A New Subspecies of Wood Rat (Neotoma mexicana) From Colorado

BY

ROBERT B. FINLEY, JR.

University of Kansas Publications

Museum of Natural History

Volume 5, No. 30, pp. 527-534, 2 figures in text

August 15, 1953

University of Kansas

LAWRENCE

1953

University of Kansas Publications, Museum of Natural History

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

Volume 5, No. 30, pp. 527-534, 2 figures in text

August 15, 1953

University of Kansas

Lawrence

1953

PRINTED BY

FERD VOILAND, JR., STATE PRINTER

TOPEKA, KANSAS

1953

24-7674

[Pg 529]

A New Subspecies of Wood Rat (Neotoma mexicana) from Colorado

By

ROBERT B. FINLEY, Jr.

Field and museum studies of the wood rats of Colorado have revealed the
existence of an unnamed subspecies of Neotoma mexicana in eastern
Colorado south of the Arkansas River. The characters of the new
subspecies are most distinctive in the northeastern part of its range
near Two Buttes and Higbee. It differs in cranial characters from N. m.
fallax and N. m. inopinata and averages slightly larger, but cannot
be distinguished by coloration of the pelage.

This heretofore undescribed subspecies may be known as:

Neotoma mexicana scopulorum subsp. nov.

Type.—Mus. Nat. Hist., Univ. Kansas, No. 37137, old adult male, skin
and skull; from 37° 47' N, 103° 28' W, three miles northwest of Higbee,
4300 feet, Otero County, Colorado; trapped 16 May 1950 by R. B. Finley,
Jr., original number 500516-1.

Range.—Cañons, mesas, and foothills south of the Arkansas River, east
to Two Buttes, Colorado, and south to Clayton, New Mexico. The extent of
the range to the southwest in New Mexico has not been determined.

Diagnosis.—Size large for the species; interorbital constriction near
middle of frontal rather than anteriorly; supraorbital ridges of frontal
concave laterally; skull large, strongly arched at base of rostrum;
rostrum wide; nasals wide anteriorly; upper incisors wide, light yellow;
molars large, tooth-rows long; zygomatic arches wide and heavy;
interparietal short, wide, and posterior margin straight or with a
slight posterior median angle.

Description.—Adults in dense unworn pelage taken in February at Two
Buttes Reservoir: size large for the species; tail approximately 76 per
cent as long as head and body; hind feet of medium length. Pelage:
moderately long, thick; tail covered with short hairs; longest vibrissae
80 mm. Color: sides near Raffia (11 E 6) (capitalized color names and
designators are of Maerz and Paul, A Dictionary of Color, McGraw-Hill
Book Co., New York, 1930) overlaid with black, the general effect being
grayish buff (13 G 6); back darker, moderately to heavily overlaid with
black; indistinct dark eye ring; underparts whitish, fur basally gray
except patch of fur pure white to base almost always present on upper
throat; dark line around mouth; tail bicolor, black above, whitish
below; feet white to ankles.

Skull: large for the species, strongly arched at base of rostrum;
rostrum heavy; zygomatic arches widely spreading, heavy, squarish;
braincase moderately ridged and angular; nasals wide anteriorly, lateral
margins nearly parallel or converging evenly posteriorly, tapered
abruptly at posterior ends which[Pg 530] reach posteriorly beyond anterior
plane of orbits; dorsal branches of premaxillae extending 0.5 to 1.2 mm.
posterior to nasals; interorbital region moderately channeled, narrowly
constricted near middle of frontal (instead of anteriorly); supraorbital
ridges concave laterally, diverging more strongly posterior to
interorbital constriction (frontal 8.7 to 9.5 mm. wide at posterior ends
of supraorbital ridges); temporal ridges widely flaring on parietals;
occipital ridges prominent; interparietal broadly rectangular between
temporal ridges, usually short in median line of skull, posterior margin
straight or with slight median posterior angle; incisive foramina
tapered toward both ends, sometimes narrower anteriorly than
posteriorly; anterior palatal spine usually forming a blade thickened on
ventral edge, and right and left sides usually incompletely fused; nasal
septum with a posterior notch separating vomer from maxillary; posterior
margin of palate usually bearing single or double point, sometimes
straight; interpterygoid fossa moderately wide, lateral margins concave;
sphenopalatine vacuities large; auditory bullae of medium size;
basioccipital with low median ridge or crest; upper incisors wide,
yellow or yellow-orange; molars large, M1 wider than M2; maxillary
tooth-rows long, nearly parallel; anterointernal fold of M1 deep,
cutting more than half way across first enamel loop.

