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2010 SOUTHEASTERN NATURALIST 9(4):757–772
Identification of Peromyscus gossypinus at Poinsett State
Park, South Carolina
Pearl R. Fernandes1,*, Justin L. Reynolds1, Nicole Segedin-Garrett1,
and Michael J. Dewey2
Abstract - The overlap of external morphometric measurements between Peromyscus
leucopus (White-footed Mouse) and P. gossypinus (Cotton Mouse) makes
species determination challenging. Peromyscus were live-trapped at Poinsett State
Park, Sumter County, SC, and identified using field and laboratory techniques. Our
measurements for hind-foot length, total length, and tail length overlapped with
published values for both species. Body mass, as measured in the field, was a good
criterion that identified our animals as Cotton Mice (n = 29). This identification
was confirmed by digital radiography, microsatellite DNA markers, and glucose
phosphate isomerase analysis. Our results indicated that a combination of field and
laboratory techniques is a valuable approach for positively identifying morphologically
In many habitats of the eastern United States, mice of the genus Peromyscus
(Gloger) are the most abundant small mammals. The genus exhibits
a considerable range of morphological, behavioral, and physical variation,
and thus serves as a model for studies of population biology, community
ecology, adaptive physiology, and evolutionary biology (Carleton 1989). It
is also a useful model in genomic approaches to physiological and behavioral
adaptations to habitat (Dewey and Dawson 2001). In natural ecosystems,
Peromyscus are important not only as prey for mammals, snakes, and birds,
but are also predators of insects that cause damage to crop trees (Bellcoq
and Smith 1992). Two species, Peromyscus leucopus (Rafinesque) (Whitefooted
Mouse) and P. gossypinus (Le Conte) (Cotton Mouse), make up the
species group leucopus of Peromyscus, and are believed to have diverged
only recently (Carleton 1989, Hall 1981, Hooper 1968, Osgood 1909). This
close relationship is supported by a genetic identity of 0.84 (Zimmerman et
al. 1978) as well as electrophoretic (Price and Kennedy 1980) and karyologic
(Baker et al. 1983) similarities. The species are inter-fertile via captive
breeding (Dice 1937) and hybridize in the wild (Barko and Feldhamer 2002,
Dice 1940). The White-footed Mouse is generally found at elevations below
900 m in relatively xeric woodlands (Laerm and Boone 1994) or in higher,
drier uplands (Howell 1921) of the eastern United States. The Cotton Mouse
1Division of Science, Mathematics and Engineering, University of South Carolina
Sumter, 200 Miller Road, Sumter, SC 29150. 2Peromyscus Genetic Stock Center,
Department of Biological Sciences, University of South Carolina, Columbia, SC
29208. *Corresponding author - email@example.com.
758 Southeastern Naturalist Vol. 9, No. 4
is found primarily in mesic lowland hardwood, and swamp forests of the
Southeastern United States (Barbour and Davis 1974, Dice 1940, Hoffmeister
1989, Laerm and Boone 1994, Le Conte 1853, McCarley 1954, McCay
2000, Wolfe and Linzey 1977). The White-footed Mouse ranges from Maine
southward through the eastern half of the United States to South Carolina,
Georgia, and Alabama, and westward through New Mexico to Central Arizona.
The Cotton Mouse occurs from Louisiana and southern Florida north
in the Mississippi Valley to southern Illinois and southern Kentucky, and
through southern South Carolina to eastern North Carolina and Virginia.
West of the Mississippi, the Cotton Mouse’s range extends to northeastern
Oklahoma and eastern Texas (Whitaker and Hamilton 1998). In areas where
both species occur, the White-footed Mouse is generally found in upland
woods, and the Cotton Mouse inhabits lowland woods (McCarley 1963,
Taylor and McCarley 1963).
