Deer Mouse Populations Research Articles

Microhabitat selection of Peromyscus leucopus and P. maniculatus in mid-successional vegetation

Abstract We studied syntopic populations of white-footed mice (Peromyscus leucopus) and deer mice (P. maniculatus) in mid-successional vegetation to determine microhabitat selections on experimental plots in northeastern Kansas. Microhabitat selections, based on woody stem densities (WSD), differed between species and differed from expected for both species. Peromyscus leucopus used habitats with medium and high WDS more than expected, whereas P. maniculatus used habitats with low WSD more than expected. Our results indicate that as plant succession proceeds on experimental plots, habitats preferred by P. leucopus should increase, whereas habitats preferred by P. maniculatus should decrease.

NATURAL SELECTION DRIVES ALTITUDINAL DIVERGENCE AT THE ALBUMIN LOCUS IN DEER MICE, PEROMYSCUS MANICULATUS

In populations that are distributed across steep environmental gradients, the potential for local adaptation is largely determined by the spatial scale of fitness variation relative to dispersal distance. Since altitudinal gradients are generally characterized by dramatic ecological transitions over relatively short linear distances, adaptive divergence across such gradients will typically require especially strong selection to counterbalance the homogenizing effect of gene flow. Here we report the results of a study that was designed to test for evidence of adaptive divergence across an altitudinal gradient in a natural population of deer mice, Peromyscus maniculatus. We conducted a multilocus survey of allozyme variation across a steep altitudinal gradient in the southern Rocky Mountains that spanned several distinct biomes, from prairie grassland to alpine tundra. As a control for the effects of altitude, we also surveyed the same loci in mice sampled along a latitudinal transect through the prairie grassland that ran perpendicular to the east-west altitudinal transect. We used a coalescent-based simulation model to identify loci that deviated from neutral expectations, and we then assessed whether locus-specific patterns of variation were nonrandom with respect to altitude. Results indicated that the albumin locus (Alb) reflects a history of diversifying selection across the altitudinal gradient. This conclusion is supported by two main lines of evidence: (1) Alb was characterized by levels of divergence across the altitudinal transect that exceeded neutral expectations in two consecutive years of sampling (in contrast to the spatial pattern of variation across the latitudinal transect), and (2) levels of divergence at the Alb locus exhibited a positive association with altitudinal distance in both years (in contrast to the pattern observed at unlinked loci). We conclude that clinal variation at the Alb locus reflects a balance between gene flow and diversifying selection that results from elevational changes in fitness rankings among alternative genotypes.

Adrenal gland differences associated with puberty and reproductive inhibition in Peromyscus maniculatus

Laboratory populations of the prairie deermouse ( Peromyscus maniculatus) reach a growth asymptote due primarily to the failure of more than 90% of the young born into the population to mature sexually. This inhibition is reversible; when young are removed from the inhibiting influence of the population more than 75% will reproduce within 80 days of pairing. Interestingly, the mechanism of this inhibition does not involve the degree of adrenal hypertrophy as reported in rats and housemice. We report here that the adrenal morphology of reproductively inhibited deermice raised within laboratory populations is different from patterns seen with normal puberty; namely in the area, histology, and apparent activity of the weak androgen-secreting zona reticularis. Our data indicate that the frequency of adrenal cortex cellular apoptosis is not different between the adrenal zones or between reproductively capable versus inhibited animals and therefore does not account for the differences in the numbers of cells with pycnotic nuclei in the zona reticularis of inhibited animals. We also found that the concentration of serum DHEA is significantly reduced in reproductively inhibited animals suggesting that the zona reticularis of inhibited animals may be less active than in controls. We present data to indicate that the adrenal zona reticularis of 30-day-old control animals is likely to be more active than reproductively inhibited animals of the same age. Our data also indicate that older reproductively inhibited animals of both sexes are also likely to have a much less active zona reticularis. These differences may be implicated in the mechanism of reproductive inhibition.

