2008 SOUTHEASTERN NATURALIST 7(1):125–134
Osteological Variation within the Baldwin County,
Georgia, Population of Didelphis virginiana
David B. Patterson1 and Alfred J. Mead1,*
Abstract - We analyzed intraspecific and sexually dimorphic osteological variation
within the Baldwin County, GA, population of Didelphis virginiana (Virginia
opossum). Where possible, 20 measurements (11 cranial/mandibular and 9 postcranial)
were obtained from each of 59 road-killed adult opossums (47 males, 12
females). Although range overlaps exist between the sexes for all measurements,
males are significantly larger (P ≤ 0.05) for the following characteristics: greatest
length of skull, condylobasal length, basal length, postpalatal length, nasal length,
bicanine width, zygomatic breadth, length of mandible, scapula length, humerus
length, radius length, femur length, and fibula length. Males exhibit significantly
less (P ≤ 0.05) variation in mandibular tooth row length and significantly greater
(P ≤ 0.05) variation in scapula length and fibula length. Intraspecific variation
ranges from 14–54% for cranial/mandibular and 29–73% for postcranial measurements.
Sexual dimorphism is most pronounced in cranial/mandibular dimensions.
Comparisons with multi-state samples indicate that the continent-wide population
of the Virginia opossum is very similar in terms of osteological dimension. The
combination of low variation index and nonsignificant difference between the
sexes in mandibular tooth row length suggests that this measure may be useful for
distinguishing fossil opossum species.
Introduction
Although there have been several studies on Didelphis virginiana
Kerr 1792 (Virginia opossum) collected throughout Middle and North
America (e.g., Allen 1901, Coues 1872, Elftman 1929, Gardner 1973,
Jenkins 1971, Jenkins and Weis 1979, Lowrance 1949, Petrides 1949,
Tague 2003, White 1989), little is written about the variability of skeletal
characteristics within a localized population. Allen (1901) analyzed
skeletal and external characteristics of approximately 350 pelts and 100
opossum skulls (D. virginiana included in sample) collected from across
the United States and Middle America. Lowrance (1949, 1957) assessed
developmental characteristics, maturation rate, and correlation of linear
postcranial dimensions to skull weight and length using 150 specimens
collected from eastern Kansas. Petrides (1949) outlined a method for
age determination using 49 carcasses collected from a 10-county area
in Ohio. Gardner (1973) studied the systematics of the Virginia opossum
and D. marsupialis Linnaeus (southern opossum) using 2800+ specimens
1Department of Biological and Environmental Science, CBX 081, Georgia
College and State University, Milledgeville, GA 31061. *Corresponding author -
al.mead@gcsu.edu.
126 Southeastern Naturalist Vol.7, No. 1
from Middle and North America. Most recently, Tague (2003), using 95
specimens collected from a 12-state geographic area within the United
States (e.g., Florida, Texas, Illinois, Pennsylvania), addressed intraspecific
pelvic variability.
The analysis of osteological variation in the Virginia opossum is
complicated by delayed epiphyseal fusion and closure of cranial sutures.
Allen (1901) observed that the degree of sagittal and occipital crest development
was age-related and concluded that variation between specimens
from different localities was likely due to age rather than discernable
geographic differences. Washburn (1946) determined that epiphyseal fusion
was anatomically regionally sequential, progressing from the girdle
to the elbow, feet, wrist, hip, ankle, and then knee. Lowrance (1949)
documented the same pattern in a different sample and concluded that
the determination of skeletal maturity must be based on tooth eruption.
Petrides (1949) also found that sagittal crest development and epiphyseal
closure were not as useful in age determination when compared to the
sequence of molar eruption. Gardner (1973) reanalyzed Allen’s sample
along with an additional 2700+ specimens and concurred that cranial
development occurred throughout life and skull dimensions are linked to
the nutritional richness of the local environment. He also noted that skulls
exhibit high individual variation correlated with age and sex, and tooth
wear in adults may be related to abrasiveness of diet rather than age.
More recently, Tague (2003) pointed out that delayed epiphyseal fusion
and differential growth rates for males and females could greatly influence
measurements of sexual dimorphism.
