatively larger median ossification, the absence of ossification in
lateral processes, and the distinctly bilobate base and larger size, may
represent geographic differences, or individual variation. The
proportions of length, width, and depth of the stalk, and the appearance
in lateral view do not differ greatly from others examined by Hamilton,
by Dearden (1958:546), and by me.
_Specimens examined_: Five, representing four subspecies; _S. cooperi
gossii_, 6 mi. N Midway, Holt Co., Nebraska 78379, 78380; _S. cooperi
relictus_, 5 mi. N, 2 mi. W Parks, Dundy Co., Nebraska, 72601 (immature);
_S. cooperi saturatus_, 3 mi. S Demotte, Jasper Co., Indiana, 3-C-454,
collection of W. B. Quay; _S. cooperi paludis_, Meade County State Park,
Kansas, 13716.
Clethrionomys rutilus Pallas
Fig. 11
Baculum: Stalk elongate, and proximally enlarged, greatest length (2.7
mm.) 2 times greatest breadth; less than 4 times greatest depth; three
well-developed ossified processes; length of stalk 2-1/3 times length of
median process; median process with basal (and ventral) protuberence and
lateral lobes, arched in dorsoventral plane; lateral processes as large
as median process, flattened distally, having ventromedial vane on distal
half; basal tuberosities of stalk well developed, medially confluent;
posterior profile in dorsal view trilobate or convex throughout with
rounded posterior apex; dorsal concavity well developed, ventral surface
but slightly concave, medial constriction of base as little as 1/2
greatest depth; shaft straight, slender, at mid-point of stalk but
slightly wider than high; basal tuberosities largely dorsal to axis of
shaft in lateral view; lateral profile in dorsal view with an abrupt
curvature separating the gently sloping sides of the shaft from the basal
part at its greatest breadth.
The specimen of _Clethrionomys rutilus_ figured by Ognev (1950:120) is
essentially like the North American specimens examined by me in the
relative sizes of the ossifications and the general shape of the stalk.
_Specimens examined_: Four, of one subspecies; _C. r. dawsoni_, west bank
Gakona River, 1700 ft., 5 mi. NNE Gulkana, Alaska, 42865, 42866; SW end
Dezadeash Lake, 2400 ft., Yukon Territory, 42910, 42921.
Clethrionomys gapperi (Vigors)
Fig. 10
Baculum: Stalk elongate, greatest length (2.8 mm.) 1-3/4 times greatest
breadth, and 3-3/4 times greatest depth; proximally enlarged, greatest
depth 1/2 greatest breadth; three well-developed ossified processes;
length of stalk 2-1/3 times length of median process; median process
arched in dorsoventral plane, with basiventral protuberence or spine and
lateral lobes; lateral processes as large as median process, flattened
distally, arched; basal tuberosities of stalk well developed, medially
confluent; posterior profile in dorsal view trilobate or convex
throughout with a rounded posterior apex; dorsal concavity well
developed, ventral surface but slightly concave, or in some cases
slightly convex; medial constriction of base 3/5 greatest depth; shaft
straight, slender, at mid-point of stalk twice as wide as high; basal
tuberosities dorsally placed relative to axis of shaft; lateral profile
in dorsal view abruptly curved anterior to point of greatest width;
slender stalk distinct from angular enlarged base.
The most noticeable difference between the baculum of _C. rutilus_ and
_C. gapperi_ is size. The proportions of the four ossifications are
approximately the same. Ventral vanes on the lateral processes are not
developed in _C. gapperi_. _C. gapperi_ and _C. rutilus_ are more nearly
alike in their bacula than any other two species of _Clethrionomys_
examined. _Clethrionomys occidentalis_, the other New World species, is
also much like _C. gapperi_ and _C. rutilus_. The differences are of a
magnitude comparable to those between the bacula in subspecies of
_Microtus montanus_ (Figs. 19-21) for example, or in subspecies of
_Lagurus curtatus_ (Dearden, 1958:542).