Beating the odds: effects of weather on a short-season population of deer mice

We examined 11 years of data on reproductive success, survival, and population dynamics of two populations (Fortress and Grizzly) of deer mice (Peromyscus maniculatus) in the Kananaskis Valley, Alberta, to investigate the extent to which the dynamics of these populations is dictated by weather conditions. Summer population growth was not related to the population growth in the winter preceding the breeding season or to spring population density. Over the summer on the Fortress grid, population growth was positively related to adult survival, whereas on the Grizzly grid, population growth was positively related to nestling survival. Neither summer population growth nor demographic correlates of summer population growth was consistently related to weather patterns. On Fortress, adult survival during the breeding season was negatively correlated with precipitation. On Grizzly, nestling survival during the breeding season was negatively correlated with precipitation. Winter population growth was inversely proportional to the fall population density prior to the winter but neither was related to weather conditions. Climate limits seasonal breeding in these populations, but compensatory responses appear sufficient to accommodate extreme weather conditions during both the breeding and nonbreeding seasons.

Shedding and intracage transmission of Sin Nombre hantavirus in the deer mouse (Peromyscus maniculatus) model.

The mechanism(s) by which Sin Nombre (SN) hantavirus is maintained in deer mouse populations is unclear. Field studies indicate that transmission occurs primarily if not exclusively via a horizontal mechanism. Using an experimental deer mouse infection model in an outdoor laboratory, we tested whether infected rodents shed SN virus in urine, feces, and saliva, whether infected mice transmit infection to naïve cage mates, and whether infected dams are able to vertically transmit virus or antibody to offspring. Using pooled samples of urine, feces, and saliva collected from mice infected 8 to 120 days postinoculation (p.i.), we found that a subset of saliva samples, collected between 15 and 90 days p.i., contained viral RNA. Parallel studies conducted on wild-caught, naturally infected deer mice showed a similar pattern of intermittent positivity, also only in saliva samples. Attempts to isolate virus through inoculation of cells or naïve deer mice with the secreta or excreta of infected mice were uniformly negative. Of 54 attempts to transmit infection by cohousing infected deer mice with seronegative cage mates, we observed only a single case of transmission, which occurred between 29 and 42 days p.i. Dams passively transferred antibodies to neonatal pups via milk, and those antibodies persisted for at least 2 months after weaning, but none transmitted infection to their pups. Compared to other hantavirus models, SN virus is shed less efficiently and transmits inefficiently among cage mates. Transmission of SN virus among reservoir rodents may require factors that are not required for other hantaviruses.

Within‐population craniometric variability of insular populations of deer mice, Peromyscus maniculatus, elucidated by landscape configuration

Within‐population genetic variability of twelve insular and four mainland populations of deer mice (Peromyscus maniculatus) was assessed using craniometric characters, and compared to results previously obtained from RAPD data. An index of Craniometric Variance (CVar) was computed from pairwise distances among all specimens. Variations in CVar measures were then compared to landscape variables using a linear regression approach. Our results suggest that CVar decreases in presence of large number of a competitive species (the boreal redback vole, Clethrionomys gapperi; r=−0.527, p<0.037) in deer mouse populations. Island remoteness (r=−0.251, p<0.220) and the geometry of the bank opposite to each island (r=−0.459, p<0.067) were marginally correlated with CVar, but the linear combination of these two variables, forming a composite isolation index, represented the major factor explaining the observed CVar (r=−0.648, p<0.011). Using a multiple regression model, 76.3% of the CVar was explained by a combination of this isolation index and the competitors’ abundance. These results suggest that taking into account landscape barriers as well as the dispersal behavior of small mammals might provide sounder ecological variables than geographical distances alone for predicting within‐population genetic variability in a network of habitat patches.

Induced Spatial Heterogeneity in Forest Canopies: Responses of Small Mammals

We hypothesized that creating a mosaic of interspersed patches of different densities of canopy trees in a second-growth Douglas-fir (Pseudotsuga menziesii) forest would accelerate development of biocomplexity (diversity in ecosystem structure, composition, and processes) by promoting spatial heterogeneity in understory, midstory, and canopy, compared to typical managed forests. In turn, increased spatial heterogeneity was expected to promote variety in fine-scale plant associations, foliage height diversity, and abundance of small mammals. Three years following treatment, understory species richness and herb cover were greater with variable-density thinning than without. Midstory and canopy species did not have time to develop significant differences between treatments. Variable-density thinning resulted in larger populations of deer mice (Peromyscus maniculatus), a species associated with understory shrubs; creeping voles (Microtus oregoni), a species associated with herbaceous vegetation, and vagrant shrews (Sorex vagrans), a species usually associated with openings but common in old growth. No forest-floor small-mammal species, including those associated with old-growth forest, declined in abundance following variabie-density thinning. Annual variation in population size was not related to treatment. Variable-density thinning may accelerate the development of biocomplexity in second-growth forest by promoting spatial heterogeneity and compositional diversity in the plant community, increasing diversity and abundance of small mammals, and similarly affecting other vertebrate communities. When combined with long rotations, legacy retention, and management for snags and coarse woody debris, variable-density thinning has broad applicability to enhance biodiversity in managed Douglas-fir forests across the Pacific Northwest.