Figs. 1-6. Skulls of Neotoma mexicana. All × 1.

Figs. 1-2. Neotoma mexicana scopulorum, holotype.

Figs. 3-6. Neotoma mexicana fallax, 1-1/2 mi. NW Golden, 6200 ft.,
Jefferson County, Colorado, 8 June 1948, No. 29182 KU.

Adult in worn pelage taken in May at Two Buttes peak: no molt in
evidence; pelage thinner and rougher than in adults of same tooth wear
taken in Febru[Pg 531]ary in unworn pelage (described above); upper parts
duller, less heavily overlaid with black; sides less richly yellowish,
slightly more pinkish in hue; underparts with no fur white to base (as
usual for the species). The skull of this rat has narrower nasals than
other adults from Two Buttes and a longer interparietal with a posterior
median angle.

Subadult taken in April at Regnier: completing postjuvenal (first) molt;
new pelage fairly long and thick everywhere except on neck and upper
back, where covered by remaining juvenal pelage; upper parts of new
pelage duller than in adults, sides less buffy, more grayish; juvenal
pelage grayer than new pelage; new pelage indistinguishable from same
pelage (second pelage of first year) of N. m. fallax.

Comparisons.—N. m. scopulorum is extremely variable in color but
averages lighter and richer in color than N. m. fallax, and about the
same as N. m. inopinata. N. m. scopulorum can be separated from
either by the following cranial differences: skull larger, more strongly
arched at base of rostrum, interorbital constriction more posterior;
supraorbital ridges concave laterally (in contrast to straight,
diverging); interparietal shorter in median line, more widely spreading
and rectangular; zygomatic arches more widely spreading and heavier;
upper incisors wider; and molars larger. N. m. scopulorum differs from
inopinata also in paler upper incisors and less prominent basicranial
ridges.

N. m. scopulorum is paler than N. m. pinetorum. The skulls of these
two subspecies are of about the same size, but the subspecies differ in
other respects as scopulorum differs from fallax and inopinata.

Judging from the description and photograph of mexicana in Goldman's
revision of the genus Neotoma (N. Amer. Fauna, 31: 54-56, Pl. IV, 19
October 1910), scopulorum differs from N. m. mexicana in: larger
skull; longer nasals and dorsal branches of premaxillae; more posterior
interorbital constriction (supraorbital ridges more concave laterally);
wider upper incisors; and larger molars.

Measurements.—Mean and extreme measurements in millimeters of 6 males
and 5 females from 3 mi. NW Higbee and the vicinity of Two Buttes are,
respectively, as follows: total length, 357 (345-368), 345 (310-379);
length of tail, 147 (140-158), 159 (138-178); length of hind foot, 35
(32-38), 36.4 (35-38); length of ear, from notch, 25.5 (25-26), 25.7
(25-27); weight (in grams), 234 (213-253), 206 (161-246); basilar
length, 37.9 (36.8-38.9), 36.2 (34.5-38.6); length of nasals, 19.0
(18.2-20.0), 17.9 (16.4-19.6); zygomatic breadth, 23.9 (23.0-24.5), 23.3
(22.3-24.0); interorbital breadth, 5.3 (4.9-5.6), 5.1 (5.0-5.3); breadth
of rostrum, 7.2 (6.8-7.7), 6.8 (6.7-6.9); diastema, 12.8 (12.3-13.3),
12.2 (11.1-13.7); alveolar length of maxillary tooth-row, 9.5 (9.2-9.8),
9.4 (9.0-9.7); length of incisive foramina, 9.7 (9.2-10.2), 9.2
(8.6-10.2); length of palatal bridge, 8.8 (8.4-9.2), 8.5 (8.0-8.9).

