Cotton Mice, Peromyscus gossypinus LeConte (Rodentia: Cricetidae), in the Great Dismal Swamp and Surrounding Areas

James L. Boone
Museum of Natural History, Institute of Ecology, and Savannah River Ecology Laboratory University of Georgia, Athens, Georgia 30602

Joshua Laerm
Museum of Natural History, University of Georgia, Athens, Georgia 30602

ABSTRACT

Livetrapping of small mammals was conducted in the Great Dismal Swamp and other areas of North Carolina in 1990. Five Peromyscus gossypinus were caught in the Dismal Swamp proper, and 42 were caught in the Chowan Swamp adjacent to the Dismal Swamp. These are the first published records of P. gossypinus taken in the Dismal Swamp region since the 1930s.

INTRODUCTION

Rose et al. (1990) suggested that the cotton mouse, Peromyscus gossypinus LeConte, may be extinct in the Great Dismal Swamp of Virginia and North Carolina. With the exception of two specimens collected in 1933 by Dice (1940), virtually none have been captured there since the turn of the century despite the efforts of Handley (1979) in the 1950s and Rose et al. (1990) in the 1980s. Our recent collections and genetic analyses show P. gossypinus exists in the Dismal Swamp, and that based on capture rate it is uncommon in the Swamp proper, but relatively abundant in areas adjacent to the southern section of the Swamp.

Separating P. gossypinus from P. leucopus Rafinesque (white-footed mouse) can be difficult both for live and museum specimens. Dice (1940) states that in eastern Virginia these species can be separated using size characteristics but not color. Our studies (unpubl. data) show that several cranial and external characters from adult specimens are required for consistent species identification with discriminant analysis. However, a fixed allozyme difference at the Glucose-6-Phosphate Isomerase locus (GPI or PGI, Enzyme Commission No. 5.3.1.9), and nearly fixed differences at the Albumin and alpha-Glycerolphosphate dehydrogenase (a-GPD or GPD, Enzyme Commission No. 1.1.1.8) loci separate these two species (Price and Kennedy 1980, Robbins et al. 1985, Boone 1990, Boone, unpubl. data).

METHODS

We captured small mammals with Sherman livetraps in Northeastern North Carolina for studies examining subspecific affinities, population genetics, and Lyme disease (Magnarelli et al., 1992) in P. gossypinus. On April 26 and 27, 1990, we trapped in the Dismal Swamp along Highway 158 from 6.3 to 10.6 km east of Highway 32 (east of Sunbury, Gates Co., N.C.) for 587 trapnights. On June 13, 1990, we placed 200 traps in the Chowan Swamp between 2.9 and 5.3 km south of Gatesville (Gates Co., N.C.). On April 28 and 29, 1990, we trapped along the Cashie River in and around Windsor, Bertie Co., N. C. for 350 trapnights, and we placed 150 traps in and around Richlands, Onslow Co., N. C. on April 30, 1990.

Locations of trap lines and specific traps were selected to maximize the capture of P. gossypinus based on our understanding of its habitat preference and ecology learned from the capture of more than 2,100 cotton mice from throughout its entire range. Although these mice can be caught almost anywhere, they seem to exist in highest densities in thick, undisturbed (anthropogenic or natural) seasonally flooded riparian woodlands near water. On coastal barrier islands where these habitats do not exist, they seem to occur most densely in undisturbed old-growth oak-palmetto (Quercus sp. and Serenoa repens) forests. Traps were set on, in, and under logs, in trees, under stumps, in the rotten bases of trees, on the edges of ponds, on floating debris in flooded forests, as well as in old buildings and trash piles. More than one trap was set in particularly promising sites.

We used allozyme markers to identify the Peromyscus. Genetic analysis was performed using standard horizontal starch gel electrophoretic and protein staining techniques on blood, liver, and muscle for 42 enzyme and protein loci using techniques similar to those of Selander et al. (1971) as described in Boone (1990).

Body mass was measured to the nearest 0.1 g. Age (juvenile, subadult, or adult) was determined by pelage color, and reproductive status was determined by examination of external and internal reproductive structures. Non-adults and pregnant females were deleted from morphological comparisons.

RESULTS

Peromyscus gossypinus were caputred in each of the four areas examined, and P. leucopus were captured in all areas except Richlands (Table 1). Additionally, 1 golden mouse (Ochrotomys nuttalli Harlin) and 1 juvenile Virginia opossum (Didelphis virginiana Kerr) were captured in the Dismal Swamp, and 1 Blarina was captured at Windsor.

