Influences of an Urban Environment on Home Range and
Body Mass of Virginia Opossums (Didelphis virginiana)
Jeffrey D. Wright, M. Scott Burt, and Victoria L. Jackson
Northeastern Naturalist, Volume 19, Issue 1 (2012): 77–86
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2012 NORTHEASTERN NATURALIST 19(1):77–86
Influences of an Urban Environment on Home Range and
Body Mass of Virginia Opossums (Didelphis virginiana)
Jeffrey D. Wright1,*, M. Scott Burt1, and Victoria L. Jackson2
Abstract - Increasing urbanization in the United States presents new challenges and opportunities
for wildlife species. One species that is thought to benefit from urbanization
is Didelphis virginiana (Virginia Opossum). We used radio-telemetry to determine homerange
size of opossums living in an urban area and compared body mass measurements
of urban and rural animals to ascertain how urbanization affects this parameter for opossums.
Minimum convex polygon estimates for male (37.3 ± 46.0 ha; n = 3) and female
home ranges (18.8 ± 15.6 ha; n = 5) were smaller than those reported for opossums in
rural areas and similar to those from previous urban studies. Opossums living within the
city limits had an average body mass (3.0 ± 0.8 kg) that was 34% larger than those in rural
areas (2.2 ± 0.6 kg). These data, combined with previous work, suggest that urban areas
provide more resources and may be beneficial to opossum populations.
Introduction
The US becomes more urban every year (Adams et al. 2006). As of the 2000
census, 80% of the country’s population resided within urban areas or urban clusters
(US Census Bureau 2000). Urbanization presents new dynamics for wildlife,
including changes in habitat, food resources, and predation (Beissinger and
Osborne 1982, Brittingham and Temple 1992, Gosselink et al. 2003). Although
these new challenges may make urban centers uninhabitable for certain species,
others are able to take advantage of the new resources provided by urban environments
(Gill and Bonnett 1973).
A species’ home range, defined as the area that an individual uses to find food,
mates, and to care for young (Burt 1943), can provide insight into the resources
available to that individual. Home-range size is related to a variety of factors,
including body size and energetic requirements (Gittleman and Harvey 1982,
Haenel et al. 2003, McNab 1963, Weckerly 1993), annual rainfall and population
density (Fisher and Owens 2000), habitat productivity (Reylea et al. 2000),
behavioral changes associated with breeding season (Alt et al. 1980, Cooper and
Randall 2007, Farentinos 1979, Frank and Heske 1992), and access to mates (Ims
1988, Madison 1980, Ribble and Stanley 1998). Therefore, variation in a species’
home-range size and space use in different environments may yield information
about how the species adapts to new or changing habitats and may provide information
on the quality of those habitats.
Didelphis virginiana Kerr (Virginia Opossum) is a common resident
throughout the US, including urban areas (Schwartz and Schwartz 2001). This
1Division of Biology, Truman State University, Kirksville, MO 63501. 2Department of
Biology and Earth Sciences, University of Central Missouri, Warrensburg, MO 64093.
*Corresponding author - jeffrey_d_wright@yahoo.com.
78 Northeastern Naturalist Vol. 19, No. 1
species exhibits many characteristics that make it an ideal urban species (Gill
and Bonnett 1973): it is relatively small, nocturnal, and has a high reproductive
capacity. Many residents in urban areas consider the opossum a pest (Conover
1997). Although opossums are commonly found in urban areas, little is known
about the influence of urban environments on the species. Previous studies
regarding home-range size of opossums in urban areas (Oregon: Meier 1983;
Missouri: Harmon et al. 2005) reported average home-range sizes smaller than
those reported from rural areas (Gillette 1980, Gipson and Kamler 2001, Ryser
1995). However, opossums were introduced to the Pacific Coast in the 1910s
and have been in Oregon for a relatively short period of time. Consequently,
home-range sizes may not be typical of opossums in well-established populations.
Home ranges reported from Missouri also may not accurately represent
those of opossums in typical urban environments because the study area was
the St. Louis Zoological Park, which differs structurally from areas dominated
by human residences, and also provides supplemental food due to feeding of
captive animals. Furthermore, opossums were not tracked if they left the Park
grounds, which likely influenced estimated home-range sizes. No published
study has estimated home-range size for opossums living in a typical urban environment
that the species naturally colonized.
