Similarities between golden mice (Ochrotomys nuttalli) and white-footed mice (Peromyscus leucopus) have been well-studied in both field and laboratory settings. Often sympatric, these species share similar habitat, as well as other resources, yet previous researchers have found little evidence for interspecific competition. Niche partitioning may reduce direct competition through specialization of resource use. Although the golden mouse is considered a resource specialist, it is likely that the degree of habitat specialization differs by locality, and thus, the degree of interspecific competition with similar species is variable. To determine the extent to which microhabitat use differs between golden mice and white-footed mice, I measured 16 and 15 microhabitat variables during the leaf-on and leaf-off seasons, respectively, in Jackson County, Illinois. Trapping took place on 3 grids from March 2010 to September 2011. The ratio of individual golden mice (n = 74) to white-footed mice (n = 85) was unusually high during this study. Microhabitat use models were constructed for both species during both seasons using logistic regression by comparing microhabitat at trapping stations where each species was captured vs. stations without captures. Few variables described habitat occupied by golden mice. Overall, dense vegetation up to 2.0 m was most important for golden mice and ground-level structures such as logs were most important for white-footed mice. Captures and noncaptures were predicted with a high degree of accuracy by logistic regression (81.5-90.3%). Discriminant function analysis was used to identify which microhabitat variables optimally discriminated between habitat used by golden mice, white-footed mice, and neither species. More variables discriminated between species during the leaf-on season than the leaf-off season although discriminating variables during leaf-off were more important overall. Habitat where either species was captured was combined and compared against habitat where no mice were captured; microhabitat used by mice was statistically distinct within the study area. Captures and noncaptures were classified correctly more than would be expected by chance by discriminant function analysis but moderate classification success values indicated microhabitat differences between species were subtle. Microhabitat and elevated trap use varied between seasons for both species, but neither species used ground or elevated traps more than expected during the leaf-on season. Spatial segregation was more apparent during the leaf-off season when golden mice used elevated traps more than expected and white-footed mice used ground traps more than expected. Overall results suggest that golden mice exhibited a great deal of plasticity in microhabitat use seasonally, and are more habitat generalists than previous literature would suggest. Although some spatial segregation was apparent between golden mice and white-footed mice, there was no evidence for avoidance between species, which implies a lack of interference competition. It is likely that other life-history factors (such as metabolic rate, nest building, or sociality) in combination with microhabitat and vertical partitioning allow coexistence between these species rather than microhabitat segregation alone.

The golden mouse (Ochrotomys nuttalli), a state-threatened species in Illinois, is sympatric throughout most of its geographic range with the ubiquitous white-footed mouse (Peromyscus leucopus). The degree of interspecific competitive interaction between these species has been investigated by previous researchers, with often conflicting results. I live trapped and marked both species on two grids from 21 May 2008 through 8 May 2009. One grid was the control area; from the second grid I removed white-footed mice. Habitat on the grids was very similar for six measured habitat variables. My objectives were to document the extent of competition between the two species as indicated by a population density or behavioral response of golden mice to removal of white-footed mice on the experimental grid, and to determine if white-footed mouse presence or habitat variables affected the capture of golden mice. During 6,528 trap nights per grid, I captured a total of 22 individual O. nuttalli and 66 P. leucopus on the control grid, and 48 O. nuttalli and 202 P. leucopus on the removal grid. Compared to previous studies (Rose, 2008), the number of individual golden mice captured on my two sites (n = 70) was very large. Whereas I expected an increase in numbers and space use of golden mice upon removal of Peromyscus, the only statistically significant change was an apparent decrease in space use--there was no population density change. These results, which may indicate a lack of interspecific competition, are more likely the result of experimental design flaws, especially considering the following results. The likelihood of trapping a golden mouse at any given station on the control site was unaffected by any of the six habitat variables. White-footed mouse presence significantly decreased the likelihood of capturing golden mice on both sites. These results were the same for both elevated and ground traps. On the experimental site, golden mice changed from predominantly arboreal space use before Peromyscus removal to equal space use between arboreal and ground habitat after Peromyscusremoval. These results illustrate a spatially segregated habitat with competitively superior white-footed mice dominating the ground level microhabitat and golden mice staying predominantly in elevated microhabitat. My study, and others, demonstrate that golden mice in southern Illinois are likely habitat generalists (Morzillo et al., 2003), competitively inferior to Peromyscus(Feldhamer and Maycroft, 1992), and excluded from certain microhabitat; although it may be they actually prefer different microhabitats.

Golden mice (Ochrotomys nuttalli) are generally an elusive and rare species throughout their geographic range in the southeastern United States. They are considered to be habitat specialists that prefer dense understory consisting of shrubs and vines. Golden mice are less vagile, and likely disperse shorter distances than other sympatric species such as the white-footed mouse (Peromyscus leucopus). Conversely, white-footed mice are considered habitat generalists that inhabit a variety of habitat types, are more vagile, and disperse farther than golden mice. Because of this it is likely that golden mice have a lower genetic diversity and are more genetically subdivided than white-footed mice. In southern Illinois, golden mice are on the periphery of their range, which is one of the reasons they are on the state-threatened list in Illinois. It has been hypothesized that populations on the periphery of a species range will have more population structure and lower genetic diversity than populations in the core of the range. Tissue samples for golden mice and white-footed mice were collected from 24 sites throughout southern Illinois and 24 sites throughout the golden mouse core range. I analyzed 13 and 10 microsatellite markers as well as 594 and 624 base pairs of the mitochondrial control region for golden mice and white-footed mice, respectively, to characterize and compare the genetic diversity and population structure of both species. Overall haplotype diversity (0.76) and nucleotide diversity (0.20%) was lower in golden mice compared to white footed mice (0.99 and 1.97%). Results of an AMOVA using the mitochondrial control region revealed more subdivision among the 3 populations of golden mice (Φst = 0.099, P < 0.001) than among the 3 populations of white-footed mice (Φst = 0.058, P < 0.001). Microsatellite loci showed a similar trend with overall FST values of 0.027 (P < 0.001) for golden mice and 0.004 (P = 0.137) for white-footed mice. I intended to compare golden mouse individuals from southern Illinois and the core of the range, but too few individuals were collected from the core. More samples need to be collected throughout the core of the range to better understand the population genetics of golden mice in the core of the range compared to the periphery.