Measurements of the type.—Total length, 348; length of tail, 143;
length of hind foot, 35; length of ear, from notch, 25; weight (in
grams), 230; basilar length, 38.1; length of nasals, 18.8; zygomatic
breadth, 24.2; interorbital constriction, 5.5; breadth of rostrum, 7.2;
diastema, 13.0; alveolar length of maxillary tooth-row, 9.2; length of
incisive foramina, 9.7; length of palatal bridge, 8.9.

Remarks.—The large size and distinctive cranial characters of N. m.
scopulorum are fairly constant in the northeastern part of its range,
but there is a wide range of variation in color. The only two skins from
the type locality differ[Pg 532] markedly in color. Both specimens (the type
and KU 37138, adult male) were collected on 16 May 1950 and are in
moderately worn pelage. The upper parts of the holotype are much more
yellowish than in KU 37138, and are even lighter buff than adults in
unworn pelage from Two Buttes. The underparts of the holotype are more
extensively white than in almost any other specimen seen of Neotoma
mexicana. The basal gray coloration, where it is present along the
sides of the venter, forms only a narrow intermediate color band
extending not more than one third the length of the hairs. An extensive
area of the throat, breast, axillae, median belly, and inguinal region
is covered by hairs pure white to the skin. The dark line around the
mouth is present, as usual for the species. The upper parts of KU 37138
are like those of the adult in worn pelage from Two Buttes peak,
described above; the underparts have only small patches of pure white
fur on the throat and inguinal region, being elsewhere gray at the base
of the fur, as is usual for the species.

The molars of the type specimen are in an advanced state of wear, having
the pattern of the enamel folds still discernible but the depth of
remaining enamel slight. A large alveolar abscess surrounds the abnormal
left M1. There are two, much worn, peglike fragments of the tooth
projecting slightly from an ovoid alveolar cavity 5.1 mm. long and 4.3
mm. wide. As a result of the reduction of wear on the opposing m1, the
crown of m1 is much less worn than those of the other lower molars and
projects 0.8 mm. above the occlusal level of the two posterior molars. A
few barbed cactus glochids (bristles) are inbedded in the cavity around
the base of the molar remnants. Although glochids are of rather frequent
and normal occurrence between the teeth of Neotoma albigula and N.
micropus, they are not so commonly found in N. mexicana and possibly
induced the alveolar infection in this individual.

In addition to the skins in unworn and worn pelages already described
from Two Buttes, an extremely dark specimen is at hand from Two Buttes
peak, taken on 9 May 1950. This specimen (KU 37141 ) is an adult
in moderately worn pelage. The back is dark brownish gray (Taupe, 16 A
6), the sides lighter (a shade lighter than Beaver, 15 A 6). The entire
underparts are washed with reddish buff (Grain, 11 B 5) over the gray
basal coloration, with a patch of white only in the genital region. The
dark eye ring and dark line around the mouth are heavier than usual. The
underside of the tail is light gray. The white hind feet are sharply set
off from the dark gray ankles.

Each of four skulls from Regnier (three adults and one subadult) differs
from skulls from Two Buttes in having a longer interparietal with a
posterior angle. The skins of five adults collected in December at
Trinchera are less richly colored on the sides than skins from Two
Buttes and look more nearly like topotypes of N. m. fallax. The skull
of one of the five from Trinchera differs from skulls from Two Buttes in
much narrower nasals anteriorly, narrower rostrum, much narrower upper
incisors, and smaller zygomatic breadth, these characters being as in
fallax.

Four adults and one subadult from Trinidad are intergrades between N.
m. scopulorum and N. m. fallax, perhaps more nearly resembling the
former. In pelage they are indistinguishable from specimens of fallax
from Gold Hill (the type locality), less buff than most individuals of
scopulorum from Otero, Prowers, and Baca counties. The skulls of the
three fully mature adults are large with a wide zygomatic breadth, large
rostrum, and large upper incisors[Pg 533] as in scopulorum; but the upper
molars are small and the bullae are rather small and narrow as in
fallax. In the degree of arching at the base of the rostrum, the shape
of the frontal, the shape of the interparietal, and the size of the
upper molars, the specimens from Trinidad are intermediate. It seems to
me best to refer them to scopulorum.