_Specimens examined_: Nine, of two subspecies; _Clethrionomys gapperi
athabascae_, British Columbia, 42922 (Indian Creek, Mile Post 234 of
Alaskan Highway), 64281 (West bank Racing River, 89 mi. W Muskwa), 64287
(North bank Tetsa River, 56 mi. W, 11 mi. S Muskwa), 64290 (44 mi. W, 9
mi. S Muskwa), 64310 (32 mi. W, 2 mi. S Muskwa); _Clethrionomys gapperi
galei_, 31 mi. N Pinedale, Sublette Co., Wyoming, 42108; Grand Mesa,
Delta Co., Colorado, 60014 and 60015 (5-1/2 mi. E, 12 mi. S Collbran),
60022 (8 mi. E, 1/2 mi. S Skyway).
Clethrionomys occidentalis (Merriam)
Fig. 12
Baculum: Stalk elongate, greatest length (2.8 mm.) 2-1/2 times greatest
breadth, 6 times greatest depth; three well-developed ossified processes;
median process larger than lateral processes, 1/2 the length of stalk,
curved, basally broad, ventrally keeled, trilobate posteriorly; lateral
ossifications large, flattened distally, curved; posterior profile of
stalk posteriorly slightly emarginate, thus bilobate in outline; in
end-view dorsal concavity deeper than ventral, constriction less than 1/2
greatest depth, tuberosities confluent, visible in dorsal view at each
side; shaft slender, especially in depth, straight; at mid-point of stalk
almost twice as wide as deep, slight terminal inflation.
The general proportions of the stalk and the relatively large, uniquely
shaped processes, are characteristic of most specimens of the genus
_Clethrionomys_ examined.
_Specimen examined_: _C. occidentalis californicus_, one from Mary's
Peak, Benton Co., Oregon, 30, F. W. Sturges' collection.
Clethrionomys glareolus Schreber
Fig. 13
Baculum: Stalk elongate, greatest length (2.9 mm.) twice the greatest
breadth in the specimen examined, flattened proximally, greatest length
almost 6 times greatest depth of base; three well-developed ossified
processes; median process arched in a dorsoventral plane, with basal
notch and lateral lobes; lateral processes as long as median process,
bowed in dorsal view, flattened distally, with ventromedial vane; basal
tuberosities of stalk weakly developed, medially confluent; posterior
profile in dorsal view evenly rounded; in end-view dorsal concavity
shallow in comparison to most species but deeper than ventral concavity,
constriction 3/4 greatest depth; shaft straight, at mid-point slightly
wider than high, elongate, widest point of stalk less than 1/4 of total
length from proximal end, slight lateral inflation at tip; lateral
profile in dorsal view sloping at first abruptly and then gradually from
widest point of stalk anteriorly onto shaft.
The specimen of _Clethrionomys glareolus_ figured by Ognev (1950:31) in
dorsal view as I interpret it, resembles my specimen in the rounded base;
in the elongate, distally inflated shaft; in the initially abrupt slope
of the lateral profile in dorsal view from the greatest width of stalk
anteriorly; and in the presence of three well ossified processes. Ognev's
specimen differs from mine in the median process being more elongate
relative to its width, and rounded proximally, lacking lateral lobes and
basal notch; in lateral processes being less curved; in the greater
terminal inflation of the shaft; and in the closer approximation of the
terminal processes to the shaft. The baculum of _Clethrionomys glareolus_
as described and figured by Didier (1954:243-244) resembles my specimen
in general proportions, but is more pointed proximally and more curved in
dorsoventral plane. Didier states that the baculum is rather variable in
form in this species, in different regions, but that a large number of
specimens must be examined to assess the geographic nature of this
variation.
_Specimen examined_: One from Zermatt, Valais, Switzerland, 67100.
Clethrionomys rufocanus Sundevall
Fig. 9
Baculum: Base of stalk broad but relatively flattened dorsoventrally,
greatest length (3.2 mm.) less than 1-1/2 greatest width, 4 times
greatest depth; three well-developed ossified processes; median process
arched in dorsoventral plane, having basal notch and lateral lobes;
lateral processes as long as median process, flattened distally, with
ventromedial vane; basal tuberosities of stalk weakly developed, medially
confluent; posterior profile in dorsal view convex with rounded posterior
apex; dorsal surface of base almost flat, ventral concavity broad and
shallow; constriction 3/4 greatest depth (not including an unusual
irregularity on the ventral surface of the base); shaft straight, at
mid-point of stalk distinctly wider than high, slender at distal end,
widest point of stalk almost 1/3 of total length from proximal end, tip
of shaft rounded; lateral profile in dorsal view gradually sloping from
widest point anteriorly onto shaft.
