greatest breadth, 4 times greatest depth; median process ossified, size
small, 1/5 length of stalk, higher than wide, having small anterodorsal
prominence in both specimens examined; lateral processes cartilaginous,
relatively small, displaced posteriorly, attenuate; posterior margin in
dorsal view broadly rounded, or having blunt median apex, convex
throughout; basal tuberosities moderately well developed, medially
confluent, barely visible in dorsal view when mature; in end-view median
constriction 4/5 greatest depth, ventral concavity deeper than dorsal
concavity, both comparatively shallow; stalk at mid-point 1-1/2 times as
wide as deep; shaft relatively slender, bowed dorsally at tip, relatively
straight otherwise; lateral profile in dorsal view a gradual concave
slope from point of greatest width anteriorly to distal part of shaft.
_Specimens examined_: Two, from Douglas Co., Kansas, 76834 (2 mi. N
Baldwin), 68545 (1 mi. NE Pleasant Grove).
Microtus (Pitymys) parvulus (Howell)
Fig. 40
Baculum: Stalk broad, greatest length (2.4 mm. in specimen examined)
1-3/4 times greatest breadth, 4 times greatest depth; median process
ossified, size small, less than 1/4 length of stalk, wider than high,
terminally flattened; lateral processes cartilaginous, relatively small,
attenuate; posterior margin in dorsal view flattened, irregularly curved
with concavities medially and laterally; basal tuberosities well
developed, medially confluent; visible in dorsal view; in end-view median
constriction 2/3 greatest depth, ventral concavity well-formed, no dorsal
concavity; stalk at mid-point twice as wide as deep; shaft relatively
slender, bowed dorsally toward tip; in dorsal view lateral profile a
gradual concave slope from point of greatest width anteriorly to distal
part of shaft; tip of shaft enlarged.
The baculum of _M. parvulus_ resembles that of _M. pinetorum_ more than
it resembles the baculum of any other microtine studied, differing
primarily in smaller size.
_Specimen examined_: One, from 1 mi. W Micanopy, Alachua Co., Florida,
Univ. Florida No. 1508.
Microtus (Pitymys) quasiater (Coues)
Figs. 29 and 30
Baculum: Stalk broad, greatest length (2.6-3.2 mm.) 1-1/3 to 1-2/3 times
greatest breadth, 3-1/3 to 3-2/3 times greatest depth; median process
ossified, with ventral depression, process 1/4 to 1/3 length of stalk,
appressed to tip of shaft, wider than high proximally, relatively broad
terminally; lateral processes cartilaginous, small, attenuate; posterior
profile of stalk in dorsal view broadly rounded, bilobate, or trilobate,
median lobe formed by posterior projection of dorsal shelf between
enlarged lateral tuberosities that form outer lobes, posterolateral faces
of these tuberosities visible in dorsal view of stalk; in end-view dorsal
surface slightly concave, ventral concavity broad and deep, median
constriction 1/2 greatest depth; shaft flattened except tip that is more
terete, and bowed dorsally; at mid-point, stalk twice as wide as high;
shaft relatively slender terminally, narrower than median ossification.
The baculum of _M. quasiater_ is the largest and has the best developed
base and median process of the three American species of the subgenus
_Pitymys_. The three species closely resemble each other in basic form.
_Specimens examined_: Five, all from Veracruz; Teocelo, 4500 ft., 30709,
30711; 4 km. N Tlapacoyan, 1700 ft., 24466; 5 km. N Jalapa, 4500 ft.,
19869, 19878.
Microtus (Pitymys) fatioi (Mottaz)
Fig. 26
The baculum of a single specimen (KU 67103) of _M. fatioi_ from Zermatt,
Valais, Switzerland, was examined. The baculum is immature, as evidenced
by its small size, slender stalk and absence of ossified processes,
therefore no characterization is included.
The baculum of another Old World species of the subgenus _Pitymys_, _M.
pyrenaicus_ from France, figured and described by Didier (1954:242-243),
differs from all New World _Pitymys_ examined in processing ossified
lateral processes.