Longitudinal studies of Sin Nombre virus in deer mouse-dominated ecosystems of Montana.

Sin Nombre virus (SNV), hosted by the deer mouse (Peromyscus maniculatus), is the primary etiologic agent of Hantavirus pulmonary syndrome (HPS) in North America. To improve our understanding of the epidemiology of HPS in the western United States, we conducted studies of population dynamics and SNV antibody prevalence in deer mouse populations for 6 years on 12 mark-recapture grids in Montana. Monthly numbers of deer mice ranged from zero to over 170 on 1-hectare grids. SNV antibody prevalence was higher than observed in studies in other parts of the United States, averaging 13% (0% to 50%), and peaking in May or June each year. Antibody-positive mice were older (heavier) (78% of positives were adults versus 52% of negatives) and more likely to be males (61% of positives versus 53.4% of negatives). A higher proportion of antibody-positive deer mice of all age-mass classes had scars than did antibody-negative mice. Month-to-month survivorship of antibody-positive adult mice was similar to that of antibody-negative mice, but survival of young antibody-positive deer mice was lower than antibody-negative deer mice. This is the first study to clearly suggest a detrimental effect of SNV infection on deer mice.

Spatial and Temporal Patterns of an Irrupting Population of Deer Mice

Spatial and Temporal Patterns of an Irrupting Population of Deer Mice

SPATIAL AND TEMPORAL PATTERNS OF AN IRRUPTING POPULATION OF DEER MICE

We assessed the spatial and temporal pattern and scale of an irruption by a population of deer mice (Peromyscus maniculatus ) in the summer of 1997 in New Brunswick, Canada. We tested the prediction that spatial scales finer than the extent of the irruption would not reveal domains of population growth. Increases in the abundance of mice were seen across an extensive set of study grids (separated by .15 linear kilometers); however, growth rates were autocorrelated spatially over short distances ( ,300 m). The extensive irruption may have been a result of finer-scale irruptions occurring simultaneously.

Life on the edge: the demography of short‐season populations of deer mice

We documented populations of deer mice in the Kananaskis Valley, southwestern Alberta, Canada from 1979 to 1997 to determine whether these short‐season populations were more, or less, variable than populations in more temperate environments. We then examined patterns of reproduction, age‐specific survival, and immigration to explain variation in summer population growth. Population densities showed no multi‐annual periodicity and were generally low. At maximum, numbers doubled over the breeding season, but declined over the breeding season in 4 of 16 yr. Variability in population density was low, and similar to that of Peromyscus populations in more temperate environments. No demographic parameters were related to spring population densities, and immigration rates were low when conditions for survival of nestlings and adults were favorable. Variation in summer population growth was attributed primarily to variation in nestling survival among years.

Antibody to sin nombre virus in rodents associated with peridomestic habitats in west central Montana.

Most human cases of hantavirus pulmonary syndrome are acquired in the peridomestic environment, yet studies of the ecology and infection dynamics in the reservoir host, the deer mouse (Peromyscus maniculatus), have focused on sylvan populations. We describe a 2.5-year study of hantavirus infection in rodents associated with peridomestic habitats in west central Montana. Antibodies reactive with Sin Nombre virus (SNV) were found in five species. Overall SNV antibody prevalence was highest among deer mice (25% of individuals tested). As has been demonstrated for sylvan populations, the antibody-positive component of the deer mouse population consisted of a higher proportion of adults and males. However, the prevalence of antibodies to SNV was higher in this study than has been reported in most sylvan studies. The average monthly proportion of deer mouse blood samples with antibodies to SNV ranged from approximately 20% to 25% and was highest in the late spring/early summer. The higher SNV antibody prevalence in peridomestic compared with sylvan settings may be related to behavioral differences and/or potentially longer survival of the virus deposited inside buildings. Peridomestic settings presented higher concentrations of virus and may present a higher risk of human infection than do sylvan settings.

