Table Of ContentPROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON
112(4):813-856. 1999.
Identification and distribution of cotton rats, genus Sigmodon
(Muridae: Sigmodontinae), of Nayarit, Mexico
Michael D. Carleton, Robert D. Fisher, and Alfred L. Gardner
(MDC) Division of Mammals, National Museum of Natural History, Smithsonian Institution,
Washington, D.C. 20560, U.S.A.; (Ba)F, ALG) Patuxent WildUfe Research Center, Biological
Resources Division, U.S. Geological Survey, National Museum of Natural History,
Washington, D.C. 20560, U.S.A.
—
Abstract. Morphological, chromosomal, distributional, and ecological data
are presented for three species of Sigmodon (S. alleni, S. arizonae, and S.
mascotensis) from the state of Nayarit, Mexico. The species were collected in
all possible pairwise combinations of sympatry, including the first record of
such documented for S. arizonae and S. mascotensis. Emphasis is devoted to
the discrimination of S. arizonae and S. mascotensis, from each other and from
typical S. hispidus, using qualitative features of the skin and skull and mor-
phometric analyses of craniodental measurements. Based on these results and
examination of type specimens, additional synonyms of S. mascotensis are
identified, with reassignment of two forms, tonalensis Bailey (1902) and ob-
velatus Russell (1952), currently mistaken as subspecies of S. hispidus. Sig-
modon mascotensis emerges as a species distributed from southern Nayarit and
Zacatecas to extreme western Chiapas, where it inhabits deciduous or semi-
deciduous tropical vegetation having a pronounced dry season. These reallo-
cations and other reidentifications remove any documentation for S. hispidus
along the entire Pacific versant of Mexico.
A
useful form of research communica- xico, and the taxon of interest is the genus
tion that sees less application today is the Sigmodon, the ecologically abundant cotton
brief expeditionary account or short faunal rats that inhabit open landscapes from the
note. Aside from the practical enhance- southern United States, through Mexico and
ments in knowledge of a taxon's distribu- Middle America, to northern South Amer-
tion and habitat, such reports offer the ad- ica (Hall 1981, Voss 1992). Situated along
vantage of bringing regionally focussed no- coastal westcentral Mexico, the state of
menclatural clarity to complex taxonomic Nayarit encloses a varied topography and
problems that seem incomprehensible over diverse natural environments, a biogeo-
a broader geographic scale. One recalls that graphical setting that has proven pivotal for
the prolific literature appearing over the illuminating the systematics of other small
past two decades on the Peromyscus boyHi mammals (for example, Fisher & Bogan
group emanated from Hooper's (1955) im- 1977, Gardner 1977, Bogan 1978, Diersing
memorable commentary in "Notes on & Wilson 1980, Carleton et al. 1982, Wil-
Mammals of Western Mexico," in which he son 1991).
recorded the sympatric occurrence of vari- The excellent series of Nayarit cotton rats
ous "morphological types" of boylii at sev- collected by personnel of the U.S. Fish and
eral collecting localities in Jalisco, Nayarit, Wildlife Service in the middle 1970s war-
and Sinaloa (see systematic reviews by rant report in view of Zimmerman's (1970)
Carleton 1989, and Bradley et al. 1996). seminal report on Sigmodon taxonomy. His
The regional focus here is Nayarit, Me- study, and the subsequent contributions of
814 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON
&
Severinghaus Hoffmeister (1978) and History, Smithsonian Institution, Washing-
Hoffmeister (1986), revealed the greater ton, D.C. (USNM, the abbreviation for the
species diversity and distributional com- former United States National Museum). A
plexity of Sigmodon found in the south- few originated from the pioneering Biolog-
western United States and northwestern ical Survey of Mexico conducted by E. A.