The specimen of _Clethrionomys rufocanus_ figured by Ognev (1950:97)
resembles my specimen in the presence of three well ossified processes.
Ognev's specimen differs however in the lack of a proximal notch on the
median process, the lesser proportion of the stalk included in the basal
enlargement, the more posterior position of the point of greatest width,
and the presence of a concavity in the posterior profile of the stalk in
dorsal view. These differences in the stalk may be owing to a difference
in age (my specimen perhaps being older).
_Specimen examined_: One from 1 mi. NW Oho-ri, 6 M., Korea, 60438.
Phenacomys intermedius Merriam
Figs. 7 and 8
Baculum: Stalk slender, greatest length (2.9 mm.) 2-1/4 to 2-1/2 times
greatest breadth, 4 times greatest depth; three well-developed ossified
processes, median one almost 1/2 length of stalk, curved, broad basally
and slightly larger in all dimensions than either lateral process;
lateral processes flattened distally, curved; base of stalk well
developed, basal tuberosities medially confluent or separated by medial
emargination, posterolateral faces flattened or rough; emarginations in
the four adults examined; posterior profile in dorsal view bluntly
pointed or flattened except for emargination posterially, abruptly curved
at point of greatest width; shaft arising broadly from distal side of
base of stalk; in end-view hour-glass shaped, medial constriction
pronounced, both dorsal and ventral concavities deep; shaft having
relatively straight but distally convergent sides; at mid-point of stalk,
1 to 1-1/2 times as wide as deep; tip bluntly rounded, or slightly
inflated.
The specimens from Quebec differ from the one from Wyoming in smaller
size, relatively smaller lateral digital processes, larger more medial
basal emargination, and slender shafts. The baculum of _Phenacomys
intermedius_ differs much from that of _Phenacomys longicaudus_,
described by Hamilton (1946:381) and by Dearden (1958:547). Dearden
states that the three bacula examined by him of _Phenacomys longicaudus_
differ markedly from the specimen described by Hamilton. It seems to me
that in major features the resemblance is greater between the specimens
of _Phenacomys longicaudus_ examined by these two authors than between
their specimens and specimens of other microtines, including _Phenacomys
intermedius_. Neither Hamilton nor Dearden record the exact localities of
capture, the collections in which the specimens are deposited, or the
catalogue numbers of specimens. Consequently verification of
identifications and observations is difficult.
_Specimens examined_: Five, of two subspecies; _P. intermedius
intermedius_, 5.4 mi. S Moran, Teton Co., Wyoming, 3-C-309, collection of
W. B. Quay; _P. intermedius celatus_, four (including one immature
specimen) from Authiernord, Abitibi-ouest Co., Quebec, specimens in
collection of Bristol Foster designated by numbers 2041-2044 of S.
Anderson's field catalogue. Smith and Foster (1957:107) were of the view
that _Phenacomys ungava_ (including the above specimens from Quebec) may
be specifically distinct from _Phenacomys intermedius_.
Ondatra zibethicus (Linnaeus)
Not figured
Baculum: In the single specimen examined, less mature than that figured
by Hamilton (1946:384), the digitate processes are cartilaginous, the
basal tuberosities are less well developed, and the shaft is slenderer
throughout. The cartilaginous processes are of the same proportions as
ossified processes in the figure mentioned. The shaft is also convex
ventrally in lateral profile. The view of the side here considered to be
anatomically the ventral side (adjacent to the urethra) is labelled
dorsal view in Hamilton's specimen.
_Specimen examined_: One, from Reserve, Brown Co., Kansas, 72405.