The status of _Pitymys_, as a genus or as a subgenus, is uncertain. Hall
and Cockrum (1953:448) considered the North American _Pitymys_ and
_Pedomys_ as subgenera of _Microtus_. They did not state specifically the
basis for this point of view, but mention the fact that these two
subgenera (_Pitymys_ and _Pedomys_) closely resemble each other
cranially. These authors did not study nor comment upon the status of the
Old World _Pitymys_. It may be asked whether the Old World and New World
_Pitymys_ have developed as fossorial _Microtus_ independently, or from
an ancestor common to both groups and not common to any other _Microtus_.
Matthey (1955:202) found 62 chromosomes (2N) in both the New World
_Pitymys pinetorum_ and the Old World _Pitymys duodecimcostatus_. This
suggests, but does not prove, common ancestry.
Neofiber alleni True
Fig. 49
Baculum: Stalk massive, greatest length (4.7 mm.) 1-3/4 times greatest
breadth, 4 times greatest depth; ossification in digitate processes
variable; in one (KU 27123) of two specimens examined lateral processes
ossified and median process unossified, as in two specimens examined by
Hamilton (1946:379) from "southern Florida"; in my other specimen (KU
27268) that is possibly more mature, median process ossified although
less deeply stained than lateral ossifications or stalk; posterior
profile in probable dorsal view roughly rounded; in end-view probable
dorsal concavity deep, ventral concavity broad but shallow, and with
center convex; median constriction 3/5 greatest depth; shaft heavy, least
depth 2/3 greatest depth of base; stalk, at mid-point, slightly wider
than deep and more than 1/3 width of base; lateral profile in dorsal view
sharply incurved distal to point of greatest breadth, shaft therefore
relatively distinct from basal part of stalk; slight subterminal
constriction; tip less reduced in the two specimens examined than in two
figured by Hamilton. In preparation, the tissues that make it possible to
distinguish with certainty the dorsal and ventral surfaces of the baculum
were removed in both specimens.
_Specimens examined_: Two, of the subspecies _Neofiber alleni alleni_, 2
mi. S Gainesville, Alachua Co., Florida, 27268; 1 mi. E Courtenay,
Merritt Island, Brevard Co., Florida, 27123.
Lagurus curtatus (Cope)
Fig. 46
Baculum: Stalk slender, greatest length (2.5 mm.) 2 to 2-2/3 times
greatest breadth, 4 to 5 times greatest depth; three ossified processes;
median one more than 1/3 length of stalk, curved dorsally toward tip,
proximally flattened and having acute lateral angles in dorsal view,
wider than deep except in distal half; lateral processes smaller than
median one, slenderer, shorter, of approximately same depth, also curved
dorsally; base of stalk well developed, basal tuberosities medially
confluent, in part visible in dorsal view, in end-view wider ventrally
than dorsally, dorsal and ventral concavities of equal depth and both
wide, medial constriction 1/2 greatest depth; posterior profile in dorsal
view broadly bilobate; lateral profile with abrupt transition from basal
tuberosities to gradually converging, slightly curved sides of shaft;
shaft terminally inflated.
Dearden (1958:543) described and figured the bacula of six subspecies of
_Lagurus curtatus_ and two Asiatic species, _Lagurus lagurus_ and
_Lagurus luteus_. He examined at least 34 specimens of _L. curtatus_ and
found geographic variation in size, breadth of shaft distally, and
proportions of digital ossifications to each other and to the stalk. The
description that I have given above pertains to _L. c. levidensis_.
The baculum of the Asiatic _Lagurus (Lagurus) lagurus_ figured by Ognev
(1950:554) agrees with that of _Lagurus (Lemmiscus) curtatus_, described
here, in the relatively elongate shaft and slender stalk, the proportions
of the processes, and the well-formed and moderately enlarged base of the
stalk. The bacula of three _Lagurus lagurus_ examined by Dearden
(1958:545) were of older individuals than the specimen that Ognev figures
and differ from it and from bacula of _Lagurus curtatus_ (all subspecies)
in the unusual, almost heart shaped, median process, and in larger size.
_Lagurus luteus_ examined by Dearden (1958:545) differs from both
_Lagurus lagurus_ and _Lagurus curtatus_ in lacking lateral digital
ossifications and in having shorter median digital ossifications and
wider base of stalk.