Mexico. In this report, we document the Goldman and E. W. Nelson (1897 expedi-
kinds and distribution of Sigmodon species tion to Tepic; see Goldman 1951), but most
in Nayarit {S. alleni, S. arizonae, and S. were collected recently (1975-1977) by
mascotensis); review morphological and personnel associated with the U.S. Fish and
chromosomal characteristics for identifying Wildlife Service (now part ofthe USGS Pa-
the species, with emphasis on discrimina- tuxent Wildlife Research Center), pursuant
tion of S. arizonae and S. mascotensis; and to a faunal analysis of Nayarit mammals.
amplify the known geographic range of S. Other specimens reported here, including
mascotensis in western Mexico, including holotypes and type series, are housed in the
the reallocation offorms currently classified American Museum ofNatural History, New
as subspecies of S. hispidus (namely, Sig- York (AMNH); the Field Museum of Nat-
modon hispidus tonalensis Bailey, 1902, ural History, Chicago (FMNH); Museum of
and Sigmodon hispidus obvelatus Russell,
Natural History, University of Kansas,
1952).
Lawrence (KU); University of Michigan
Museum of Zoology, Ann Arbor (UMMZ);
Materials and Methods
The Museum, Michigan State University,
The 214 specimens of Nayarit Sigmodon East Lansing (MSU); and Texas Coopera-
that form the nucleus of this report are con- tive Wildlife Collections, College Station
tained in the National Museum of Natural (TCWC).
—
Table I. Gazetteer of collecting localities (see Fig. 1).
Elevation Coordinates
Locality (meters) NAV
1. Tacote, 1.4 mi N 15 22°387105°27'
2. Rio Canas, near La Concha 20 21°317105°26'
3. Acaponeta 46-75 22°297105°21'
4. Cuautla, 1 mi S 22°127105°38'
5. Santa Cruz, 6 km S 21°567105°35'
6. Playa Colorado 21°537105°34'
7. Playa Los Corchos, 4 mi E 21°437105°25'
8. San Bias 21°327105°17'
9. Aticama, 4 km S 50 21°277105°11'
10. Chacala 30 21°107105°13'
11. Lo de Marcos, 1 mi S 20°577105°21'
12. El Venado, 3.5 mi E 100 22°577104°57'
13. Arroyo de Jiguite 100 2r497104°48'
14. El Casco, 1.2 mi S 60 21°457104°51'
15. La Villita, 1 km S 760 21°357104°56'
16. Tepic 1000 21°307104°53'
17. San Pedro Lagunillas, 2 mi E 1300 21°127104°43'
18. Estanzuela 1380 21°167104°28'
NW
19. Coapan, 1.8 mi 1560 21°097104°29'
20. Ahuacatlan, 8 mi S 1500 20°587104°28'
21. Mesa del Nayar 1300 22°127104°39'
22. Ocota Airstrip 1900 21°507104°13'
23. Rancho Sapotito 1100 21°207103°58'
VOLUME 112, NUMBER 4 815
Fig. 1. State of Nayarit, Mexico, illustrating collecting localities of Sigmodon specimens housed in the
National Museum of Natural History (numbers refer to sites listed in Table 1).
Twenty-three Nayarit localities are rep- and later verified against the same topo-
resented in the preserved material (Table 1, graphic series.
Fig. 1). Coordinates of collecting sites were Animals were generally preserved as
later determined from 1:50,000 topographic conventional study skins and skulls but also
maps used by the field teams and annotated as complete skeletons and formalin-fixed,
with the localities visited; most elevations whole carcasses stored in alcohol. Prepa-
were recorded in the field with an altimeter ration of standard chromosomal spreads,
816 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON
definition of fundamental number (FN), and extending from the arches; breadth across
descriptive terminology for chromosomal occipital condyles (BOC); depth of brain-
morphology follow Patton (1967) and case (DBC); distance between temporal
—
&
Gardner Patton (1976). The 32 voucher ridges (DTR) vertical distance between
specimens for the karyotypic variation re- the dorsal margin of the superior temporal
USNM
ported are contained in and repre- ridge and the ventral edge of the inferior
sent the localities listed next. temporal r—idge (see Fig. 8); length of ros-
Sigmodon alleni: Nayarit, Arroyo de Jigui- trum (LR) measured obhque to the mid-
te, Rio Santiago, 1 6 (523935); 4 km S longitudinal cranial axis, from the inner-
Aticama, 1 S (524480), 2 $ (524478, most bevel of the right zygomatic notch to
524479); Chacala, 1 9 (523934); 1.2 mi the end of the nasals at their midsa—gittal
S El Casco, 1 ? (511699); 1 mi S Lo de junction; breadth of rostrum (BR) dis-
Marcos, 1 S (523943), 3 ? (523940- tance across the lateralmost convexity of
523942). the nasolacrimal capsules; postpalatal
length (PPL); length of bony palate (LBP);
S. arizonae: Nayarit, Mesa del Nayar, 1 $
breadth of bony palate (BBP); length of in-
(511700); Ocota Airstrip, 2 S (523948,
cisive foramen (LIF); length of diastema
523950), 4 9 (523946, 523947, 523949,
(LD); breadth of zygomatic plate (BZP);
523951). Sinaloa, Microondas La Mur- —
length of zygomatic spine (LZS) distance
alla, 1 6 (524487), 1 $ (524486).