Two first-year adults from Fisher Peak and Long Cañon are
indistinguishable from topotypes of fallax of similar age and also
resemble a young adult and a subadult from Trinidad, but all are
insufficiently mature to show subspecific characters distinctly. Until
adequate series are available from southwestern Las Animas County it
seems best to regard all specimens from the three localities as
representatives of a single uniform population which is intermediate
between fallax and scopulorum but more nearly like the latter.
Unfortunately no other specimens are available from the foothill zone
south of the Arkansas River where morphological intergradation and
ecological transition between fallax and scopulorum might reasonably
be expected to occur.

Three specimens from the north side of the Arkansas River, about 26
miles below Canon City, Pueblo County, are like fallax in size, dorsal
profile of the skull, and shape of the interorbital constriction; but
they approach scopulorum in shape of the interparietal, size of the
rostrum, and size of the molars. They are intergrades referrable to
fallax.

Neotoma mexicana was first reported from Oklahoma by W. Frank Blair in
1939 (Amer. Midl. Nat., 22:126) who referred a specimen from Tesequite
Canyon, Cimarron County, to N. m. fallax. I have seen one specimen (MZ
80469) from Tesequite Canyon and refer it to scopulorum.

Of scopulorum, each of eight skulls, of the 28 skulls examined, has an
anteroexternal enamel fold on the m3 and one (BSC 35222/47487 )
has an anterointernal fold on the m3. Of the other 19 mandibles, a few
are too old to show such a fold, which tends to be obliterated with wear
in later age, and the others lack the fold.

Two other wood rats (N. albigula warreni and N. micropus canescens)
occur at many of the same localities as N. m. scopulorum. The dens of
scopulorum almost always are situated among rocks, but the dens of
warreni and canescens are in a variety of other situations as well
as among rocks. Houses of sticks or cactus joints piled up around the
base of a juniper (Juniperus monosperma), thicket of skunkbush (Rhus
trilobata), clump of soapweed (Yucca glauca) or tree cactus (Opuntia
arborescens) have been found to shelter only N. a. warreni or N.
micropus canescens. When these wood rats are associated with
scopulorum among the rocks, their dens can be recognized by the
compact midden of innumerable cactus spines. The dens of scopulorum
have only a few loosely scattered spine areoles or none at all.

I am grateful to the officials of the following institutions for
permission to examine specimens from the collections under their care:
Denver Museum of Natural History; Biological Survey Collection, U. S.
Fish and Wildlife Service; American Museum of Natural History; Museum of
Vertebrate Zoology, University of California; Museum of Zoology,
University of Michigan. The drawings of the skulls were made by Victor
Hogg.

Specimens examined.—Total 66, distributed as follows:

Colorado. Baca County: "Furnace Canyon" [= Furnish Canyon], 1 (DMNH);
Regnier, 4500 ± ft., 4 (2 DMNH, 2 KU); Two Buttes Reservoir,[Pg 534] 4200 ±
ft., 5 (3 DMNH, 2 KU). Las Animas County: Fisher Peak, "about 8000
ft." [6 mi. SE Trinidad], 1 (BSC); Long Cañon (near Martinsen), 1 (BSC);
Mesa de Maya, 1 (MZ); 9 mi. W jct. Purgatory [= Purgatoire] & Chaquaqua
[= Chacuaco] rivers ("Red Rock Canyon," collector's field notes), 1
(MVZ); Trinchera, 6 (5 DMNH, 1 AMNH); Trinidad, 5 (BSC); 20 mi. E
Walsenburg, "Huerfano Co." [probably Las Animas County], 1 (DMNH).
Otero County: 3 mi. NW Higbee, 4300 ft., 4 (KU). Prowers County: Two
Buttes peak, 4600 & 4650 ft., 2 (KU).

New Mexico. Union County: Clayton, 9 (BSC); 9 mi. NE Des Moines on the
"Carramba River" [= Cimarron River], 1 (DMNH); Folsom, 6 (BSC); Raton
Range (Oak Cañon), 8 (BSC); Sierra Grande, 9 (BSC).

Oklahoma. Cimarron County: Tesequite Canyon, 1 (MZ).

Museum of Natural History, University of Kansas, Lawrence. Transmitted
April 20, 1953.

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