We found that Dice's (1940) suggestion that these Peromyscus species can be distinguished by size is not strictly true. Our comparison of genetic markers and morphology indicates that although P. gossypinus tends to be larger and heavier than P. leucopus, there is considerable overlap. For the mice caught east of Sunbury, body mass of P. gossypinus ranged from 20.9 to 35.5 g ( = 26.3 g, n = 5), whereas P. leucopus ranged from 14.6 to 24.6 g (= 19.1 g, n = 26). In the Chowan Swamp, body mass of P. gossypinus ranged from 17.1 to 36.8 g (= 25.9 g, n = 42); the one adult P. leucopus weighed 15.9 g. In the Windsor area, P. gossypinus ranged from 19.2 to 37.9 g (= 28.4 g, n = 22), and P. leucopus ranged from 17.1 to 24.1 g (= 20.4 g, n = 4) The P. gossypinus from Richlands ranged from 21.2 to 39.4 g (= 29.2 g, n = 33). Therefore, if Rose et al. (1990) used size to identify Peromyscus, some of the specimens identified as P. leucopus by might actually have been P. gossypinus.

DISCUSSION

Our results probably differ those of Rose et al. (1990) as a result of different trapping location, design, and methods. In the southern portion of the Dismal Swamp, Rose et al. (1990) used pitfall traps set on a grid. We used only Sherman live-traps and our collection locations were selected to target habitats thought to be optimal of P. gossypinus without concern for determining density or other demographic parameters. Therefore, we were not confined to a grid, and we were able to trap in areas, and place traps in sites, that would be inappropriate to use with pitfall traps in a demographic study. Furthermore, our trapping was only conducted in the southern-most part of the Swamp, an area more accessible to migrants from the Chowan Swamp where P. gossypinus is abundant, whereas the majority of Rose et al.'s (1990) effort was concentrated in the northern section of the Swamp where P. gossypinus may be absent.

Although P. gossypinus is abundant in areas near the Dismal Swamp, it is probably not currently abundant in the swamp proper. Handley (1979) stated that P. gossypinus densities fluctuate widely in the swamp, and this population may simply be at a low point in its cycle. This species now occurs in the Great Dismal Swamp, but current management practices in the Great Dismal Swamp National Wildlife Refuge that promote clearings and vegetational heterogeneity may endanger it because we have observed that P. gossypinus occurs in greatest density in mature undisturbed riparian forests.

ACKNOWLEDGMENTS

The Savannah River Ecology Laboratory under contract DE-AC09-76SR00819 between the U. S. Department of Energy and the University of Georgia Institute of Ecology, the University of Georgia Museum of Natural History, Sigma Xi, and the Theodore Roosevelt Memorial Fund administered by the American Museum of Natural History provided support for this work, and Kevin Roe assisted with the trapping.

LITERATURE CITED

Boone, J. L. 1990. Reassessment of the taxonomic status of the cotton mouse (Peromyscus gossypinus anastasae) on Cumberland Island, Georgia, and implications of this information for conservation. M. S. Thesis. University of Georgia, Athens. 70pp. Abstract

Dice, L. R. 1940. Relationship between the wood-mouse and the cotton-mouse in eastern Virginia. Journal of Mammalogy 21:14-23.

Handley, C. O., Jr. 1979. Mammals of the Dismal Swamp: a historical account. Pages 297-357 in The Great Dismal Swamp. P. W. Kirk, Jr., ed. University Press of Virginia, Charlottesville.

Magnarelli, L. A., J. H. Oliver, H. J. Hutcheson, J. L. Boone, and J. F. Anderson. 1992. Antibodies to Borrelia burgdorferi in rodents in the eastern and southern United States. Journal of Clinical Microbiology 30:1449-1452.

Price, P. K., and M. L. Kennedy. 1980. Genetic relationships in the white-footed mouse, Peromyscus leucopus, and the cotton mouse, Peromyscus gossypinus. American Midland Naturalist 103:73-82.

Robbins, L. W., M. H. Smith, M. C. Wooten, and R. K. Selander. 1985. Biochemical polymorphism and its relationship to chromosomal and morphological variation in Peromyscus leucopus and Peromyscus gossypinus. Journal of Mammalogy 66:498-510.

Rose, R. K., R. K. Everton, J. F. Stankavich, and J. W. Walke. 1990. Small mammals of the Great Dismal Swamp of Virginia and North Carolina. Brimleyana 16:87-101.

Selander, R. K., M. H. Smith, S. Y. Yang, W. E. Johnson, and J. B. Gentry. 1971. Biochemical polymorphism and systematics in the genus Peromyscus. I. Variation in the old-field mouse (Peromyscus polionotus). Studies in Genetics VII. University of Texas Publication 7103:49-90.

Table 1. Peromyscus gossypinus and P. leucopus captured in North Carolina, 1990.

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     Location                                 Species
                          -----------------------------------------------
                              P. gossypinus              P. leucopus
                          ----------------------   ----------------------
                         Number     Captures/      Number      Captures/
                         caught  1000 trapnights   caught  1000 trapnights
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Gates Co., Dismal Swamp     5         8.5            27         46.0
Gates Co., Chowan Swamp    42       210.0             2         10.0
Bertie Co., Windsor        33        94.3             5         14.3
Onslow Co., Richlands      42       280.0             0          0.0
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