S. mascotensis: Nayarit, 8 mi S Ahuacatlan between the anterior orbital rim and the tip
of the zygomatic spine; length of auditory
1 S (523953), 2 ? (523952, 523954);
bulla (LAB); coronal length of maxillary
Arroyo de Jiguite, Rio Santiago, 4 S
toothrow (LMl-3); coronal width of upper
(523962, 523963, 523965, 523966), 1 $
(523964); Rancho Sapotito, 2 S (511703, first molar (WMl); depth of up—per incisor
(DI); depth of mandible (DM) distance,
511704), 3 ? (511702,511705,511706).
taken on the lateral surface of the dentary,
We recorded 5 external, 18 cranial, and from the rim of the ml alveolus to the ven-
3 dental dimensions (in millimeters, mm) to tralmost projection of the mental symphy-
summarize patterns of variation within and sis.
between the populations sampled. Total Relative age was coarsely indexed by de-
length, length of tail vertebrae, hindfoot gree of molar wear according to the four
length, length of pinna from notch, and age-classes recognized by Carleton and
—
mass (weight in grams, g) were transcribed Musser (1989) juvenile (J), young (Y),
from skin labels. Crania were viewed under full (A), and old-adult (O). The distinction
a dissecting microscope when measuring between juvenile and young adult cohorts
mm
the 21 craniodental variables to 0.01 based on upper third molar eruption consti-
with hand-held, digital calipers accurate to tuted a more objective criterion of age rec-
0.03 mm. These measurements, their abbre- ognition than did the assignment ofindivid-
viations as used in tables and figures, and uals among the three adult classes based on
their landmark definitions where clarifica- gradations of wear. Among specimens with
tion is necessary, include (see Carleton and annotation of their reproductive state, many
Musser, 1995, for illustration of most ana- we assigned as young or full adult based on
tomical endpoints): occipitonasal length tooth wear exhibited signs of reproductive
(ONL); zygomatic breadth (ZB); least in- maturity (testis scrotal, evidence of lacta-
terorbital br—eadth (lOB); breadth of brain- tion, counts of embryos or embryo scars),
case (BBC) taken behind the squamosal whereas those classified as juveniles did
root of the zygomatic arches, the caliper's not.
jaws resting on the squamosal bones just To augment sample sizes for the various
above the flange (inferior temporal ridge) morphometric comparisons, Nayarit speci-
VOLUME NUMBER
112, 4 817
mens were grouped into eight analytical 1969, Voss 1992, Zelditch et al. 1992). Cot-
samples (operational taxonomic units, ton rats are highly precocial and weaned an-
OTUs), as defined below (locality numbers imals soon enter the trappable population;
correspond to those identified in Table 1 seasonal differences in age composition of
and Fig. 1). To provide a comparative stan- a population, as well as differences in
dard, we included a homogeneous series of growth rates between age cohorts, may be
Sigmodon hispidus, sensu stricto, as a ninth remarkable (Layne 1974, Slade et al. 1984).
