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. 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