_Specimens examined_: Seven _Lagurus curtatus levidensis_ from Wyoming; 9
mi. S Robertson, Uinta Co., 26045, 26053; 8 mi. S, 2-1/2 mi. E Robertson,
Uinta Co., 26049; Farson, Sweetwater Co., 37906; 16 mi. S, 11 mi. W
Waltman, Natrona Co., 42457; 32 mi. S, 22 mi. E Rock Springs, 42465,
42466.
The following key to the bacula in some adult North American Microtinae
is intended to help point out some of the most important differences. It
should be noted that not all species can be keyed out on the basis of the
baculum. The most difficult group in this respect includes the species of
_Microtus_ that have small or no ossified lateral processes, especially
species of the subgenera _Pedomys_ and _Pitymys_, and the two species
_Microtus californicus_ and _Microtus mexicanus_ of the subgenus
_Microtus_. Another complicating factor is the variability of bacula
evident in some species even in the small samples available. It is to be
expected that additional specimens will show variations not yet observed.
KEY TO THE BACULA OF SOME NORTH AMERICAN MICROTINES
1. Length of lateral digital ossifications more than 1/3 breadth
of stalk 2
1'. Length of lateral digital ossifications less than 1/3 breadth
of stalk or absent 15
2. Size small (total length of baculum less than 5.5 mm.) 3
2'. Size large (total length of baculum more than 5.5 mm.) 14
3. Width at mid-point of stalk more than 1/3 greatest breadth of
stalk 4
3'. Width at mid-point of stalk less than 1/3 greatest breadth of
stalk, 8
4. Stalk, viewed from proximal end hour-glass shaped, and width
of stalk less than 1/2 length of stalk.... _Phenacomys
intermedius_, p. 197
4'. Stalk not both hour-glass shaped when viewed from proximal
end, and with width less than 1/2 length of stalk 5
5. Shaft thin basally, thickness less than 1/3 of greatest breadth 6
5'. Shaft thick basally, thickness 1/3 or more of greatest breadth 7
6. Stalk more or less straight, base not deflected. _Microtus
oeconomus_, p. 204
6'. Stalk spatulate, and base deflected from axis of shaft....
_Microtus guatemalensis_, p. 198
7. Base enlarged, depth nearly 1/2 of breadth.... _Lemmus
trimucronatus_, p. 193
7'. Base moderately enlarged, depth near 1/3 of breadth....
_Microtus pennsylvanicus_, p. 206, or _Microtus townsendii_, p. 204
8. Base hour-glass shaped as viewed from proximal end....
_Phenacomys intermedius_, p. 197
8'. Not so 9
9. Lateral processes separated from tip of shaft by more than the
thickness of the lateral process 10
9'. Lateral processes separated from tip of shaft by less than the
thickness of the lateral process 11
10. Lateral processes more than 1/2 the width of median
process.... _Microtus longicaudus_, p. 201
10'. Lateral processes slender, less than 1/2 the width of median
process.... _Microtus montanus_, p. 204
11. Lateral ossifications equal in length to median
ossification.... _Clethrionomys_, p. 194
11'. Lateral ossifications shorter than median ossification 12
12. Size small, less than 3.4 mm. in total length....
_Microtus oregoni_, p. 199
12'. Size medium, more than 3.4 mm. in total length 13
13. Greatest width of stalk at a point about 1/3 of length of
stalk from base.... _Microtus chrotorrhinus_ (Hamilton, 1946:382).