Both species occur in South Carolina. The White-footed Mouse was reported
in Lancaster, Spartanburg, and York counties (Cloninger et al. 1977,
Golley 1966), and from all counties in the north-central Piedmont except
Union County, where it also likely occurs (Fields 2007). The Cotton Mouse
was first reported in the Upper and Lower Coastal Plains in Lancaster
County, where it occupied moist mixed woods (Coleman 1948). Cotton
Mice have also been reported in Aiken, Barnwell, and Allendale counties,
where they use coarse woody debris for primary refuge sites (Hinkelman
and Loeb 2007, Loeb 1999, McCay 2000). In the Piedmont Plateau Province
of western South Carolina, the Cotton Mouse has been reported in
Abbeville, Edgefield, Greenwood, McCormick, and Saluda counties. In areas
of overlap, the Cotton Mouse prefers areas with more extensive woody
biomass and more complete overstory than does the White-footed Mouse
(Mengak and Guynn 2003).
Traditionally, pelage coloration, body measurements, and cranial characteristics
have been used to distinguish the Cotton Mouse from the Whitefooted
Mouse. The dorsum of the Cotton Mouse is dark golden brown with
a mid-dorsal dusky area. The White-footed Mouse is grayish brown to dull
reddish brown dorsally. Both species are white on the ventral side (Wilson
and Ruff 1999). The Cotton Mouse is more massive (17–46 g) than the
White-footed Mouse (15–25 g) and has a considerably darker dorsum (Reed
et al. 2004, Whitaker and Hamilton 1998). Hind-foot length is generally
>22 mm in Cotton Mouse and ≤22 mm in the White-footed Mouse (Dice
1940, McCarley 1954). The two species are best distinguished by skull
length, which is generally >28 mm in the Cotton Mouse and less than 25.4 mm in
the White-footed Mouse (Lowery 1974). Since skull length and other cranial
characteristics require sacrifice of animals, these criteria are not useful in
many ecological studies.
In certain habitats, morphological differences between the two species
can be rather subtle (Hoffmeister 1989, Schwartz and Schwartz 1981) due to
2010 P.R. Fernandes, J.L. Reynolds, N. Segedin-Garrett, and M.J. Dewey 759
their close relationship (Lovecky et al. 1979, McCarley 1963), so misidentifications of species occur. Identification is particularly challenging when
small sample sizes preclude use of discriminant function analysis of external
measurements (Choate 1973, Feldhamer et al. 1983). Additionally, age and
geographic variation can affect accuracy of identification based on external
measurements (Sternburg and Feldhamer 1997).
Molecular markers allow positive identification of closely related species
of Peromyscus. Protein markers have been used to study the genetic
structure of natural populations (Lewontin 1991). Genotype for protein
loci can be inferred from electrophoretic patterns because of their co-dominant
expression, constant number of enzyme subunits in related species,
and consistent patterns of tissue-specific expression (Luikart et al. 1998).
Salivary amylase variants have been used to accurately identify sympatric
species of Peromyscus (Aquadro and Patton 1980) and improve the reliability
of field-based identifications (Bruseo et al. 1999, Feldhamer et al.
1983, Lindquist et al. 2003, Rich et al. 1996, Sternburg and Feldhamer
1997). Alleles of glucose phosphate isomerase (GPI) are diagnostic between
the Cotton and White-footed Mouse (Barko and Feldhamer 2002,
Price and Kennedy 1980, Robbins et al. 1985). However, allozymes
overlook mutations that do not produce amino acid changes. These silent
mutations could contain important additional information about each species.
Microsatellite DNA markers allow detection of genetic variation at
an unlimited number of loci with much greater sensitivity than allozyme
analysis (Allendorf 1994). These DNA markers permit researchers to study
historical patterns of isolation and gene flow in populations which can be
obscured in allozyme analysis due to balancing selection at allozyme loci
(Karl and Avise 1992). In the present study, we compare effectiveness of
microsatellite markers and allozyme electrophoretic mobility of GPI with
morphological trait measurements as tools for unequivocal identification
of mice trapped at a site in south-central South Carolina. We chose
Poinsett State Park as an undisturbed habitat for Peromyscus species and
investigated: (1) which species of Peromyscus is/are present, (2) whether
individuals can be accurately identified by field techniques, and (3) how
laboratory techniques compare with field techniques for identification of
Field Site Description
Poinsett State Park includes approximately 4 square km of relatively
undisturbed habitat in central South Carolina and is 35 km southwest of
Sumter in Sumter County, at 33°48'28"N, 80°56'0"W. Elevation ranges from
25–68 m above sea level. The Park is part of Manchester Forest at the edge
of the Sandhills, but still lies within the upper portion of the Coastal Plain. A
diverse geographic landscape of north- and south-facing slopes, wet swampy
760 Southeastern Naturalist Vol. 9, No. 4
lowlands, and dry, exposed ridges characterize the Park (South Carolina
State Parks 2009). A topographical map (Fig. 1) was created using Arc GIS
software v8.1 (ESRI, NJ) and GIS data from the South Carolina Department
of Natural Resources (2002).