Urban environments can benefit species by providing more resources (e.g.,
food, shelter) than surrounding natural areas (Prange et al. 2003). In the Great
Basin, Ursus americanus Pallas (Black Bears) that used urban areas were 37%
larger on average than bears living in wildlands in the same region, and the urban
population sustained densities 37.5 times higher than those in rural areas
(Beckmann and Berger 2003). Pecari tajacu L. (Collared Peccaries) using anthropogenic
food sources also maintained a higher body mass on average than
did Collared Peccaries using only natural food sources (Bellantoni and Krausman
1993). While the Virginia Opossum is a model species to benefit from an increase
of resources in an urban environment, no study has investigated differences in
indicators of higher resource availability (e.g., body mass) between urban and
rural populations.
The goals of our study were to: 1) document home-range size of male and
female opossums throughout the year within a naturally colonized urban setting,
and 2) document body mass differences between an urban opossum population
and a nearby (less than 2 km away) rural population.
Methods
Study area
This study was conducted from May 2007 through April 2008. Our urban site
was within the city limits of Kirksville, MO. Kirksville is located in the northcentral
region of the state (40°11'37"N, 92°34'58"W) at an elevation of 299 m
above sea level. The city had a population of 16,998 in the 2000 census and an
estimated population of 17,057 in 2004 (US Census Bureau 2000). Kirksville
was classified as an urban cluster (core census blocks with >2500 residents and
2012 J.D. Wright, M.S. Burt, and V.L. Jackson 79
a population density of >1000 residents/2.59 km2; US Census Bureau 2000).
Kirksville covered roughly 27.2 km2 and was surrounded by a mix of agricultural
land and oak-hickory hardwood forest. Average temperatures ranged from a high
of 24.6 °C in July to a low of -4.3 °C in January. This area received an average
of 81.5 cm of precipitation annually (National Climatic Data Center 2008).
Our rural site was Big Creek Conservation Area (Big Creek CA), located 1.8
km west of Kirksville. Approximately 80% of the 431-ha conservation area was
native grassland, restored savanna, or old-field habitat, with the remaining 20%
comprised of oak-hickory hardwoods. The area surrounding Big Creek CA was
primarily oak-hickory forest in Thousand Hills State Park to the south and west
and private lands dominated by agriculture to the east and north. Climate and
elevation at Big Creek CA were similar to those of Kirksville.
Urban home ranges
We trapped opossums using single-door live traps (81 x 30 x 25 cm, Havahart
Products, Lititz, PA; 107 x 30 x 30 cm, Tomahawk Live Trap Co., Tomahawk,
WI) baited with cat food or slices of apple. Before traps were placed inside the
Kirksville city limits, property owners were contacted for permission. We attempted
to trap throughout the city. We deployed 10 live traps and maintained
them for up to 6 nights a week from May 2007–March 2008. Traps were placed
randomly across the city at locations where we were able to secure permission.
Our trapping efforts ensured that at least 4 opossums were collared within the city
limits at any given time during the 12-month study period.
Captured opossums were transported to a private area to be processed. We
immobilized opossums with an intramuscular injection of 5 mg/kg ketamine
hydrochloride + 2 mg/kg xylazine (Ellis et al. 1999). The trap was covered with
a towel, and the opossum was left alone during the 5–10 min induction time.
Once an individual was immobilized, we recorded sex and weight, and assigned
a unique identification number. We radiocollared opossums with HLPM-3800
(36 g; Wildlife Materials, Inc., Murphysboro, IL) and M1940B (44 g; Advanced
Telemetry Systems, Isanti, MN) radiotransmitter collars. After collaring, we returned
the opossums to the trap and released them after sunset at the original site
of capture. The amount of anesthetic used allowed animals to fully recover within
1–2 hrs. All trapping and handling methods followed guidelines approved by the
American Society of Mammalogists (Sikes et al. 2011).