Microtus (Herpetomys) guatemalensis Merriam
Figs. 42 and 43
Baculum: Stalk moderately elongate, greatest length (3.5 mm.) 2-1/3 times
greatest breadth, spatulate, flattened throughout, greatest thickness 1/3
millimeter; three ossified processes; median process having three
cornered base, curved dorsally, wider than high, 1/4 to 1/5 greatest
length of stalk; each lateral process bent at middle, as long as median
process, compressed laterally; base of stalk curved dorsally,
tuberosities marginal, hence narrow, lateral excavations of tuberous
margin not confluent medially; in end-view ventral concavity broad, no
dorsal concavity, medial constriction but slightly less than greatest
thickness (not depth); shaft wider than high throughout, at mid-point
more than 3 times as wide as high; tip of shaft slightly inflated both
laterally and dorsoventrally; lateral profile gradually sloping
anteriorly from widest point of stalk.
Specimen number 65921 (Fig. 43) differs from number 65895 (Fig. 42)
described above. Terminus of shaft of number 65921 has lateral lobes from
which arise lateral cartilaginous processes; median terminal ossification
irregular in shape, smaller, imbedded in terminally bilobate cartilage.
In the spatulate flattened stalk these two specimens are much alike. An
immature specimen, number 65908, is smaller (length of stalk 2.6 mm.)
also flattened and spatulate, has the terminal processes cartilaginous,
the lateral processes bent medially, and proportions as in the adult.
The baculum shows no noteworthy resemblance to that of any other species
of North American _Microtus_; on the other hand the differences between
_M. guatemalensis_ and some other species are no greater than the
differences between certain species included in the subgenus _Microtus_.
The baculum neither strengthens nor weakens the case for subgeneric rank
for _M. (Herpetomys) guatemalensis_.
_Specimens examined_: Three from Guatemala; 65895 (2 mi. S San Juan
Ixcoy), 65908, (3-1/2 mi. SW San Juan Ixcoy), 65921 (10 mi. E, 4 mi. S
Totonicapan).
Microtus (Arvicola) richardsoni (DeKay)
Figs. 38 and 39
Baculum: Stalk broad, greatest length (3.7 to 4.3 mm.) 1-1/2 times
greatest breadth, relatively flattened, greatest depth 1/3 greatest
breadth; single median ossified process, in smaller of two specimens this
ossification incomplete and of unusual shape (Fig. 39); length of stalk 4
times length of median process; concavities of basal tuberosities
medially confluent, constriction less than 1/2 greatest depth; widest
point of shaft less than 1/4 length of shaft from posteriormost point;
shaft wider than high except at distal end that is inflated dorsally and
sometimes laterally; both ventral and dorsal concavities of base of stalk
broad and moderately deep; posterior profile in dorsal view evenly
rounded or having marginal notch.
In the absence of ossified lateral processes my two specimens differ from
bacula of _Microtus (Arvicola) terrestris_ figured by Didier (1943:79,
1954:245, 247, 248) and by Ognev (1950:591). The median process relative
to the size of the shaft is smaller, and the shaft relative to its length
is wider in _M. richardsoni_ than in _M. terrestris_. The stalk of _M.
(Arvicola) amphibius_ figured by Didier is like that of _M. richardsoni_
in its greater breadth and median notch on posterior border.
The relationship of the New World water rat, _M. richardsoni_, to the Old
World water rats (genus _Arvicola_ of some European authors) is
uncertain. Miller (1896:66) placed all of them in the subgenus
_Arvicola_. Subsequent authors, stressing differences in the teeth, have
placed _M. richardsoni_ in the subgenus _Aulacomys_ of Rhoads. Zimmerman
(1955) has shown that teeth in some _Arvicola_ approach the more complex
pattern of _M. richardsoni_. He argues also that _Arvicola_ is
generically distinct from _Microtus_ on the grounds that the two groups
have separate origins, _Arvicola_ having descended from the genus
_Mimomys_ and _Microtus_ from some other group of microtines. This
argument also was advanced by Hinton (1926:47-48). Pending further
studies of the possible polyphyletic origin of other subgenera of the
genus _Microtus_, I refer both _M. richardsoni_ and _M. terrestris_ to
the subgenus _Arvicola_.
The evidence afforded by the bacula available is not conclusive as to
relations of Old World and New World water rats. No general agreement on
the number of species in this Palaearctic group has been reached, and
bacula of only three or four of the numerous Old World subspecies have
been figured. I have examined none.