Remote sensing and geographic information systems: charting Sin Nombre virus infections in deer mice.

We tested environmental data from remote sensing and geographic information system maps as indicators of Sin Nombre virus (SNV) infections in deer mouse (Peromyscus maniculatus) populations in the Walker River Basin, Nevada and California. We determined by serologic testing the presence of SNV infections in deer mice from 144 field sites. We used remote sensing and geographic information systems data to characterize the vegetation type and density, elevation, slope, and hydrologic features of each site. The data retroactively predicted infection status of deer mice with up to 80% accuracy. If models of SNV temporal dynamics can be integrated with baseline spatial models, human risk for infection may be assessed with reasonable accuracy.

Impact of orchard vegetation management on small mammal population dynamics and species diversity

Author(s): Sullivan, Thomas P.; Sullivan, Druscilla S.; Hogue, Eugene J. | Abstract: Voles of the genus Microtus feed on bark and vascular tissues of trees in fruit orchards across North America. Management of orchard floor vegetation with multiple applications of herbicide effectively altered habitat and reduced montane vole (M. montanus) populations in apple orchards near Summerland, British Columbia, Canada. Non-target populations of deer mice (Peromyscus maniculatus), northwestern chipmunks (Tamias amoenus), and Great Basin pocket mice (Perognathus parvus) appeared to respond positively to the treatment units. None of these non-target species was adversely affected by the actual herbicide treatments. Species diversity of small mammal communities in treated orchards was the same as that in untreated orchards and nearby old fields. Orchard agroecosystems with intensive vegetation management regimes appear to maintain diverse non-target small mammal communities.

The genetic heterogeneity of deer mouse populations ( Peromyscus maniculatus ) in an insular landscape

A survey of the genetic variability in deer mouse populations was performed using specimens collected from six different islands on a lake covering approximately 50 km2. Random amplified polymorphic DNA (RAPD) was used to measure the extent of the genetic differences in this insular system. An analysis of molecular variance (AMOVA) revealed that populations are clearly separated at this microgeographic scale (Fst = 0.13863; P < 0.001). The homogeneity of molecular variance test (HOMOVA) indicated that within-population levels vary greatly (Bp = 0.76831; P < 0.001). The within-population molecular variance was found to be mainly correlated with the accessibility of the islands, computed as the inverse of the geographic distance separating an island from the lakeshore (r = 0.916; P < 0.003).

The effects of dietary protein content on growth and maturation in deer mice

Growth and female maturation appear to be limited by the availability of dietary protein in natural populations of deer mice (Peromyscus maniculatus borealis) in the Kananaskis Valley, Alberta. We examined the effects of dietary protein content on nestling growth rates and sexual maturation of female deer mice in two laboratory experiments. In the first, mice whose mothers were fed a low-protein mixture of sunflower seeds and oats (14% protein) exhibited slow growth prior to weaning and those fed high-protein cat food (30% protein) postweaning showed compensatory growth. Preweaning but not postweaning diet quality affected the proportion of females who were sexually mature at 42 days of age. Therefore, while deficient nestling growth can be compensated for, the effects of a low-quality maternal diet during lactation may have lasting effects on the maturation of female offspring. In the second experiment, mice raised on isocaloric diets of 14, 20, and 30% protein did not differ in growth as nestlings or juveniles. Differences among the three diets in the proportion of mature females at 42 days did not correspond to dietary protein levels as predicted. Dietary protein content from 14 to 30% appear to be sufficient for juvenile mice raised in captivity.

GENETIC DIVERSITY AND CAPILLARIA HEPATICA (NEMATODA) PREVALENCE IN MICHIGAN DEER MOUSE POPULATIONS.

There have been few field tests of the hypothesis that homozygous populations are prone to high levels of disease. I tested for a negative correlation between genetic diversity and parasitism by estimating the allozyme heterozygosity, population density, and proportion of individuals infected by Capillaria hepatica (Nematoda) in nine Michigan populations of deer mice (Peromyscus maniculatus). Parasite prevalence was correlated negatively with heterozygosity when the effects of density were held constant, but was not correlated with population density after controlling for the effects of genetic diversity. These data support the prediction that inbred populations will be more susceptible to parasite infestations.