Sexual dimorphism and intraspecific variation in a species can provide
valuable information in both a paleontological and ecological context (Clutton-
Brock et al. 1977, Ralls 1977, Weckerly 1998, Willig and Hollander
1995). Quantification of variation within a modern mammalian population
provides a baseline for the analysis of extinct species. Also, osteological
sexual dimorphism within an extant population may have multiple implications
concerning the social and ecological habits of the species. A general
conclusion for the presence of sexual dimorphism is that differences in size
could indicate an intense level of mating competition; larger males being
dominant to smaller and thus gaining mating privileges. In the Virginia opossum,
males are aggressive to one another, and violent interactions may end
in death of the weaker (Gardner 1982, McManus 1974). Alternately, a difference
in size of certain skeletal characteristics between males and females
could refl ect different ecological habits for the two sexes or, as indicated for
this species, differential growth rates following sexual maturity (Gardner
1973, 1982).
The Virginia opossum provides an excellent subject for scientific research,
mainly due to large population numbers and ease of collection. It is
a relatively ubiquitous species found in large population numbers within the
2008 D.B. Patterson and A.J. Mead 127
eastern and extreme western portions of the United States. Current classifi-
cation includes four subspecies: D. v. virginiana Kerr, D. v. pigra Bangs, D.
v. californica Bennett, and D. v. yucatanensis Allen (Gardner and Sunquist
2003). Little has been written concerning the specific skeletal characteristics
of the Georgia population of the Virginia opossum (Golley 1962). In the previously
mentioned morphological studies, only 8 of approximately 1350 US
specimens were collected in Georgia (Allen 1901, Gardner 1973, Lowrance
1949, Tague 2003). The present study explored the intraspecific and intersexual
variation within a localized population of the Virginia opossum from
the Georgia Piedmont. Variation within the localized sample was compared
to published samples that included specimens from much larger geographical
areas.
Materials and Methods
The 59 specimens (47 males, 12 females) of the Virginia opossum
analyzed in this study were collected as road-kill within Baldwin County,
central Georgia Piedmont, in the winter months (January–March) of 2002
and 2004. The specimens were tagged, sexed, weighed, and skeletonized
using dermestid beetles. Baldwin County lies on the border of the range
of two subspecies, D. v. virginiana and D. v. pigra (Golley 1962). Based
on characteristics given by Gardner (1973), the specimens analyzed in this
study belong to D. v. pigra. However, Tague (2003) found no statistical differences
in osteological characteristics between the two subspecies, so the
issue of some specimens being D. v. virginiana rather than D. v. pigra should
have no bearing on this analysis.
Due to the way in which these animals died, each specimen contained
numerous broken bones. When possible, 11 cranial/mandibular and 9
postcranial measurements were obtained from each skeleton. The extent
of skeletal damage in conjunction with the initial unbalanced sample created
large disparities between the numbers of males and females for some
measurements. The cranial/mandibular measurements (from Martin et al.
2001) include greatest length of skull (GLS), condylobasal length (CL),
basal length (BL), postpalatal length (PL), nasal length (NL), bicanine width
(BW), zygomatic breadth (ZB), postorbital constriction (PC), maxillary
tooth row length (P2–M4: MxTR), length of mandible (LM), and mandibular
tooth row length (p2–m4: MnTR). The postcranial measurements (greatest
length unless otherwise noted) include scapula length (SL), humerus length
(HL, head to intercondylar sulcus), radius length (RL), ulna length (UL),
epipubis length (EL), pelvis length (LP), femur length (FL, trochanteric
sulcus to intercondylar sulcus), tibia length (TL), and fibula length (FiL).
Chicago Brand digital calipers accurate to 0.01 millimeters were used to take
measurements. All skeletons are housed in the Georgia College and State
University Recent Mammal Collection (GCM).
128 Southeastern Naturalist Vol.7, No. 1
Each specimen was placed in an approximate age group based upon
stage of tooth eruption and was considered an adult at 10–11 months
(Gardner 1973, Tague 2003). The assigned groups were derived from
molar eruption as described by Petrides (1949). All of the analyzed
specimens exhibited degrees of tooth eruption consistent with those of
mature individuals of age classes 5 and 6 of Gardner (1973). For each
measurement within both sexes, the mean, range, standard deviation, and
coefficient of variation were calculated. An intraspecific variation index
(VI = largest individual measurement ÷ smallest individual measurement)
was determined for each character. Variation between the sexes was
computed using the dimorphism ratio (DR = male mean value ÷ female
mean value). The significance of the difference between each osteological
character’s mean male and female value was determined using the Student’s
t-test and considered significant at the P ≤ 0.05 level. Levene’s test
was used to analyze the significance (P ≤ 0.05) of the relative variation
between sexes (Van Valen 2005) for each osteological character (MnTR,
Table 1. Summary of cranial osteological measurements (mm) and statistics for Didelphis
virginiana (Virginia opossum) from Baldwin County, GA. GLS = greatest length of skull, CL
= condylobasal length, BL = basal length, PL = postpalatal length, NL = nasal length, BW =
bicanine width, ZB = zygomatic breadth, PC = postorbital constriction, MxTR = maxillary
tooth row length, LM = length of mandible, MnTR = mandibular tooth row length, SD = standard
deviation, CV = coefficient of variation, VI = largest individual measurement ÷ smallest
individual measurement, and DR = male mean value ÷ female mean value. P-value pertains to
Student’s t-test for comparison of means.
