2007 SOUTHEASTERN NATURALIST 6(3):407–412 Double Captures of Peromyscus leucopus (White-footed Mouse and Ochrotomys nuttalli (Golden Mouse) Cory C. Christopher1 and Gary W. Barrett2,* Abstract - We report the first interspecific double captures (n = 14) of Peromyscus leucopus (white-footed mouse) and Ochrotomys nuttalli (golden mouse). Intraspecific double captures of white-footed mice (57.0% of all double captures) were heterosexual (likely mating pairs). Overall, 62.0% of the double captures for both species were heterosexual, suggesting that there appears to exist a strong conspecific, heterosexual odor preference. The large number of intra- and interspecific double captures (n = 103) also suggests minimal interference or exploitation competition between these two small mammal species of similar body mass and life histories. Introduction Peromyscus nuttalli Rafinesque (white-footed mouse) and Ochrotomys nuttalli Harlan (golden mouse) have similar body masses, nest-site references, home-range sizes, food preferences, and periods of activity, and both are semiarboreal and coexist in southeastern forests (Christopher and Barrett 2006, Knuth and Barrett 1984, Lackey et al. 1985, Linzey 1968, Linzey and Packard 1977, Pruett et al. 2002). However, there have been no reports of both species being simultaneously captured (i.e., interspecific captures) in the same live trap. The occurrence of two or more small mammals trapped simultaneously (double captures) in field studies has been described by numerous investigators (Bergstrom and Sauer 1986, Dunaway 1968, Getz 1972, Jenkins and Llewellyn 1981, Novak 1983, Petersen 1975, Sheppe 1967, Spencer et al. 1982). Most of these double captures were intraspecific. The highest percentage of intraspecific double captures was heterosexual (Getz 1972, Petersen 1975, Slade 1976; but see Feldhamer 1977). Heterosexual double captures were especially prevalent during the breeding season (Novak 1983). Blaustein and Rothstein (1978), Feldhamer (1977), Getz (1972), Jenkins and Llewellyn (1981), Novak (1983), Petersen (1975), and Spencer et al. (1982) discuss how social structure and foraging behavior partially explains intraspecific multiple captures. Getz (1972), for example, suggests that multiple captures indicate sociality between individuals based on sex or age relationships. Double captures also may be the result of trap sensitivities and random encounters of small mammals when entering a single trap (Bergstrom and Sauer 1986). 1Department of Biological Sciences, University of Cincinnati, Cincinnati, OH 45221. 2Institute of Ecology, University of Georgia, Athens, GA 30602. *Corresponding author - gbarrett@uga.edu. 408 Southeastern Naturalist Vol. 6, No. 3 Interspecific double captures of small mammals in single-catch livetraps are rare. However, Petersen (1975) reported interspecific double captures of Baiomys taylori Thomas (northern pygmy mouse) and Reithrodontomys megalotis Baird (western harvest mouse), northern pygmy mouse and R. fulvescens Allen (fulvous harvest mouse), and western and fulvous harvest mice. Feldhamer (1977) reported double captures of Peromyscus maniculatus Wagner (deer mouse), and Perognathus parvus Peale (Great Basin pocket mouse). Evans and Holdenreid (1943) reported similar findings regarding double captures between species of these genera in California. We report here for the first time interspecific double captures of white-footed and golden mice. Past investigations suggest that the low frequency of interspecific double captures of small mammals most often is attributed to antagonistic behavior, social behavior during the breeding season, and population density (Blaustein and Rothstein 1978, Jenkins and Llewellyn 1981, Novak 1983, Petersen 1975). We questioned whether interspecific double captures in our study could be related to a reduction in or lack of competition for resources (nesting sites, food, or space) within the forest habitat. Methods Study area This investigation was conducted at the Horse Shoe Bend (HSB) Ecological Research Site located in Clarke County near Athens, GA (33º57'N, 83º23'W). HSB is a 14.2-ha (35-acre) riverine peninsula formed by a meander of the North Oconee River and is composed of bottomland and upland forest habitat (see Klee et al. 2004 for details). Both the upland and bottomland habitats contained