13'. Greatest width of stalk at a point less than 1/3 of length of
stalk from base.... _Lagurus curtatus_, p. 210
14. Size of baculum larger, base more than 3 mm. wide, processes
all well developed.... _Ondatra zibethicus_, p. 198
14'. Size of baculum smaller, base less than 3 mm. wide, processes
poorly developed in some animals.... _Neofiber alleni_, p. 209
15. At least one digital ossification present 16
15'. Digital ossifications not present.... _Dicrostonyx
groenlandicus_, p. 193
16. Breadth of stalk at least 1/2 length of stalk 17
16'. Breadth of stalk less than 1/2 length of stalk 19
17. Length of stalk greater than 3.6 mm. and less than 1-1/2
times its greatest breadth.... _Microtus richardsoni_, p. 199
17'. Length of stalk usually less than 3.6 mm., or if more than
3.6 mm. (up to 4.0 mm.) then length 1-1/2 times or more its
greatest breadth 18
18. Median process attenuate distally in dorsal view, and
relatively long (more than twice its own breadth), 1/5 to 3/5 the
length of stalk; breadth of stalk usually 2/3 or more length of
stalk.... _Microtus miurus_, p. 200
18'. Median process relatively blunt distally in dorsal view,
relatively short (usually less than 1/4 length of stalk), breadth
of stalk usually less than 2/3 length of stalk....
_Pitymys_, p. 208, _Pedomys_, p. 207, or _Microtus mexicanus_, p. 205
19. Distal processes small and firmly ankylosed to distal end of
shaft.... _Phenacomys longicaudus_, p. 197
19'. Distal processes if present not firmly ankylosed to distal
end of shaft 20
20. Dorsal concavity of base as viewed from proximal end usually
deeper than ventral concavity.... _Microtus mexicanus_, p. 205
20'. Dorsal and ventral concavities of base equal in depth or
ventral one the deeper 21
21. Total length of baculum more than 3.6 mm.... _Microtus
californicus_, p. 205
21'. Total length of baculum less than 3.6 mm.... _Synaptomys
cooperi_, p. 194
DISCUSSION
Owing to shortness of lower incisors and present geographic distribution
of the species, Hinton (1926:35) considered the Tribe Lemmi (lemmings) to
be more primitive than the Tribe Microti (voles). The surviving lemmings
are specialized in many features and therefore are considered as advanced
end-products of an evolutionary radiation of a primitive microtine stock,
of which all earlier stages are extinct.
Hinton regarded _Dicrostonyx_ as the most primitive of the genera of
lemmings on account of its more complex molar teeth (complexity was
considered to be primitive), and on account of the presence of three
primitive longitudinal rows of tubercles in unworn molars. The other
three genera were arranged in order of increasing specialization as
follows: _Synaptomys_, _Myopus_, _Lemmus_.
If the baculum tended to retain its primitive character while
specializations in the external anatomy developed, and if the above
arrangement is correct the most primitive bacula would be found in
_Dicrostonyx_ and in _Synaptomys_. The baculum in these two genera in
comparison to that in _Myopus_ (as figured by Ognev, 1948:512) and
_Lemmus_ has a slenderer stalk and smaller digital ossifications or none
at all. The baculum in the genera of lemmings increases in robustness and
the development of processes from _Dicrostonyx_, to _Synaptomys_, to
_Myopus_, to _Lemmus_--the same order outlined above for total of
specialization. The two extremes in this series are near the extremes of
variation in bacula to be found in all microtines. The baculum in
lemmings as a group cannot then be considered more primitive than in
voles as a group, although the voles are usually considered to be more
advanced. The situation in the voles, as we shall see, casts a different
light on the matter.
The voles, Tribe Microti, were considered by Hinton (1926:40) to be more
advanced than the lemmings because the incisors of the voles are longer
and the root of their last lower molar is lingual to the root of the
incisor. Hinton thought also that the murine ancestors of microtines had
shorter incisors and that the backward extension of the incisors in the
voles is a more ancient feature than the hypsodonty of the molars. A
trend in the molar teeth has been toward greater hypsodonty. The voles in
which the molars are least hypsodont are thus considered primitive. These
include the living genera _Clethrionomys_, _Phenacomys_, _Ondatra_,
_Dolomys_, _Ellobius_, and _Prometheomys_. Therefore, the baculum, in
these assumedly primitive genera, would be expected to resemble the
baculum in the lemmings or at least the most primitive lemmings. This is
not the case.