Character Sex Mean Range N SD CV VI DR P
GLS M 126.99 101.11–139.17 16 11.42 8.99 1.38 1.17 <0.001
F 109.00 104.07–112.65 5 3.58 3.28
CL M 121.98 100.52–132.84 15 9.98 8.18 1.32 1.14 <0.001
F 106.94 100.57–111.73 5 4.25 3.97
BL M 117.94 95.58–132.15 15 10.61 9.00 1.39 1.15 <0.001
F 102.21 95.29–106.49 5 4.35 4.26
PL M 47.02 37.25–52.02 15 4.96 10.55 1.48 1.20 <0.001
F 39.26 35.06–41.24 5 2.50 6.37
NL M 58.11 47.87–64.50 23 4.57 7.86 1.35 1.11 <0.001
F 52.58 51.13–54.49 6 1.56 2.97
BW M 10.95 8.80–13.52 23 1.02 9.32 1.54 1.09 0.01
F 10.07 9.71–10.57 4 0.39 3.87
ZB M 68.45 57.45–74.81 14 5.05 7.38 1.42 1.19 0.003
F 57.34 52.69–61.99 4 3.88 6.77
PC M 11.82 11.23–12.84 20 0.43 3.64 1.16 0.99 0.826
F 11.89 11.06–12.75 5 0.65 5.47
MxTR M 34.22 28.53–36.98 20 2.00 5.84 1.30 1.02 0.347
F 33.42 32.71–36.22 6 1.66 4.97
LM M 100.30 84.24–111.34 28 7.67 7.65 1.34 1.15 <0.001
F 87.16 82.91–91.39 6 3.99 4.58
MnTR M 37.21 34.53–39.35 41 1.06 2.85 1.14 1.01 0.479
F 36.84 35.44–39.19 10 1.47 3.99
2008 D.B. Patterson and A.J. Mead 129
SL, HL, RL, UL, FL, TL, and FiL) with at least 8 measurements for both
sexes (Schultz 1985).
Results
Summaries of the cranial/mandibular and postcranial measurements and
statistical values are presented in Tables 1 and 2. Although the ranges overlap
for all the cranial measurements, males average larger in all characters
except PC. GLS, CL, BL, PL, NL, BW, ZB, and LM are significantly larger
in males. The average male cranial coefficient of variation (CV) (7.38) exceeds
the average female value (4.59). Males exhibit greater CVs for GLS,
CL, BL, PL, NL, BW, ZB, MxTR, and LM. Females show greater variability
in PC and MnTR, of which the MnTR relative variation is significantly
greater. Intraspecific variation (VI) ranges from 1.14 (MnTR) to 1.54 (BW).
The range of sexual dimorphism (DR) varies from 0.99 (PC) to 1.20 (PL).
Overlapping ranges also exist for all postcranial measurements. All
mean male values are larger than mean female values. The SL, HL, RL,
LP, FL, TL, and FiL measurements are significantly larger in males. The
average male postcranial CV (8.22) exceeds the female’s (7.31). Males
exhibit greater relative variation in SL, HL, LP, FL, TL, and FiL, with SL
and FiL variability being significantly greater. Females exhibit larger CVs
for RL, UL, and EL. Intraspecific variation (VI) ranges from 1.29 (LP) to
Table 2. Summary of postcranial osteological measurements (mm) and statistics for Didelphis
virginiana (Virginia opossum) from Baldwin County, Georgia. SL = scapula length; HL
= humerus length; RL = radius length; UL = ulna length; EL = epipubis length; LP = pelvis
length; FL = femur length; TL = tibia length; FiL = fibula length. See Table 1 for additional
abbreviations.