OTU (U.S.A., Florida, Wakulla Co., St. In view of such demographic factors, we
Marks National Wildlife Refuge; n = 53; found age variation to be equally conspic-
USNM
526059-526106, 527358, 527359, uous in all species samples of Sigmodon re-
527362-527364). Since most collectors at ported here, an impression quickly formed
this locality also participated in the Nayarit from superficial observation of crania and
survey, one can expect procedural confor- their wide range in size, shape, and rugos-
mity in the measurement of external vari- ity. Casual visual impressions are rein-
ables. forced by statistical comparisons among
Sigmodon alleni: OTU I, n = 20, localities age groups in the large sample of S. hispi-
8-15. dus (Table 2). Nearly all measurements dis-
S. arizonae: OTU 2, n = 9, localities 2-7; play regular, incremental increases in size
OTU 3,n = 25, locaHty 17; OTU A, n = across the four age classes we defined, pro-
6, locality 22. ducing age-correlated differences that con-
S. mascotensis: OTU 5,n = 27, locality 13; tribute substantially to nongeographic vari-
OTU 6, n = 46, locality 17; OTU l,n = ation within our locality samples. Notable
12, localities 18-20; OTU 8, n = 6, lo- exceptions include dimensions of the mo-
cality 23. lars (LMl-3, WMl), which once erupted
S. hispidus: OTU 9, n = 53, Florida. decrease in crown height with occlusal use
but do not grow in size; in contrast, girth
Standard descriptive statistics (mean,
of the incisor (DI) enlarges appreciably as
range, standard deviation) were obtained
cotton rats age.
for the OTUs. Principal components and ca-
The contribution of sexual dimorphism
nonical variates were extracted from the
to intrasample variation, on the other hand,
variance-covariance matrix and computed
is hardly apparent, at least given the unbal-
using natural logarithmic transformations of
anced nature of analyzable material usually
the 21 craniodental variables. Loadings are consolidated from museum collections.
expressed as Pearson product-moment cor- Only two (lOB, DTR) of the 21 cranio-
relation coefficients of the principal com-
dental measurements yielded significant
ponents or canonical variates with the orig-
differences according to sex (Table 2), and
inal skeletal and dental variables. All uni-
these are sufficiently infrequent and inci-
variate and multivariate computations were
dental to suggest sampling error as an ex-
generated using Systat (Version 7.0, 1997),
planation.
a series of statistical routines programmed
Other than procedural elimination of the
for microcomputers.
youngest age class, juveniles, we did not
adjust for size in the morphometric analy-
Discrimination of Nayarit Sigmodon and
ses. Although variation attributable to post-
Comparisons with Sigmodon hispidus
— weaning growth may be substantial within
Intrasample age and sex variation. samples of Sigmodon, it is typically negli-
Field and lab workers who study Sigmodon gible relative to the interspecific contrasts
populations have regularly commented on that proved to be taxonomically important.
the considerable ontogenetic variation en- In this respect, patterns of morphometric
countered (e.g., Chipman 1965, Baker differentiation among cotton rats, their con-
—
818 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON
—
Table 2. Arithmetic means ofcraniodental variables and results ofone-wayANOVAs for sex and age cohorts
in a large sample of Sigmodon hispidus from Florida (OTU 9, n = 53).
Sex Age
M F J Y A o
Variable (31) (22) /(sex) (3) (13) (29) (8) /(age)
ONL
34.8 34.3 1.0 30.2 32.8 34.9 37.0 33 J***
ZB 19.5 19.3 1.5 17.2 18.7 19.5 20.3 21 7***
lOB 5.2 5.1 4.2* 4.8 5.0 5.2 5.3 7 g***
BBC 14.4 14.2 2.1 13.5 14.2 14.4 14.7 g 3***
BOC
7.5 7.5 0.4 7.5 7.4 7.4 7.7 1.9
DBC 10.7 10.7 0.5 10.1 10.6 10.7 11.2 8.6***
DTR 3.1 2.9 7.2** 2.4 3.0 3.1 3.3 12.4***
LR 11.6 11.5 0.2 9.8 10.9 11.7 12.5 21.4***
BR 6.7 6.6 0.9 6.1 6.3 6.7 7.3 22 g***
PPL 12.2 12.0 0.4 10.5 11.3 12.2 13.3 34 1***
LBP 6.6 6.5 1.7 6.1 6.3 6.7 6.7 6.2**
BBP 7.7 7.7 0.2 7.0 7.5 7.8 8.1 24.1***
LIF 7.8 7.7 0.1 6.7 7.3 7.9 8.5 19 g***
LD 5***
9.6 9.3 1.1 8.1 8.7 9.7 10.5 3]^
BZP 3.9 3.7 2.8 3.2 3.7 3.9 4.1 9 7***
LZS 4.6 4.4 1.3 3.6 4.3 4.6 4.9 11 3***
LAB 6.3 6.3 0.0 6.1 6.1 6.4 6.5 6.3**
LMl-3 6.4 6.4 0.2 6.4 6.4 6.4 6.3 0.5
WMl
2.1 2.1 0.5 2.1 2.1 2.1 2.1 0.5
DI 2.0 2.0 0.4 1.6 1.9 2.0 2.1 49.0***
DM 6.4 6.3 1.6 5.6 6.1 6.5 6.6 14 9***
= P < 0.05; **=/>< 0.01; *** = p < 0.001.