Opossums are generally nocturnal, leaving their dens after sunset and returning
before sunrise (Allen et al. 1985, Meier 1983, Ryser 1995). Therefore, we
located opossums at randomly selected times between sunset and sunrise 4 nights
a week. Times were chosen each day using a random number generator to select
a number between the hour of sunset and 3 hours prior to sunrise (to allow time
for all radiocollared opossums to be located before sunrise). The chosen number
was used as the starting hour for data collection. Opossums were tracked on a
rotating schedule in which the last opossum located the night before was the first
opossum located the following evening. We attempted to locate each opossum
once per night.
80 Northeastern Naturalist Vol. 19, No. 1
We used a portable receiver, omnidirectional vehicle-mounted antenna, and
3-element Yagi antenna (Telonics, Inc, Mesa, AZ) to determine locations for
each opossum. The omnidirectional antenna was used to scan areas from the
vehicle. Once a general area containing a target individual was determined,
locations were taken using the 3-element Yagi antenna. We obtained locations
by triangulation of ≥3 bearings (White and Garrott 1990). Animals that were
moving quickly could not be triangulated. At each bearing site, we used an
eXplorist 200 GPS receiver (Magellan Navigation, Inc.) and sighting compass
(Amer Sports, Vantaa, Finland) to obtain the UTM coordinates of the site and
bearing to the signal, respectively, and recorded all bearing information on a
digital voice recorder. Generally, determining a location took <10 min once the
animal’s general location had been identified and the first bearing was taken.
Opossums were tracked throughout the year until their signal could not be located
or the individual died.
We used Locate III (Nams 2006) to generate location data from the bearing
information. We only used locations for analysis if the estimated error around a
point was <5 ha. We used the Animal Movement extension in ArcView (Hooge
and Eichenlaub 1997) and calculated minimum convex polygon (100% MCP)
and adaptive kernel (ADK; 95% isopleths) home-range estimates, and ADK
core-use-area (50% isopleths) estimates for each animal. We used both estimation
methods because many previous studies on home ranges of opossums used
100% MCP; we wanted to compare methods, and we also wanted to provide
both types of data for future analyses. Based on previous studies (Harmon et al.
2005, Ryser 1995), >20 independent locations per animal were needed for accurate
estimation of home-range size, and we confirmed this using our own data.
With 20 locations, home-range estimates represented >90% on average of the
final estimated home range of animals with >25 locations. Due to non-homogeneous
variances between groups, we log10 transformed home-range data for
comparisons between males and females. Unlike non-parametric statistics, log10
transformation preserved some of the original data’s variance that was then
included in the statistical testing (D. DeCock, Department of Mathematics and
Computer Science, Truman State University, pers. comm.). This transformation
resulted in statistically homogenous variances and allowed home-range sizes
for males and females to be compared for both estimators using t-tests (SPSS
Inc., Chicago, IL).
Using t-tests, we also compared home range estimates by sex with those from
the literature. The Student’s t-test was used for comparisons with both sexes from
Gipson and Kamler (2001) and Gillette (1980), and with males from Meier (1983)
because they met the assumption of equal variances. All other comparisons were
made using Welch’s t-test, which allows for comparisons between populations
with unequal variances.
Opossum body mass
During February–April 2008, we captured, weighed, marked, and released
opossums at Big Creek CA. We compared body mass of this group to body mass
2012 J.D. Wright, M.S. Burt, and V.L. Jackson 81
from opossums in Kirksville to determine if average mass differed between
opossums living in rural and urban locations. Only sexually mature animals
were used for this comparison. Our comparison was only made between males
because we only captured 1 female at Big Creek CA during our trapping. While
previous studies have shown that adult opossums do not differ in body mass between
the sexes (Gehrt et al. 1997, Gipson and Kamler 2001), we did not want
to make this assumption. We conducted a t-test to compare average body mass
from each area. We also compared the body mass of male and female opossums
from Kirksville using a t-test to determine if there was a difference between
sexes at this location. Only females without young in their marsupium at the
time of capture were used.
Results
Urban home ranges
From May 2007 to May 2008, 17 adult opossums (9M, 8F) were captured
within the city limits of Kirksville. Due to loss of radio signals and error associated
with point estimates, we collected enough locations (>20) for estimations
of home ranges for only 8 animals (3M, 5F; Table 1). One animal, EST0435,
shifted its home range once during the tracking period, and we were able to estimate
2 spatially separate home ranges, one in fall 2007 and another in winter
2007/2008.