Vegetative cover in the Park varies from pine, mixed pine-hardwood,
secondary growth hardwood to mature swamp forest. South-facing slopes
(upland) contain mainly Pinus taeda L. (Loblolly Pine), Quercus falcata
Michx. (Southern Red Oak), Q. marilandica Münch. (Blackjack Oak),
Q. margarettae (Ashe) Small (Dwarf Post Oak), and Q. laevis Walter
(Turkey Oak). Dominant vegetation in the swampy lowlands includes
Figure 1. Location and topography of Poinsett State Park in Sumter County, SC.
2010 P.R. Fernandes, J.L. Reynolds, N. Segedin-Garrett, and M.J. Dewey 761
Southern Red Oak, Turkey Oak, Q. nigra L. (Water Oak), Liquidambar
styraciflua L. (Sweet Gum), Nyssa sylvatica Marsh. (Black Gum), and
Oxydendrum arboreum (L.) DC. (Sourwood) in the canopy and an understory
of Arundinaria tecta (Walter) Muhl. (Switch Cane), Lyonia lucida
(Lam.) K. Koch (Fetterbush), Leucothoe axillaris (Lam.) G. Don (Coastal
Doghobble), and Kalmia latifolia L. (Mountain Laurel). Lowland sites
had thick overstory, dense vine thickets, and various amounts of coarse
woody debris (CWD).
A preliminary feasibility study conducted in the late spring of 2002 identified two appropriate trapping sites for Peromyscus at Poinsett State Park.
Within each 1-ha plot, two upland (hill slopes) and three lowland (swamp)
sites were selected based on visual inspection of the terrain. A road separated
upland from lowland sites. At each site, ten traps were set at approximately
10-m intervals in a 2 x 5 grid pattern (W. Dawson, Department of Biological
Sciences, University of South Carolina, Columbia, SC, 2002 personal
comm.). Trapping was conducted from May to June in 2002, and May to August
in 2003. During each sampling period, 20 traps were set in upland sites
and 30 in lowland sites. Total number of trap days was 20 in 2002 and 55 in
2003. Peromyscus were captured with small Sherman live traps (17 cm by 5.4
cm by 6.5 cm; H.B. Sherman Co., Tallahassee, fl) baited with peanut butter.
Traps were set on, in, or under tree bases, fallen decaying logs, stumps, root
boles, vine thickets, open areas, hill slopes, and low elevations near stream
edges. Traps were set on Monday, left in place for 4 days, and brought back
to the laboratory on Friday for washing. Traps were checked each morning
between 0700 and 0900 h and replaced with clean traps after each capture. In
2003, Sherman traps were placed inside sections of polyvinyl chloride (PVC)
pipe with chicken wire ends and a hole for mouse entry. This modification
prevented damage from raccoons. Trapped mice were weighed, sexed, and
assessed for characters of dorsal and ventral pelage, mass, hind-foot length,
total length, and tail length. (Feldhamer et al. 1983, Sternburg and Feldhamer
1997). Measurements were taken to the nearest gram or millimeter. Date and
location of capture were also noted. Captured mice were marked by tail clipping
for recognition of recaptures. Tail tips were numbed with ethyl chloride
spray before a maximum of 4 mm was snipped for molecular analysis. Tips
were placed in ice, returned to the laboratory, and stored in a -20 °C freezer.
Trapped mice were released at the site of capture. All mouse procedures conformed
to IACUC Protocol # 1175 at the University of South Carolina.