The bacula that I have examined of _Clethrionomys_ and _Phenacomys_ have
well-developed digital ossifications. In this they resemble the baculum
of the genus _Lemmus_, the most advanced genus of lemmings according to
Hinton. The baculum of _Dolomys_ has not been studied. The baculum in
_Ondatra_, and in _Prometheomys_ as illustrated by Ognev (1948:552), also
possesses well-developed processes. The baculum of _Ellobius_ is small
and lacks processes (as figured by Ognev, 1950:662). No ossification was
found in a single specimen of _Ellobius_ examined by me although the
entire glans penis was removed and cleared without dissection. So far as
known then, with the exception of _Ellobius_ and _Phenacomys longicaudus_
(Dearden, 1958:547), the primitive microtines having rooted molars
possess bacula having three well-developed ossified processes.
Voles of the genus _Microtus_ vary in the structure of the baculum almost
as much as do the lemmings. Within the single subgenus _Microtus_ some
individuals of _Microtus mexicanus_, for example, have minute ossified
lateral processes and other individuals lack these processes; _Microtus
pennsylvanicus_ and some other species have proportionately large lateral
ossifications. If the well-developed condition of the baculum in the
microtines having rooted molars is primitive, then within the genus
_Microtus_ those species having well-developed bacula may be considered
primitive.
The genera _Lagurus_ and _Neofiber_ have moderately developed or
well-developed lateral processes. _Neofiber_ exhibits a tendency, not
prominent elsewhere, to have a proportionately smaller median process
rather than reduced lateral processes.
American species of _Microtus_ (genus and subgenus) that have moderately-
to well-developed ossified lateral processes are _M. townsendii_, _M.
oeconomus_, _M. pennsylvanicus_, _M. montanus_, and _M. chrotorrhinus_.
_Microtus_ of other subgenera having this type of baculum include _M.
(Herpetomys) guatemalensis_, _M. (Chilotus) oregoni_, and _M. (Chionomys)
longicaudus_.
American species of _Microtus_ (genus and subgenus) in which the lateral
ossifications are weakly developed or absent (although cartilaginous
lateral processes are present) include _M. mexicanus_ and _M.
californicus_. In other subgenera, species of _Microtus_ having reduced
lateral ossifications are _M. (Pedomys) ochrogaster_, _M. (Pitymys)
pinetorum_, _M. (Pitymys) parvulus_, _M. (Pitymys) quasiater_, _M.
(Arvicola) richardsoni_, and _M. (Stenocranius) miurus_.
The microtines are essentially holarctic in distribution. Both of the
tribes, the lemmings and the voles, as well as primitive representatives
of each tribe (not considering _Ellobius_) occur in both the Old World
and New World. It is not certain on which continent (or continents) the
Microtinae first differentiated. It is certain, however, that at various
times, both early and late in the evolution of the subfamily,
representatives have crossed from Eurasia to North America or _vice
versa_. Each of 10 or more microtines in the New World is more closely
related to some microtine in the Old World than to any other microtine in
the New World.
The similarities or differences in the baculum in Old World and New World
representatives placed in the same genus or subgenus, or thought to be
"companion species" have been commented upon in accounts of _Lemmus_,
_Dicrostonyx_, _Clethrionomys_, _Lagurus_, _Arvicola_, _Stenocranius_,
_Chilotus_, _Chionomys_, _Pitymys_, and in accounts of _Microtus
agrestis_ as compared with _M. pennsylvanicus_, and _Microtus oeconomus_
(both Old World and New World).
The baculum in the Microtinae more closely resembles the baculum in the
Cricetinae of the Old World than in the Murinae, or than in any other
rodents known to me. This resemblance suggests relationship between
Microtinae and Cricetinae.
LITERATURE CITED
ARGYROPULO, A. I.
1933a. Die Gattungen und Arten der Hamster (_Cricetinae_ Murray, 1866)
der Palaearktik. Zeitschr. f. Saeugetierkunde, 8:129-149, 27 figs. in
text.
1933b. Ueber zwei neue palaearktische Wuehlmaeuse. Zeitschr. f.
Saeugetierkunde, 8:180-183, 3 figs. in text.
CALLERY, R.
1951. Development of the os genitale in the golden hamster,
_Mesocricetus (Cricetus) auratus_. Jour. Mamm., 32:204-207, 1 fig. in
text.