DENSITY DEPENDENCE, PREY DEPENDENCE, AND POPULATION DYNAMICS OF MARTENS IN ONTARIO

Ecological factors influencing demographic parameters of mammalian carnivores are poorly understood, due to the difficulty of simultaneously measuring predator and prey populations over an extended period. We used cohort analysis based on age-specific harvest data to estimate population densities over 20 yr for martens (Martes americana). Marten abundance increased threefold over the study period, probably due to relaxation in harvest intensity at the beginning of the study interval. Changes in rates of population growth by martens were positively correlated with population densities of three species of small mammals recorded over the same time span: deer mice (Peromyscus maniculatus), red squirrels (Tamiasciuris hudsonicus), and flying squirrels (Glaucomys sabrinus). We found no evidence of short-term cycles in marten abundance, nor were there significant cross-correlations at lags greater than 1 with prey population densities. Martens also showed evidence of density-dependent population growth. Such density dependence beyond the demographic effect of variation in prey density was possibly due to agonistic interactions among territory holders. Such mixtures of prey dependence and density dependence often have a stabilizing influence in theoretical models, which could contribute to the observed stability of deer mouse, red squirrel, and marten populations in Algonquin Park. Harvest intensity was negatively related to yearly variation in marten population growth. Mortality due to trapping averaged 37.9% over two decades, with no detectable relationship to changes in marten population density. Hence, harvesting acted as a stochastic external variable that was additive to density-dependent and prey-dependent effects.

Deer Mouse Predation on the Biological Control Agent, Urophora spp., Introduced to Control Spotted Knapweed

Field observations made in 1993 suggested that rodents were preying on spotted knapweed (Centaurea maculosa) seedheads, possibly targeting the gall fly larvae (Urophora SPP.) which overwinter within them. I conducted a brief study to determine the cause of seedhead predation and quantify gall fly predation. Stomachs were examined from 19 deer mice (Pero- myscus maniculatus) captured in the fall of 1993 and winter of 1997. All individuals had preyed upon gall fly larvae. The mean number of gall fly larvae found in 10 deer mouse stomachs in the winter of 1997 was 212.8. The minimum number of larvae consumed by these 10 animals for 1 night of foraging was 2686. Availability of a concentrated protein that is a readily accessible and abundant resource during winter may elevate deer mouse populations in knapweed-in- fested habitats. Increases in densities of deer mice due to gall fly presence could bring about shifts in composition of small mammal communities.

Silvicultural Use of Herbicide in Sub-Boreal Spruce Forest: Implications for Small Mammal Population Dynamics

Vegetation management in northern coniferous forest ecosystems may have implications for small mammal communities. This study was designed to test the hypothesis that herbicide-induced habitat alteration would reduce small mammal populations in early-successional stages of sub-boreal spruce forest. A secondary objective was to determine the long-term influence of herbicide treatment on reproduction, survival, and growth attributes of deer mouse (Peromyscus maniculatus) and southern red-backed vole (Clethrionomys gapperi) populations. Small mammal populations were intensively sampled in 4 pairs of control and treatment sites dominated by early-successional vegetation (5-10 yr after clearcut harvesting) near Prince George, British Columbia, Canada. Sampling was conducted during 1987 (pretreatment year) 1988-89 (first and second posttreatment years), and in 1991-92 (fourth and fifth posttreatment years). Higher mean numbers of red-backed voles (P = 0.03) and shrews (Sorex spp.; P = 0.001) were present on control than treatment sites during 1988-92. Mean abundance of meadow voles (Microtus pennsylvanicus; P = 0.69) and deer mice (P = 0.20) were similar on control and treatment sites throughout the study. Short-tailed weasels (Mustela erminea) and long-tailed weasels (Mustela frenata) were commonly captured on both control and treatment sites. There were no consistent differences between control and treatment populations in body mass, total biomass, or proportion of adult male and female red-backed voles and deer mice in breeding condition. However, there was a higher mean number of successful pregnancies in control than treatment populations of red-backed voles (P = 0.02) but not deer mice (P = 0.18) during posttreatment years. Mean Jolly-Seber estimates of survival of red-hacked voles (P = 0.01) but not deer mice (P = 0.70) were higher in treatment than control populations. The magnitude of observed demographic effects from herbicide treatment were well within the mean values of natural fluctuations of these variables. Herbicide treatment of areas dominated by early-successional vegetation in northern coniferous forests should be staggered in time and space to allow recovery (2-3 yr) of vegetation and small mammal species sucl as red-backed voles and shrews.