abundant Smilax spp. (greenbrier), Lonicera maackii (Rupr.) Herder (amur honeysuckle), L. japonica Thunb. (Japanese honeysuckle), and Ligustrum sinense Lour. (Chinese privet). Quercus nigra L. (water oak), Liquidambar styraciflua L. (sweet gum), and Liriodendron tulipifera L. (tuliptree) dominated both habitat types, whereas Q. alba L. (white oak) and Fagus grandifolia Ehrh. (American beech) were abundant in the upland habitat, and Betula nigra L. (river birch) was abundant in the lowland habitat. Research design Eight trapping grids (each 0.21 ha) were established in bottomland and upland forest habitat types (n = 4 each) to quantify population abundance of white-footed and golden mice. Each grid consisted of 12 trapping stations established in a 2 x 6 grid pattern. Trapping stations were 10 m (± 2 m) apart. Each station consisted of two Sherman live traps (7.62 x 7.62 x 25.4 cm each); one trap was on a wooden platform 1.5 m high on the trunk of a tree, and the second trap was on the ground within one meter of the base of the same tree. Live trapping was conducted weekly from 29 March to 16 November 2001, and from 17 March to 3 November 2004. Traps were baited with black oil sunflower seed, set before dark, and examined the following morning. 2007 C.C. Christopher and G.W. Barrett 409 Captured white-footed and golden mice were either marked by toe clipping (ASM Animal Care and Use Committee 1998) or by ear tags for identification. Trap-springing weights were not used as described by Bergstrom (1986) because both species of small mammals are of similar body mass (Christopher and Barrett 2006). Each trap was set for sensitivity at the time of setting. It is possible, however, that some traps were less sensitive than others. Such an insensitive trap might not be sprung until two individuals had entered the trap. We determined the sex, weight, reproductive condition (vaginal orifice open or closed, testes abdominal or scrotal), and general health for each captured animal. The date and location of capture was recorded for each individual. Captured animals were immediately released following examination at the site of capture. Mean weekly population abundance was estimated by the minimum-number-known-alive (MNKA) method (Krebs 1996). Results and Discussion Mean weekly population abundance for both white-footed and golden mice reached a peak in early spring and declined steadily until late autumn both years. In 2001, the maximum mean abundance for white-footed mice per grid (0.21 ha) was 23.1 during 22–28 April, and the maximum mean abundance for golden mice per grid was 12.3 during 20–26 May. White-footed and golden mice populations steadily declined to a mean of 1.5 during 11–17 November and 0.3 during 4–10 November, respectively. Similar trends were observed in 2004 when white-footed mice mean population abundance peaked at 34.3 during 28 March–3 April, and golden mice peaked at 6.1 during 12–27 March. Population abundance then declined to a mean abundance per grid of 1.9 for white-footed mice and 0.3 for golden mice during early November. Christopher and Barrett (2006) and Jennison et al. (2006) provide details regarding population dynamics of each species during 2001 and 2004, respectively. There were 254 individual captures of white-footed mice during 2001 and 486 captures during 2004. Golden mice were captured 121 times during 2001, and there were 89 captures during 2004. There were also 64 double captures during 2001, and 39 double captures during 2004. Thus, there were 103 intra- and interspecies double captures during the 2001 and 2004 trapping seasons (Table 1). No juveniles or weanlings were included in the total number of double captures. Of these double captures, 57.0% involved heterosexual double captures of white-footed mice; the highest percentage of double captures of golden mice (8.7%) was also heterosexual. This heterosexual intraspecific combination of double captures agrees with the findings of Getz (1972), Novak (1983), and Petersen (1975) involving double captures of Microtus pennsylvanicus Rhoads (meadow vole); northern pygmy and western harvest mice; and white-footed mice, respectively. However, Feldhamer (1977) reported that 14 of 16 double captures of Microtus montanus Baird (montane vole) were males. 