Character Sex Mean Range N SD CV VI DR P
SL M 67.73 55.43–79.08 32 6.52 9.63 1.43 1.13 <0.001
F 59.98 55.79–62.32 9 3.04 5.06
HL M 70.50 59.05–79.59 43 5.36 7.60 1.36 1.09 <0.001
F 64.69 58.50–71.37 11 3.34 5.16
RL M 87.22 69.06–98.11 41 6.44 7.38 1.45 1.10 0.008
F 79.12 67.76–88.71 8 6.32 7.99
UL M 71.25 56.75–80.56 40 5.16 7.24 1.42 1.04 0.271
F 68.67 59.08–73.73 10 6.61 9.63
EL M 40.22 31.45–49.30 19 4.78 11.88 1.73 1.03 0.614
F 38.86 28.57–46.06 7 6.21 15.98
LP M 87.73 76.49–98.72 23 5.60 6.38 1.29 1.06 0.031
F 83.14 78.62–87.73 6 3.61 4.34
FL M 82.64 67.95–98.71 43 6.81 8.24 1.45 1.07 0.007
F 77.31 70.26–84.24 10 4.47 5.78
TL M 88.58 71.93–101.75 41 6.6 7.45 1.43 1.06 0.018
F 83.48 71.15–89.54 11 5.57 6.67
FiL M 84.94 72.50–98.39 40 6.97 8.21 1.36 1.06 0.007
F 79.91 74.95–86.64 10 4.15 5.19
130 Southeastern Naturalist Vol.7, No. 1
1.73 (EL). The range of sexual dimorphism (DR) varies from 1.03 (EL) to
1.13 (SL).
Discussion
Although variability of the Virginia opossum skeleton has been documented
in previous analyses (Coues 1872, Gardner 1973, Lowrance 1949,
Tague 2003), these studies have included specimens from large geographic
regions. In comparison to the regional, multi-state sample of Tague (2003),
the Baldwin County sample exhibits nearly identical male and female mean
values for GLS, ZB, and LM. For males, CVs are higher than those of Tague
(2003) for GLS and LM. For females, CVs are lower for GLS, ZB, and LM.
When compared to the Middle American sample of Gardner (1973), male
and female GLS, CL, ZB, PC, and LM values average larger in the Baldwin
County sample, perhaps refl ecting the differences between the subspecies
D. v. pigra and D. v. californica. For males, CVs are higher than those of
Gardner (1973) for GLS and LM. For females, CVs are lower for GLS and
LM. The VI for GLS, CL, ZB, PC, and LM in the Baldwin County sample
averages 15.5% (13–19%) less than that for comparably aged specimens
within Gardner’s (1973) Middle American sample. Maximum and minimum
values are not provided in Tague’s (2003) analysis, so VI values could not
be compared.
Coues (1872) noted the sexually dimorphic nature of the upper
canines in the Virginia opossum. Allen (1901) also noted the differences
in canines and described the female skull as generally narrower and more
slender. Gardner (1982) quantified the canine sexual dimorphism and
proposed a method of sexing skulls using canine dimensions and molar
eruption and wear. The present study illuminates additional dimorphic cranial
measurements. The Baldwin County sample displays a range of sexual
dimorphism (DR) from 1.09 to 1.20 for the cranial characters found to be
significantly different between the sexes. The DR values for GLS, CL,
ZB, and LM average 4.3% greater in comparison to the Middle American
sample presented by Gardner (1973). Also, DR values for GLS and LM
are 3.5% and 3.6% greater, respectively, in comparison to the eastern US
sample of Tague (2003). However, DR values for ZB are identical between
the Baldwin County sample and Tague’s (2003) regional sample.
In comparison to the multi-state sample of Tague (2003), the Baldwin
County sample exhibits humeral and femoral mean values that are 5.1%
and 4.7% smaller for males and 5.0% and 5.4% smaller for females. For
males, CVs are higher than those of Tague (2003) for HL and FL. For females,
CVs are lower for HL and FL. Comparable measurements are not
available for the Middle American sample of Gardner (1973). Postcranial
sexual dimorphism is not as well documented as that for cranial characteristics.
Lowrance (1957) noted sexual dimorphism in epipubis length in
2008 D.B. Patterson and A.J. Mead 131
the Kansas sample. Also, Tague (2003) found that males were significantly
larger for 14 of 16 pelvic measurements. The DR values for HL and FL are
virtually identical for the Baldwin County sample and Tague’s (2003) regional
sample.