MCP home-range size (mean ± SD) averaged 37.3 ± 46.0 ha and 18.8 ± 15.6
ha for urban males and females, respectively, and did not vary by sex (t = 0.64,
d.f. = 7, P = 0.54). ADK home-range estimates (95% isopleth) averaged 57.8 ±
62.5 ha and 32.5 ± 37.6 ha for males and females, respectively. Again, homerange
size did not vary by sex (t = 0.65, d.f. = 7, P = 0.54). Average home-range
size did not vary by method (F = 0.962; d.f. = 1, 16; P = 0.341). Core-use areas
averaged 7.47 ± 8.27 ha for males and 4.06 ± 3.49 ha for females and did not vary
by sex (t = 0.63, d.f. = 7, P = 0.55). Core-use areas were located around known
den locations.
Table 1. Home ranges (ha) of opossums living within the city limits of Kirksville, MO. Opossums
were radiotracked from May 2007–May 2008. Only individuals with >20 independent locations
were used for home-range estimates. Home-range estimates were generated using the Animal
Movement extension in ArcView. The 100% minimum convex polygon (100% MCP) and 95%
adaptive kernel (ADK) estimators are provided.
Animal ID Sex Period 100% MCP 95% ADK Locations
DW0315 F 19 June–15 July 29.4 48.1 24
BUR0417 F 1 July–27 August 41.1 72.0 29
MT0075 F 2 July–15 October 25.7 41.1 34
EST0435 F 27 August–23 October 10.1 18.0 31
2 November–4 February 2.8 9.4 21
STN0483 F 4 March–28 April 3.5 6.4 20
SHL0165 M 20 September–12 December 15.0 25.9 23
ILL0015 M 9 October–16 March 6.7 14.7 22
SKO0546 M 28 January–20 May 90.1 132.9 32
82 Northeastern Naturalist Vol. 19, No. 1
Male home-range size in our study was only significantly smaller than that of
Ryser (1995; t = 3.06, df = 5.3, P = 0.01). Female home-range size was smaller
than that reported by Gipson and Kamler (2001; t = -3.24, d.f. = 8, P = 0.006),
Ryser (1995; t = -4.30, d.f. = 22.4, P < 0.001), and Gillette (1980; t = -1.95, d.f.
= 20, P = 0.03) and larger than that of Harmon et al. (2005; t = 2.02, d.f. = 6.4,
P = 0.05). Variance in our study was high, which is similar to data reported in
previous studies (Table 2). The high variance may be due, in part, to differences
in seasonal home-range size. Our largest recorded home-range size came from
a male whose location data overlapped the breeding season (February and May:
Reynolds 1945, Schwartz and Schwartz 2001), whereas 2 of the 3 smallest were
recorded primarily in winter.
Opossum body mass
We compared body mass between 8 adult male opossums living at Big Creek
CA and 8 adult male opossums residing in Kirksville. Males opossums in Kirksville
were significantly heavier than those from Big Creek CA (t = 2.19, d.f. =
13, P = 0.047). Opossums in Kirksville (3.23 ± 0.93 kg) outweighed their rural
counterparts (2.35 ± 0.54 kg) by 37%. Weights of males (3.23 ± 0.93 kg) and females
(2.70 ± 0.70 kg) within Kirksville were not statistically different (t = 1.16,
d.f. = 12, P = 0.27).
Discussion
Home-range sizes of female opossums living in Kirksville were smaller on
average than those of females from the 3 previous studies conducted in rural settings
(Gillette 1980, Gipson and Kamler 2001, Ryser 1995) and slightly larger
than those of opossums living at the St. Louis Zoo (Harmon et al. 2005). A variety
of mammals have reduced home ranges in urban environments, including
Vulpes vulpes L. (Red Fox; Gosselink et al. 2003, Marks and Bloomfield 2006),
Lynx rufus Schreber (Bobcat; Riley 2006), Odocoileus virginianus Zimmerman
(White-tailed Deer; Cornicelli 1992, Grund et al. 2002, Kilpatrick and Spohr
2000), and Canis latrans Say (Coyote; Atwood et al. 2004). Home-range size
may be a function of resource availability (Harestad and Bunnell 1979, McNab
1963). During our study, opossums were observed eating pet food and trash, and
Table 2: Average home-range sizes (ha) of male and female opossums during this study and previous
studies in rural and urban environments. All home-range estimates calculated using the 100%
minimum-convex-polygon (MCP) estimator (mean ± SD).