Two adults trapped in 2003, and identified as Cotton Mice based on pelage
color, weight greater than 26 g, and hind-foot length greater than 22
762 Southeastern Naturalist Vol. 9, No. 4
mm (Barko and Felhamer 2002, Hoffmeister 1989), were selected for digital
radiography (DEXIS, Redwood, CA). X-rays of left lateral views of skulls
and mandibles were conducted. Skull length was calibrated using the premeasured
head length of the mouse and compared with published data for
regional species of Peromyscus (Laerm and Boone 1994, Reed et al. 2004)
to confirm identification.
Microsatellite DNA markers
DNA was extracted from tail snips of all trapped mice using Qiagen
DNeasy Kit (Cat. No.69506). DNA from adults from 2002 and all mice
from 2003 were amplified by polymerase chain reaction (PCR), using
the method of Prince et al. (2002). Primers were PO-97F (TGGCATTCAAAGTTTTATCTC),
PO-97R (CCTGGAGC TTTATCTAGAA),
PO-21F (TCTGCAAGTTGGAGGTAGAGA), and PO-21R (GGGAGCTGAGGGTTCAA),
which have been used successfully with P. polionotus
subgriseus (Wagner) (Oldfield Mouse) from the Peromyscus Genetic Stock
Center. DNA from Oldfield Mouse, P. maniculatus bairdii (Wagner) (Deer
Mouse), White-footed Mouse, and Cotton Mouse were used as controls.
Ten μl of PCR products were run on a 2% agarose gel with a 100 base pair
(0.5 μg) DNA ladder (New England Biolabs Inc.) as a size standard. DNA
bands were stained using ethidium bromide, and gels were photographed
on an ultraviolet trans-illuminator.
GPI analysis on tail tissue of 12 adult mice captured in 2003 followed
the methods of Eppig et al. (1977). These samples were run with samples
from known reference species on cellulose acetate gels. Mobility was scored
according to VanZant et al. (1983).
Mice judged to be adults, based on brown dorsal pelage and weight
greater than 18 g (Barko and Feldhamer 2002), were used to compute
mean, standard error, and range for each morphometric measurement.
Unpaired t tests with α = 0.05 were used to compare mass of adult males
between 2002 and 2003, and between all males and all females. We
compared our 95% confidence interval (mean ± 1.98 SD) and range for
each morphometric measurement with published data (mean and range)
for Cotton and White-footed Mouse. Published values outside the 95%
confidence interval were considered significant differences between
our sample and the relevant species. Due to our small sample size, we
used digital radiography, microsatellite markers, and glucose phosphate
isomerase analysis as additional techniques to confirm field identification
and overcome the problem of low statistical power to detect true differences
between the species.
2010 P.R. Fernandes, J.L. Reynolds, N. Segedin-Garrett, and M.J. Dewey 763
A total of 34 mice (29 adults and 5 juveniles) were captured over two
years. Twenty six (76%) were caught within 6 m of stream edges, and 8
(24%) were trapped between 8 and 15 m from stream edges. No mice were
caught in the upland sites, although 40% percent of traps were set in those
locations. Table 1 presents the results of the trapping data. Of 17 adults in
2002, 8 were male (45%) and 9 were female (55%). Trap success in the lowland
in 2002 was 0.078 animals per trap night. In 2003 only 12 adult males
and 1 juvenile were captured. Trap success for the lowland in 2003 was
0.016. In 2003, heavy raccoon activity destroyed many traps. No mice from
2002 were recaptured in 2003.
Measurements of mass, hind-foot length, total length, and tail length
for trapped mice are summarized in Table 2. There was no statistical difference
in mass of males between years (t = 0.59, d.f. = 18, P = 0.563). There
was also no significant difference in pooled weights of all males and all females
in year 2002 (t = 0.560, d.f. = 27, P = 0.58), so all data (n = 29) were
pooled for statistical analysis. Mass ranged from 28.0–46.0 g (mean = 35.5,
SE = 0.83), hind foot length from 20.0–26.0 mm (mean = 23.30, SE = 0.3,
n = 28), total length from 136.0–237.0 mm (mean = 163.81, SE = 3.91, n =
28), and tail length from 54.0–80.0 mm (mean = 66.78, SE = 1.42, n = 28).