spicuous age variety notwithstanding, are mascotensis and S. arizonae on the first
much like those derived for other closely principal compone—nt (loadings uniformly
related, congeneric species of New World large and positive Table 3); the generous
&
muroids (e.g., Carleton Musser 1989, proportions of the latter's molars provide
&
1995; Voss Marcus 1992, Hoofer et al. most discrimination on the second compo-
WMl
1999). nent (LMl-3 and correlations rela-
—
Morphometric variation. Covariation tively large and negative). The resulting
patterns derived from the 21 original cra- constellations of specimen scores conform
niodental variables suggest two kinds of to the now familiar ordination pattern evi-
large Sigmodon among Nayarit populations denced between morphologically similar,
(Fig. 2). These clusters correspond to the closely related (congeneric) species of Mu-
species S. arizonae and S. mascotensis roidea; furthermore, their elongate spread
(Figs. 3, 4), identifications independently and orientation, obUque to the bivariate plot
corroborated by agreement of karyotyped of PC I and PC 11, suggest the interplay of
specimens with currently understood differ- consistent interspecific shape differences
ences in diploid number (2N = 22 or 28; and age-related size increases (Voss et al.
Zimmerman 1970) and by contrasts in cer- 1990, 1992). Included among our samples
tain qualitative features (see below). The is one locality where the two species were
"correct" phenetic association of relevant collected in sympatry (2 mi E San Pedro
holotypes arizonae, major, and mascoten- Lagunillas).
—
sis (Fig. 2) bolsters the use ofthese names Unambiguous specific discrimination is
for the Nayarit populations. Greater size in preserved in the discriminant function anal-
all dimensions accounts for separation of S. ysis of the eight OTUs representing S. ar-
VOLUME 112, NUMBER 4 819
A A S. mascotensis o S. arizonae
1 1 i 1 1
A
- -
^ A ^
A
--^"^
.A mascotensis .^
. l^\
-
1 - /^ A,*\.^^^ AA
A
A
#
^ arizonae
- ">f ^ \ A ^ A ^ ^ + major
o
-^"^ ^ A^A •
A ^
o ^A A
Q_
— A
-^O^
O ^ o o
A
^M^
A
—
-2
-
•
•
•
-3 - -1.
-3-2-10
1 1 1 1 i
2 3
1
PC
(75.8 %)
I
Fig. 2. Plot of first and second principal components extracted from analysis of 21 log-transformed cran-
iodental variables measured on intact specimens of Nayarit Sigmodon arizonae {n — 29) and S. mascotensis {n
= 79). Open symbols indicate specimens collected in close proximity at 2 mi E San Pedro Lagunillas, 1300 m;
crossesreferto relevanttype specimens discussed in the text. Regression lines ofPC II on PC I differsignificantly
between species in their y-intercepts (0.54 versus —1.50; F = 144.9, P < 0.001) but not their slopes (0.42 versus
0.51; F = 0.3, P = 0.553). See Table 3.