In the Baldwin County sample, MnTR exhibits the smallest VI values.
MnTR also has a DR of 1.01. The relative constancy of MnTR is interesting
given the dimorphism noted in the other cranial measurements. The relative
lack of inter- and intra-sexual variation is likely correlated to the similar
feeding habits of the sexes (Gardner 1982) and the fact that the mandibular
tooth row is fully developed at sexual maturity and does not lengthen as the
animal continues to mature physically. The low VI and DR for the MnTR
suggest that it may be the most useful skeletal character for distinguishing
fossil opossum species.
The sexually dimorphic measurements obtained in this study suggest
that the Baldwin County sample contains males with larger cranial (GLS,
CL, BL, PL, NL, and LM) and shoulder (SL and HL) dimensions. The
larger head and shoulders in males could indicate a functional adaptation
correlated to male-male competition for mating privileges (Allen
1901, Gardner 1982, McManus 1974). As mentioned previously, parts
of the Virginia opossum skeleton have been shown to increase in size
throughout life (Gardner 1973, Lowrance 1949, Tague 2003). The rate
of development appears to slow significantly at sexual maturity, with the
females exhibiting a lower rate of growth compared to males (Gardner
1973). Developmental heterochrony would explain the observed sexual
dimorphism; however, the inability to age specimens beyond sexual maturity
complicates the analysis.
The present study appears to show that more males than females (47
males: 12 females) are being killed on Baldwin County roads during the
winter months, suggesting that males are more active and wider ranging
than females during this time of year. Allen et al. (1985) found that on the
Georgia Piedmont, male Virginia opossums are significantly wider ranging
than females. Gardner (1982) and Ryser (1995) noted that males are
more active during the winter resulting in greater highway mortality. Ryser
(1995) found that in Florida, female home-range size was approximately
half that of males, and males tend to travel farther than females as a result
of natal dispersal, mate searching, and the need for increased food acquisition
related to larger body mass. The peak of the breeding season for
Virginia opossums in Georgia is early February (Golley 1962). It is likely
that the males in Baldwin County were actively seeking mating opportunities
during the winter months of 2002 and 2004, increasing the probability
of death on roadways.
Within the Baldwin County population of the Virginia opossum, males
are generally larger and exhibit greater CVs than females (however,
only 2 of 8 measurements are considered statistically significant using
132 Southeastern Naturalist Vol.7, No. 1
Levene’s test). For the characteristics analyzed, this sample averages
slightly larger than the Middle American sample yet shows 15% less osteological
variability (VI), supporting the hypothesis that variation would
be greater in a geographically expansive sample of mixed populations
when compared to a local sample. Alternately, Gardner’s (1973) Middle
American sample (77 males, 91 females) is larger than the Baldwin
County sample and the observed differences in osteological variability
may be a function of sample size rather than geographic area. Also, due
to the presence of different subspecies, the possibility arises that differences
noted here are due to previously unreported subspecific differences.
The Baldwin County sample is identical for cranial and, on average, 5.0%
smaller for humeral and femoral measurements in comparison to a regional
US sample. However, the comparatively higher male CVs and lower female
CVs indicate the need for further exploration. The Baldwin County
sample averages 2.4% greater for cranial DR and is nearly identical for
postcranial DR in comparison to the regional sample. The differences between
these two samples may reflect the tendency to collect and prepare
large specimens for museum collections (source of specimens for regional
sample), potentially muting the recognition of sexual dimorphism that
may exist within the species. Alternately, the under-representation of females
in the Baldwin County sample may have significantly influenced
the DR values for this sample. Recognizing these differences, this study
suggests that the continent-wide population of the Virginia opossum is
quite similar in osteological dimension.
Acknowledgments
We thank the students enrolled in the 2002 and 2004 mammalogy classes at
Georgia College and State University for their work in collecting and processing
the road-killed opossum carcasses. We appreciate the valuable comments provided
by Heidi Mead, Dennis Parmley, and Bill Wall who read earlier drafts of this manuscript.
Jennifer Rhode provided valuable insights concerning population analyses.
Jason Stover generously assisted with the Levene’s analysis. The senior author
thanks Dan, Paula, and Carrie Ann Patterson for their support and encouragement.
This manuscript benefited greatly from critical comments by Robert Tague and two
anonymous reviewers.
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