Pooled data were compared with published morphological measurements
for the Cotton and White-footed Mouse and presented in Table 3. The 95%
confidence interval for mass was 26.72–44.34 g, for hind-foot length was
20.07–26.53 mm, for total length was 123.56–204.06 mm, and for tail
length was 52.13–81.41 mm.
Digital radiography, microsatellite DNA markers, and GPI analysis
Skull lengths from radiographs were 28.3 and 28.5 mm, values consistent
with Cotton mice. The PO-97 primers produced amplicons of 200 base
pairs in Oldfield and Deer Mouse controls and 170 base pairs in Cotton and
Table 1. Summary of trapping results at Poinsett State Park.
Trapping information Lowland Upland Lowland Upland
Trap dates 20 20 55 55
Trap number 30 20 30 20
Trap nights 600 400 1650 1100
Number of females 9 0 0 0
Number of males 8 0 12 0
Number of juveniles 4 0 1 0
Number of recaptures 26 0 14 0
Trap success rate 0.078 0 0.016 0
764 Southeastern Naturalist Vol. 9, No. 4
Table 3. Comparison of morphological measurements between Peromyscus leucopus and P. gossypinus populations.
Mass (g) Hind-foot length (mm) Total length (mm) Tail length (mm)
Location n Range Mean Range Mean Range Mean Range Mean Species Source
Poinsett 28–29 28–46 35.5 20–26 23.3 136–237 163.8 54–80 66.7 Present study
Missouri 24 28.8 20.9 leucopus Barko et al.(2000)
Missouri 4 34.8 24.0 gossypinus Barko et al.(2000)
North Carolina 4 17–24 20.4 leucopus Boone and Laerm (1993)
North Carolina 22 19–37 28.4 gossypinus Boone and Laerm (1993)
Illinois 73 21.0 20.6 leucopus Feldhamer et al.(1998)
Illinois 5 26.7 22.4 gossypinus Feldhamer et al. (1998)
Illinois 18–22 leucopus Hoffmeister (1989)
Illinois 22–25 gossypinus Hoffmeister (1989)
Southeastern US 108 16–21 19.4 49–83 65.0 leucopus Laerm and Boone (1994)
Southeastern US 110 20–24 22.1 58–91 73.0 gossypinus Laerm and Boone (1994)
Louisiana 17–23 19.1 17–21 20.0 134–177 157.0 51–87 69.0 leucopus Lowery (1974)
Louisiana 25–45 31.1 20–26 22.0 137–210 171.0 55–94 75.0 gossypinus Lowery (1974)
Texas 14 19.4–21 20.0 53–76 62.9 leucopus McCarley (1954)
Texas 18 21.8–23 22.4 55–80 71.0 gossypinus McCarley (1954)
Missouri 11–28 19–25 139–212 63–101 leucopus Schwartz and Schwartz (1981)
Missouri 19–25 20–25 161–209 69–101 gossypinus Schwartz and Schwartz (1981)
Illinois 168 14.5–32 21.2 18–22 20.3 leucopus Sternburg and Feldhamer (1997)
New York 30 16–28 18–23.3 21.0 157–189 170.0 60–92 76.0 leucopus Whitaker and Hamilton (1998)
Florida 15 17–46 21–23 22.0 152–189 166.0 63–80 71.0 gossypinus Whitaker and Hamilton (1998)
Tennessee 19 15–25 19.5–22 20.0 152–181 165.0 59–83 72.0 leucopus Whitaker and Hamilton (1998)
Tennessee 30 25–39 20–26 23.3 160–205 185.0 63–97 80.0 gossypinus Whitaker and Hamilton (1998)
Table 2. Summary of morphological data of adult Peromyscus trapped at Poinsett Park. All values in mm except for mass.