(zonae, S. mascotensis, and S. hispidus (Fig. yses of variance of the 21 craniodental mea-
5). Separation of Floridian S. hispidus on surements among the three species. Less
the first canonical variate extracted primar- conspicuously, the relatively greater length
ily results from—differences in three vari- of the facial region in S. hispidus is reflect-
ables (Table 4) the larger size of the au- ed in the moderate, positive loadings for
ditory bullae (LAB), the narrower distance lengths of rostrum and incisive foramen,
between the temporal ridges (DTR), and, whose univariate means match those of the
perhaps in correlation with the latter, the bigger S. arizonae. The generally larger
shallower braincase (DBC). LAB also gen- values and comparable range (mostly —0.5
erated the largest /-value in one-way anal- to —0.7) of correlations on the second ca-
820 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON
—
Table 3. Results of principal component analysis sis is straightforward based on pelage color
and one-way ANOVAs performed on all intact speci- (see below) or diploid number (2N = 52;
mteennsiss o(fn S=ig7m9o)dfornomarNiazyoanraiet,(Mne=xic29o)(asenedFSi.g.ma2)s.co- Zimmerman 1970). Yet in cranial size and
discrete characteristics, S. alleni (Fig. 6) un-
Correlations expectedly proved somewhat difficult to
ANOVAs
Variable PC I PC II f(species) distinguish from S. mascotensis', for exam-
ONL 0.97 0.15 20.9*** ple, most univariate measurements of the
ZB 0.98 0.02 55 J*** two overlap extensively (Appendix) and
lOB 0.75 0.07 4.1* disclose relatively few significant differenc-
BBC 0.88 -0.20 70.0*** es between means (Table 5). Principal com-
BOC 0.78 -0.22 22 J***
DBC 0.81 0.08 21 0*** ponent analysis of log-transformed vari-
DTR 0.82 -0.01 50.0*** ables nonetheless supplied clear separation
LR 0.94 0.16 19 4*** of the two species based on crania alone
BR 0.95 0.16 17.8*** (Fig. 7). The variables most influencing dis-
PPL 0.96 0.13 23.9*** crimination represent measurements taken
LBP 0.78 0.13 8.1** on the braincase (DBC, DTR), zygoma
BBP 0.94 -0.13 4]^ 9*** (LZS), and molars (LMl-3, WMl), each of
LIF 0.94 0.01 44.3***
LD 0.96 0.18 22 9*** which is notably greater in S. mascotensis
BZP 0.86 0.28 2.7 (Table 5; Appendix). Holotypes of interest
LZS 0.88 0.16 12.9*** within the region appropriately associate
LAB 0.85 -0.33 144.5***
with their respective Nayarit populations,
LMl-3 0.64 -0.70 189 3***
WMl those of mascotensis and colimae more so
0.53 -0.79 206.6***
DI 0.92 0.15 11.1** and that of alleni less so. Although the dis-
DM
0.97 0.10 28.2*** position of the type specimen of alleni was
Eigenvalue 15.9 1.6 somewhat peripheral as divulged in PCA,
%
Variance 75.8 7.7
discriminant function analysis based on all
* = P < 0.05; ** = />< 001" *** = P < 0.001. nine OTUs (not illustrated) indisputably
placed the holotype with the Nayarit sample
of the taxon.
nonical variate are more suggestive of gen- Although external dimensions were ex-
eral size and again underscore the robust cluded from multivariate examination, they
proportions of S. arizonae apparent in vi- generally reflect the same pattern of inter-
sual inspection of skulls; the substantial di- specific size contrasts noted for cranioden-
WMl)
mensions of its molars (LMl-3, are tal variables and supply helpful guidance
particularly noteworthy in this regard. Sam- for first-approximation field or museum
ples of S. mascotensis approximate that of identification (Appendix). The absolutely
S. hispidus on the second axis. short and relatively narrow hind foot of S.
A
posteriori classifications of type spec- alleni, for example, readily separates that
imens historically associated with the S. species from young examples of S. masco-
hispidus complex, as implemented in dis- tensis. As noted by Bailey (1902), hindfoot
criminant function analysis of the same size and tail length, absolute and relative,
eight OTUs, accord with some present tax- help to distinguish S. mascotensis from ex-
onomic alignments and dispute others. The amples of S. h. hispidus and h. berlandieri.
nomenclatural significance of these statis- Relative length oftail in S. mascotensis (TL
tical assignments is considered in the Dis- ca. 45% of TOTL) also exceeds that in the
cussion—and Taxonomic Summary (Fig. 17, larger-bodied S. arizonae (TL ca. 40% of
Table 6 see below). TOTL), a species which otherwise stands
Sorting examples of Nayarit S. alleni apart for its exceptional mass and size in all
from those of S. arizonae and S. mascoten- other external variables quantified.