Study Location State Male MCP Female MCP
Gillette (1980) Rural Wisconsin 78.6 ± 64.3 38.9 ± 23.2*
Gipson and Kamler (2001) Rural Kansas 114.0 ± 56 28.0 ± 22*
Ryser (1995) Rural Florida 141.6 ± 103.1* 64.4 ± 40.7*
Harmon et al (2005) Urban Missouri 13.4 ± 3.3 5.1 ± 6.2*
Meier (1983) Urban Oregon 32.4 ± 35.1 9.0 ± 5.6
This study Urban Missouri 37.3 ± 46.0 18.8 ±15.6
*Estimates are significantly different from those from the same sex in our study.
2012 J.D. Wright, M.S. Burt, and V.L. Jackson 83
frequently utilized buildings as den sites. Although we were unable to compare
female body mass directly, no study, including our own, has found opossums to
have sexually dimorphic body mass. Therefore, our observation of significantly
heavier males in Kirksville likely can be extrapolated to females as well. In a
Massachusetts population, female opossums over-wintering in natural areas were
12.8 times more likely to die of starvation than those in urban settings (Kanda
et al. 2009). These observations, coupled with the significantly heavier mass
suggest that urban areas contain a greater amount of resources that opossums effectively
accessed.
Variance in our home-range estimates was high, possibly due to differences in
seasonal home-range size. Male opossums in a Florida population doubled their
home-range size during the breeding season (Ryser 1992). Both male and female
opossums in a rural Wisconsin population had smaller home-range sizes in winter
(Gillette 1980). Genetic evidence from a population living in a fragmented landscape
revealed that for some males, capture location and offspring were separated
by 5–33 km (Beasley et al. 2010). Unfortunately, our dataset was not large
enough to evaluate breeding and non-breeding season home ranges separately.
Home-range sizes of males in urban and rural environments may differ when
season is included, but the large variation between breeding and non-breeding
home-range size inflated variance and reduced statistical power. Future studies
should plan for and incorporate seasonal variation into their analysis.
A possible explanation for the significant difference in mass observed during
this study was that the 2 populations might have had different age structures.
Petrides (1949) reported that wild individuals generally reached ≈2 kg by the
end of their first year of life. Therefore, having a sample of sexually mature animals
>2 kg could indicate older animals. Survival of opossums in Massachusetts
was higher in urban areas (Kanda et al. 2009), thus our urban sample may have
included older animals than our rural sample. However, in the Massachusetts
population and other opossum populations studied, almost no animals >2 years
of age have been observed (Gehrt et al. 1997, Gipson and Kamler 2001, Harmon
et al. 2005, Kanda et al. 2009, Woods and Hellgren 2003). Our difference may
indicate that more young-of-the-year were captured at Big Creek CA. However,
few opossums had a body mass <2 kg, indicating that few young-of-the-year
were incorporated into the analysis.
The US continues to lose natural areas to urbanization each year. While this
conversion can negatively affect wildlife populations, some species take advantage
of the new resources these areas offer. The Virginia Opossum clearly benefits
from the new dynamics found in urban areas, and their populations have the potential
to continue expanding as new areas are developed.
Acknowledgments
We thank L. Mechlin and P. Goldman for assistance with all aspects of the project,
the Missouri Department of Conservation for the equipment they provided, D. DeCock
for assistance with statistical analysis, and M. Caby for help with home-range analysis.
We thank A. Loida, C. Myers, and H. White for their hard work in the final phase of the
84 Northeastern Naturalist Vol. 19, No. 1
project. We thank all of the Kirksville residents for allowing access to their property.
Finally we thank T. Wright, B. Wright, and A. Shimkus for funding the project.
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