Males-2002 (n = 8) Females-2002 (n = 9) Males-2003 (n = 12) All mice (n = 29)
Character Mean ± SE Range Mean ± SE Range Mean ± SE Range Mean ± SE Range
Mass (g) 33.8 ± 1.42 28.0–41.0 36.2 ± 0.81 34.0–41.0 35.1 ± 1.66 29.0–46.0 35.5 ± 0.83 28.0–46.0
Hind-foot length 22.8 ± 1.45 20.0–25.0 23.3 ± 0.50 21.0–25.0 23.6 ± 0.501 21.0–26.0 23.3 ± 0.302 20.0–26.0
Total length 165.6 ± 5.70 147.0–190.0 154.5 ± 3.60 136.0–174.0 169.0 ± 7.601 146.0–237.0 163.8 ± 3.912 136.0–237.0
Tail length 74.0 ± 2.00 67.0–80.0 66.0 ± 1.80 67.0–78.0 63.1 ± 2.001 56.0–75.0 66.8 ± 1.422 56.0–80.0
1n = 11 (1 data point missing). 2n = 28 (1 data point missing).
2010 P.R. Fernandes, J.L. Reynolds, N. Segedin-Garrett, and M.J. Dewey 765
White-footed Mouse controls (Fig. 2). All Poinsett State Park samples produced
the 170 base pair amplicon. The PO-21 primers produced amplicons of
170, 150, and 100 base pairs in Oldfield Mouse, Deer Mouse, and White-footed
Mouse controls, respectively (Fig. 3). No amplicons were obtained in Cotton
Mouse control and 10 samples tested from Poinsett State Park. Some samples
from 2002 degraded and could not be analyzed by gel electrophoresis. The
12 samples analyzed in 2003 were homozygous for a slower migrating allele
(GPI-1a) than that (GPI-1b) seen in White-footed Mouse, Deer Mouse, and Oldfield Mouse controls (Fig. 4). Control Cotton Mouse tissue was not available.
Identification of the Cotton Mouse is difficult due to the similarity in
external appearance and overlap in morphological measurements with the
Figure 2. Agarose gel electrophoresis of PCR products with PO-97 primer. Abbreviations
are P, M, L, and G for products from P. polionotus, P. maniculatus, P. leucopus,
and P. gossypinus, respectively, (-) for a negative control without DNA, Std for a
DNA size marker, and numbers for DNA from 10 samples in 2002 (panel a) and 12
samples in 2003 (panel b). This PO-97 marker discriminates P.leucopus and P. gossypinus
from other species but fails to identify P. gossypinus.
766 Southeastern Naturalist Vol. 9, No. 4
White-footed Mouse (Hoffmeister 1989, Schwartz and Schwartz 1981). Due
to their wide distribution and geographic variation, significant differences in
pelage colors and body measurements occur within each species (Blair 1950,
Figure 3. Agarose gel electrophoresis of PCR products with PO-21 primer. Abbreviations
are P, M, L and G for products from P. polionotus, P. maniculatus, P. leucopus,
and P. gossypinus, respectively, (-) for a negative control without DNA, Std for a
DNA size marker, and numbers for DNA from 10 samples in 2002 (panel a) and 12
samples in 2003 (panel b). The PO-21 marker discriminates P. gossypinus from all
2010 P.R. Fernandes, J.L. Reynolds, N. Segedin-Garrett, and M.J. Dewey 767
Dice 1940), making reliable identification challenging. The Cotton Mouse
has been reported to average a higher body mass, longer hind foot, and larger
skull than the White-footed Mouse (Lowery 1974), and these characteristics
were of partial value in identifying our individuals. Body mass measurements
for Poinsett State Park animals were within the range reported for
Cotton Mice and larger than White-footed Mice (Boone and Laerm 1993,
Lowery 1974, Schwartz and Schwartz 1981, Whitaker and Hamilton 1998,
Wolfe and Linzey 1977). Mean body mass measurements were similar to
those reported for Cotton Mice in Missouri (Barko et al. 2000). One “general
rule” for identifying a potential Cotton Mouse is hind-foot length greater than
22 mm and/ or body mass greater than 26 g (Hoffmeister 1989). Applying
this rule for body mass confirmed our individuals as Cotton Mice. However,
5 (17%) mice had a hind-foot length of <22 mm, and 4 (14%) could have
been misidentified. Our range for hind-foot length overlapped with several
published ranges for White-footed Mice (Hoffmeister 1989, Laerm and
Boone 1994, Lowery 1974, McCarley 1954, Schwartz and Schwartz 1981,
Sternburg and Feldhamer 1997, Whitaker and Hamilton 1998) and did not
support the findings of Dice (1940) and McCarley (1954), who found hindfoot
length to be the most useful morphological character in distinguishing
between the Cotton and White-footed Mouse. As can be observed in Table 3,
published ranges for total and tail length overlapped substantially between
the two species and varied considerably throughout geographical ranges for
each species. Our measurements overlapped considerably with published
Figure 4. Allelic mobility of glucose phosphate isomerase (GPI) for several species
of Peromyscus. Abbreviations are L, L1, and L2 for P. leucopus, M for P. maniculatus,
P for P. polionotus, and 12 samples from Poinsett State Park collected in 2003.