—
VOLUME NUMBER
112, 4 821
Fig. 3. Dorsal and ventral cranial views (about 1.75X) of adult Sigmodon: left pair, S. mascotensis (USNM
510026), a male from 2 mi E San Pedro Lagunillas, Nayarit; middle pair, S. hispidus berlandieri (USNM
157382), a male from 8 mi E Deming, New Mexico; and right pair, S. arizonae (USNM 510040), a female from
2 mi E San Pedro Lagunillas, Nayarit.
—
Qualitative cranial traits. Zimmerman were advanced by Severinghaus & Hoff-
(1970) identified several consistent cranial meister (1978) and Hoffmeister (1986), par-
differences among Sigmodon populations ticularly for separation of S. hispidus from
that corresponded to the chromosomal con- S. arizonae in the southwestern United
trasts he documented and to the three spe- States. We here extend the utility of select
cies he recognized among his samples cranial features to the Nayarit populations
namely, S. hispidus, S. arizonae, and S. formerly included under S. hispidus.
mascotensis. Other useful qualitative traits The vertical distance between the supe-
822 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON
originates the temporalis, an important mas-
ticatory muscle (Rinker 1954). In S. hispi-
dus (and 5. alleni), the temporal fossa ta-
pers conspicuously toward the rear of the
skull, forming a trapezoidal shape over the
posterior half of the parietal and squamosal
bones; whereas, the fossa in S. mascotensis
and S. arizonae occupies a larger area, ap-
proximately rectangular in outline on the
lateral braincase wall (Fig. 8). As expected
for osseus features that provide muscular
attachment, some change in fossa shape ac-
companies individual aging, such that ju-
venile animals of all three species exhibit a
stronger rearward convergence of the su-
perior and inferior temporal ridges; conse-
quently, the size and shape differences not-
ed are best appreciated when comparing
crania of similar age classes.
The anterodorsal edge of the zygomatic
plate forms a spinous process in all Sig-
modon examined here, but its projection,
and the degree of concavity imparted to the
plate's anterior border, can aid specific iden-
tification, as demonstrated by Severinghaus
and Hoffmeister (1978) for Arizonan pop-
ulations of S. hispidus and S. arizonae.
Fig. 4. Lateral cranial view of adult Sigmodon:
top, S. mascotensis; middle, S. hispidus berlandieri; Among Nayarit samples, the spine is lon-
and bottom, S. arizonae (same specimen numbers as gest and most acute in specimens of S. ar-
in Fig. 3). izonae, shortest in those of S. alleni. Con-
figuration of the anterior zygoma in S. mas-
cotensis more closely resembles S. arizonae
nor and inferior temporal ridges (the latter but is not so extreme. Such interspecific
called the occipital crest by Zimmerman contrasts are partially conveyed by certain
1970) provides a reliable means for dis- variable loadings (BZP, LZS) generated
criminating examples of S. hispidus from from principal component and discriminant
those of S. arizonae and S. mascotensis. As function analyses (Tables 3, 4), as well as
noted by Zimmerman, this distance is visi- by their mean differences (Appendix). Nev-
bly narrower in S. hispidus relative to the ertheless, the expression ofthe spinous pro-
latter two (see Appendix), a difference un- cess has a strong age component (Table 2),
derscored by our multivariate results in which must be considered when comparing
which distance between the temporal ridges and identifying individuals. In S. hispidus,
(DTR) heavily influenced the separation of compared with S. arizonae, the spinous pro-
OTUs along the first canonical variate (Fig. cess is not only shorter, but its dorsal border
5, Table 4). The temporal ridges, together appears wider and its tip is usually blunt
&
with the lateral arc of the transversely ori- (Severinghaus Hoffmeister 1978:868, fig.
ented lambdoidal ridge, define the size and 1; Hoffmeister 1986). In some individuals
shape of the temporal fossa, an area on the of S. hispidus, the spinous process is even
lateral wall of the braincase from which expanded anteriorly to produce a knoblike