410 Southeastern Naturalist Vol. 6, No. 3 The high percentage of heterosexual double captures for each species suggests that conspecific odors in traps best accounts for this finding (Drickamer 1984, Mazdzer et al. 1976). Wolf and Batzli (2002) noted that adult white-footed mice were more likely to be captured in traps previously occupied by conspecific individuals of the opposite sex than in traps previously occupied by the same sex, especially during the breeding season. Our findings tend to confirm this observation for white-footed mice and suggest that golden mice may respond to trap odors in a similar manner. For example, 9 of 10 double captures of golden mice were heterosexual. Same-sex double captures of white-footed mice were 17.5% for males and 15.5% for females (Table 1). These data suggest lack of an antagonistic sexual behavior for white-footed mice. The 14 interspecific double captures of white-footed and golden mice is also evidence of lack of antagonistic behavior between these two species. In instances of interspecific double captures, only one golden mouse was found dead or injured. This finding suggests a high degree of compatibility between these two small mammal species even when confined to a single live trap. Because food resources were abundant (e.g., water oak acorn mast crop) in our study (Christopher and Barrett 2006), we suggest that interspecific double captures were related to greatly reduced interference competition between these two species in the forest habitat. There were significantly more double captures in 2001 (n = 64) than 2004 (n = 39; 􀁲2 = 6.07, df = 1, P < 0.05). Based on the total number of captures of both species during both years (n = 950), one can predict (Feldhamer and Maycroft 1992) the number of expected double captures using a binomial distribution (p2 + 2pq + q2), where p (white-footed mice) = 0.78 and q (golden mice) = 0.22. Although white-footed and golden mice were live-trapped together 14 times (Table 1), this value is significantly less than the 35.02 expected (􀁲2 = 21.2, df = 2, P < 0.001); intraspecific double captures were more frequent than expected based on this binomial distribution. Goodpaster and Hoffmeister (1954) reported finding a white-footed mouse nest approximately five meters from a golden mouse nest, and noted that there appeared to be no rivalry for food or nesting sites between these two species. Table 1. Frequency and composition of double captures of P. leucopus (white-footed mouse) and O. nuttalli (golden mouse) during 2001 and 2004. Composition of double capture Frequency Percentage 1 male, 1 female P. leucopus 45 43.7 2 male P. leucopus 18 17.5 2 female P. leucopus 16 15.5 1 male, 1 female O. nuttalli 9 8.7 1 male P. leucopus, 1 female O. nuttalli 8 7.8 1 male P. leucopus, 1 male O. nuttalli 4 3.9 1 female P. leucopus, 1 female O. nuttalli 2 1.9 2 male O. nuttalli 1 1.0 Total 103 100.0 2007 C.C. Christopher and G.W. Barrett 411 Christopher and Barrett (2006) document how coexistence between these species is made possible by differential use of three-dimensional habitat space, rather than by competition for food and nesting sites. Differences in bioenergetics and food choices tend to confirm this observation (Knuth and Barrett 1984, O’Malley et al. 2003). These findings indicate that there exists extensive niche overlap in time and space between these two species. In addition to the high percentage of conspecific heterosexual double captures for each species, likely attributed to conspecific odors in traps, perhaps sociality, rather than exploitation or interference competition, also helps to explain the frequency of interspecific double captures and compatibility between these two small mammal species of similar life histories and body mass. Acknowledgments We thank G. Cameron, S. Castleberry, and G. Feldhamer for review of this manuscript. We thank T.L. Barrett for manuscript preparation. We also thank J. Chastant, C. Jennison, R. Klee, T. Luhring, A. Mahoney, K. Meeks, A. Pruett, C. Schmidt, S. Shivers, and M. Shuman for field assistance during 2001 and 2004. This study was funded in part by the Eugene P. Odum Professorship endowment provided to G.W. Barrett. Literature Cited ASM Animal Care and Use Committee. 1998. Guidelines for the capture, handling, and care of mammals as approved by the American Society of Mammalogists. 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