_Specimens examined_: Two, from Wyoming; 42454 (31 mi. N Pinedale, 8025
ft., Sublette Co.), 37903 (23-1/2 mi. S, 5 mi. W Lander, 8600 ft.,
Fremont Co.).
Microtus (Chilotus) oregoni (Bachman)
Fig. 45
Baculum: Stalk broad, greatest length (2.2 mm.) 1-3/4 times greatest
breadth, 3-1/2 times greatest depth; three well-developed ossified
processes; median process 2/5 length of stalk, rounded or tapered
terminally, proximal end opposed to tip of stalk and flattened obliquely;
lateral processes 2/3 length of median process, deeper than wide, curved;
tuberosities of stalk well developed, confluent medially, visible in
dorsal view; in end-view dorsal concavity narrow, moderately deep,
rounded, ventral concavity wide, deep, flattened; base wider ventrally
than dorsally; shaft tapering more or less uniformly, terminally
inflated.
In the relative sizes, to each other and to the stalk, of the three
digitate ossifications _M. oregoni_ resembles closely the Old World
representative of the same subgenus, _M. (Chilotus) socialis_, as figured
by Argyropulo (1933b:181). In _M. oregoni_ the greatest width of the
baculum is more proximal on the stalk than in the _M. socialis_ figured
by Argyropulo but closely resembles the baculum of the _M. socialis_
figured by Didier (1954:242). In possessing a shallow emargination in the
base of the stalk and in possessing a median process that is smaller than
the lateral processes, _M. socialis_, as figured by Didier, differs from
_M. oregoni_. The baculum figured by Argyropulo (_loc. cit._) of
_Sumeriomys colchicus schidlovskii_ [ = _Microtus (Chilotus) socialis
schidlovskii_ according to Ognev, 1950:392] differs from other _Chilotus_
that have been studied in having an unusually elongate median process and
a more distal placement of the widest part of the stalk.
_Specimens examined_: Three, of the subspecies _M. oregoni oregoni_, from
5 mi. N Orick, Humboldt Co., California, 3-C-248, collection of W. B.
Quay; from Mary's Peak, Benton Co., Oregon, 66, collection of F. W.
Sturges; and from Sec. 3, T. 11S, R. 5W, Benton Co., Oregon, 79183.
Microtus (Stenocranius) gregalis (Pallas)
Fig. 34
Baculum: Length of stalk (2.4 mm.) 1-3/4, times greatest breadth, 4-1/3
times greatest depth; median ossified process well developed, more than
1/3 length of stalk, higher than wide, slightly bowed, closely appressed
to terminus of shaft; basal tuberosities of stalk moderately developed,
confluent medially, posterior profile of medial apex rounded in dorsal
view, lateral indentations present, hence trilobate outline; in proximal
end-view base wider ventrally, ventral concavity broader than dorsal
concavity but of equal depth, medial constriction 2/3 greatest depth;
shaft slender in distal part, inflated terminally, and wider than high at
mid-point of stalk; lateral profile a smooth slope of gradually
decreasing curvature from point of greatest width to near distal end.
The baculum of this species figured by Ognev (1950:461) differs in having
lateral ossified processes, and a more rounded base of the stalk.
Resemblance to the New World _Stenocranius_ is discussed below.
_Specimen examined_: One from "Eastern Europe," 8059.
Microtus (Stenocranius) miurus Osgood
Figs. 32 and 33
Baculum: Length of stalk (2.8 mm.) 1-1/2 times greatest breadth, 3-1/2
times greatest depth; median process ossified, 2/5 to 3/5 length of
stalk, laterally compressed, sometimes arched in dorsoventral plane;
lateral processes cartilaginous, slender; basal tuberosities well
developed, averaging less enlarged than shown in Figure 32, but more
angular in lateral outline than shown in Figure 33; tuberosities
confluent posteriorly; posterior profile smoothly rounded to trilobate,
curvature at point of greatest breadth usually acute; in proximal
end-view base wider dorsally, deep dorsal concavity, shallow ventral
concavity, medial constriction 3/5 of greatest depth; shaft slender
anteriorly, at mid-point of stalk twice as wide as high, at tip higher
than wide, laterally inflated; lateral profile in most specimens abruptly
curved anterior to point of greatest breadth.