768 Southeastern Naturalist Vol. 9, No. 4
ranges for the White-footed Mouse (Feldhamer et al. 1998, Hoffmeister
1989, Laerm and Boone 1994, Lowery 1974, McCarley 1954, Schwartz and
Schwartz 1981, Whitaker and Hamilton 1998), and we could not positively
identify our individuals using these criteria. Laerm and Boone (1994) utilized
stepwise discriminant analysis to correctly classify all their specimens
and overcome the problem of morphological overlap. Our small sample size
precluded use of this analysis. From our results, body mass was the only field
measurement that correctly identified all our individuals as Cotton Mice.
However, age and reproductive condition can affect body mass (Barko et al.
2000, Sternburg and Felhamer 1997).
Identification based on body mass was confirmed by digital radiography
to compute skull length, microsatellite DNA markers, and glucose phosphate
isomerase analysis. Skull length is the best way to identify these species
(Lowery 1974), and both specimens examined were Cotton Mice. The microsatellite
marker PO-97 did not distinguish between the Cotton Mouse and
White-footed Mouse, but was valuable in distinguishing these mice from the
two sister species, Oldfield Mouse and Deer Mouse. The PO-21 microsatellite
marker gave PCR amplicons in all species except Poinsett samples and
Cotton Mouse controls. The apparent null allele for PO-21 in Cotton Mice
was a reliable means for distinguishing Cotton Mice from other South Carolina
Allozyme analysis (Aquadro and Patton 1980, Bruseo et al. 1999, Kilpatrick
and Zimmerman 1975, Lindquist et al. 2003, Rich et al. 1996, Robbins et
al. 1985, Sternburg and Feldhamer 1997) differentiates between Peromyscus
species, and GPI distinguishes between the Cotton and White-footed Mouse
(Barko and Feldhamer 2002, Price and Kennedy 1980). The GPI allozyme
from our Poinsett samples was different from all other species of Peromyscus
in South Carolina, Deer Mouse and Oldfield Mouse. GPI has been used
to distinguish Cotton Mice from White-footed Mice in the Midwest (Barko
and Feldhamer 2002) and our results suggest a similar distinction for the two
species in the South.
In conclusion, our results suggest that all mice captured were Cotton
Mice. Body mass measured in the field was a good criterion, but hind-foot
length, total length, and tail length were poor criteria for identification. Digital
radiography for skull length, microsatellite DNA markers, and GPI were
laboratory techniques that confirmed the field identification. We recommend
using a combination of field and laboratory techniques to ensure accurate
identification of morphologically similar species.
We owe a great deal of thanks to Gabor Szalai and the Peromyscus Genetic Stock
Center at the University of South Carolina Columbia for assisting in this study. Thank
you to Dr. Wayne Van Devender for his detailed review of the manuscript and providing
helpful suggestions with the data analysis and presentation. Also, we thank the
2010 P.R. Fernandes, J.L. Reynolds, N. Segedin-Garrett, and M.J. Dewey 769
Department of Parks, Recreation, and Tourism at Poinsett State Park for providing
the study site, Professor John Logue for help with plant identifications, Clint Cook
for help with Adobe Photoshop, Amber Woodle for assistance in the field, and Dr.
Jeff Steinmetz for reading the manuscript. Our thanks extend also to Dr. Michael W.
Nimmich for use of the DEXIS digital radiography system. The study was partially
funded by the Provost’s Faculty Development Grant from the University of South
Carolina Columbia awarded to Pearl Fernandes.
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