CHAMBERLAIN, J. L.
1954. The Block Island meadow mouse, _Microtus provectus_. Jour. Mamm.,
35:587-589, 2 tables in text.
DEARDEN, L. C.
1958. The baculum in _Lagurus_ and related microtines. Jour. Mamm.,
39:541-553, 1 fig. in text.
DIDIER, R.
1943. L'os penien des Campagnols de France du Genre _Arvicola_.
Mammalia, 7:74-79, 10 figs. in text.
1954. Etude systematique de l'os penien des Mammiferes (suite),
Rongeurs: Murides. Mammalia, 18:237-256, 14 figs. in text.
ELLERMAN, J. R.
1941. The families and genera of living rodents. Vol. II. Family
Muridae. The British Museum (Natural History), London, pp. xii + 690,
50 figs.
FRILEY, CHARLES E.
1947. Preparation and preservation of the baculum of mammals. Jour.
Mamm., 28:395-397, 1 fig. in text.
HALL, E. R., and E. L. COCKRUM.
1953. A synopsis of the North American microtine rodents. Univ. Kansas
Publ., Mus. Nat. Hist., 5:373-498, 149 figs. in text.
HAMILTON, W. J., JR.
1946. A study of the baculum in some North American Microtinae. Jour.
Mamm., 27:378-387, 3 figs. in text.
HIBBARD, C. W., and G. C. RINKER.
1942. A new bog-lemming (Synaptomys) from Meade County, Kansas. Univ.
Kansas Sci. Bull., 28:25-35, 3 figs. in text.
1943. A new meadow mouse (_Microtus ochrogaster taylori_) from Meade
County, Kansas. Univ. Kansas Sci. Bull., 29:255-268, 5 figs. in text.
HINTON, M. A. C.
1926. Monograph of the voles and lemmings (Microtinae), living and
extinct, Vol. I. British Museum (Natural History), London, pp. xvi +
488, plus 15 plates, 110 figs. in text.
MATTHEY, R.
1953. Les Chromosomes des Muridae. Revue Suisse de Zoologie,
60:225-283, avec les planches 1 a 4 groupant 84 photomicrographies, 98
figures et 5 schemas dans le texte.
1955. Nouveaux documents sur les chromosomes des Muridae. Problemes de
cytologie comparee et de taxonomie chez les Microtinae. Revue Suisse de
Zoologie, 62:163-206, avec 114 figures.
MILLER, G. S.
1896. Genera and subgenera of voles and lemmings. North American Fauna
No. 12, pp. 1-85, 40 figs. and 3 plates in text.
OGNEV, S. I.
1948. The mammals of Russia (USSR) and adjacent countries (The mammals
of Eastern Europe and Northern Asia), Vol. 6. Publ. Acad. Sci. USSR,
pp. 1-587, 260 figs., 12 maps, and 11 color plates in text (in
Russian).
1950. The mammals of Russia (USSR) and adjacent countries (The mammals
of Eastern Europe and Northern Asia), Vol. 7. Publ. Acad. Sci. USSR,
pp. 1-736, 347 figs., 15 maps, and 10 color plates in text (in
Russian).
RUTH, E. B.
1934. The os priapi: A study in bone development. Anat. Rec.,
60:231-249, 16 figs. in 3 plates.
SMITH, D. A., and J. B. FOSTER.
1957. Notes on the small mammals of Churchill, Manitoba. Jour. Mamm.,
38:98-115, 3 figs. and 3 tables in text.
WHEELER, B.
1956. Comparison of the Block Island "species" of _Microtus_ with _M.
pennsylvanicus_. Evolution, 10:176-186, 4 figs. and 2 tables in text.
WHITE, J. A.
1951. A practical method for mounting the bacula of small mammals.
Jour. Mamm., 32:125.
ZIMMERMAN, K.
1955. Die Gattung _Arvicola_ Lac. im System der Microtinae.
Saeugetierkundliche Mitteilungen, 3:110-112, 2 figs. in text.
_Transmitted August 14, 1959._
28-774
UNIVERSITY OF KANSAS PUBLICATIONS
MUSEUM OF NATURAL HISTORY