Microtus Montanus Research Articles

Population Fluctuations of the Deer Mouse (Peromyscus maniculatus) in Old-Field and Bunchgrass–Sagebrush Habitats: The Role of Agricultural Setting and Optimum Habitat

In semiarid regions, the deer mouse (Peromyscus maniculatus) is a major small mammal species occupying perennial grassland habitats that include old-fields, native bunchgrass–sagebrush, and some agricultural settings. We investigated population changes in deer mouse populations in perennial grasslands, both natural and old-field, from 1982 to 2003 in southern British Columbia, Canada. Hypotheses (H) predicted that P. maniculatus populations will have (H1) multiannual fluctuations in abundance driven by population increases from extended breeding in summer and winter; (H2) relaxed spring reorganization events in some years leading to higher overall recruitment and survival; and (H3) interspecific competition with montane voles that causes deer mice to be lower in density when voles are higher. P. maniculatus populations in old-field and grass–sagebrush sites had clearly defined periods of high “peak” mean numbers (32–52/ha) and other times of low mean numbers (20–22/ha). Based on mean annual peak density in autumn, deer mouse populations exhibited fluctuations of 3–4 years in both habitats, but this pattern was not always present. The greater numbers of P. maniculatus in high than low years was directly related to population increases from extended breeding seasons and an increased number of lactating females, thereby supporting H1. Spring breeding season declines occurred but were similar or less in high than low years of mean abundance and were relaxed in comparison to forest populations of deer mice in other studies. Thus, H2 was supported for recruitment with high numbers of young-of-the-year breeding and total number of juvenile recruits but for survival was equivocal. Total summer survival was consistently higher in high than low population years but juvenile productivity in all years was poor. Mean abundance of P. maniculatus and M. montanus in old-field sites were highly correlated, and hence H3 was not supported. This latter result is the first, to our knowledge, of P. maniculatus coexisting in a similar pattern of population fluctuations with a Microtus species in a mainland grassland habitat. Higher than average precipitation in the year preceding a peak population of deer mice may have enhanced herbaceous vegetation and contributed to population increases in both habitats. We conclude that the old-field habitat associated with this agricultural setting provides optimum habitat for P. maniculatus and facilitates multiannual population fluctuations in this species.

Small mammal relative abundance within riparian ecosystems of the Blue Mountains

There is a growing interest and investment in restoring riparian areas in the Pacific Northwest to protect biodiversity and water quality, and to restore quality habitat for threatened fish species. However, these management activities change vegetation conditions and potentially impact terrestrial species in these ecosystems. Our objective was to estimate associations between small mammals and 4 broad cover types—conifers, shrubs, grasses, and forbs—in riparian areas within the Blue Mountains of northeast Oregon, USA. We estimated abundances and spatial association for 10 small mammal species using a multispecies hierarchical abundance model and mark-recapture data collected on 36 sites from 2014 to 2017. We predicted that forest-associated species would be more abundant with greater conifer cover, that increases in shrub cover would favor most species, and that increases in grass and forb cover would favor grassland-associated species. Yellow-pine chipmunks (Tamias amoenus) were positively associated with conifer cover, while long-tailed voles (Microtus longicaudus) and montane voles (Microtus montanus) were negatively associated with conifer cover. Associations with shrub cover were positive for bushy-tailed woodrats (Neotoma cinerea), deer mice (Peromyscus maniculatus), and yellow-pine chipmunks, but negative for montane voles. Bushy-tailed woodrats and montane voles were negatively associated with grass cover. This study is the first to characterize the riparian small mammal communities in this region and provides insights on the effects of restoration activities on riparian ecosystems in dry interior forests and grasslands. We suggest that riparian plantings, a ubiquitous restoration practice, will increase the relative abundances for some, but not all, small mammal species. We also demonstrate use of a new multi-species abundance model that can be a powerful tool for analyzing mark-recapture data because it allowed us to extend the scope of our analysis to data poor species by pooling information with data rich species.

Dynamics of montane vole (Microtus montanus) populations in set-asides, field margins, and orchards: Keystone or pest species?

Voles of the genus Microtus are major small mammal species responsible for feeding damage to agricultural crops in Europe and North America. Microtus also occupy non-crop habitats such as field margins, hedgerows, edges, riparian zones, set-asides, and fallow fields where the conundrum of “keystone species” vs. “pest species” arises. We ask if montane vole (M. montanus) populations will occupy small set-asides and potentially move into field margins adjacent to tree fruit orchards during periods of high numbers? In replicated sites in south-central British Columbia, Canada, in 2009–2012, we tested the hypotheses (H) that (H1) small (0.25 ha) perennial grassland sites would be high-quality habitat for voles and generate a multi-annual population fluctuation in abundance; (H2) voles will follow the same population pattern in set-asides and field margins; and (H3) voles occurring in set-aside and field margins will disperse into adjacent tree fruit orchards. In set-asides, voles increased to a mean peak population of 290 voles/ha and then declined to 50/ha thereby providing support for H1. Montane voles generally followed the same pattern of population changes in set-asides and field margins thereby supporting H2. A much reduced abundance of voles in field margins was likely owing primarily to the areal versus linear extent of the two habitats as both had similar mean abundance of herbs, thatch, and structural diversity. Vegetation management in orchards contributed to very low numbers of voles, and hence no relationship in population pattern with the set-asides. Movement of voles from set-asides to field margins was low and from set-asides and field margins to orchards was almost negligible, thereby providing no support for H3. There was no feeding damage to crop production during winter periods. Conservation of even small (0.25 ha) set-asides of perennial grasslands may provide high-quality habitat for fluctuating populations of montane voles without necessarily impacting crop production in adjacent orchards that have appropriate vegetation management. Microtus species may be considered keystone architects of biodiversity in at least some agricultural contexts.

Genomic Resources for the North American Water Vole (Microtus richardsoni) and the Montane Vole (Microtus montanus).

Voles of the genus Microtus are important research organisms, yet genomic resources are lacking. Such resources would benefit future studies of immunology, phylogeography, cryptic diversity, and more. We sequenced and assembled nuclear genomes from two subspecies of water vole (Microtus richardsoni) and from the montane vole (Microtus montanus). The water vole genomes were sequenced with Illumina and 10× Chromium plus Illumina sequencing, resulting in assemblies with ∼1600,000 and ∼30,000 scaffolds, respectively. The montane vole was also assembled into ∼13,000 scaffolds using Illumina sequencing. Mitochondrial genome assemblies were also performed for both species. Structural and functional annotation for the best water vole nuclear genome resulted in ∼24,500 annotated genes, with 83% of these having functional annotations. Assembly quality statistics for our nuclear assemblies fall within the range of genomes previously published in the genus Microtus, making the water vole and montane vole genomes useful additions to currently available genomic resources.

Population changes and limitation in the montane vole (Microtus montanus) in perennial old-field grasslands: insights from a long-term study

AbstractWe livetrapped populations of Microtus montanus from 1982 to 2003 in semiarid perennial old-field grasslands of southern British Columbia. We evaluated two, nonmutually exclusive hypotheses (H) to explain their population dynamics: first (H1), that extended breeding during the summer or winter will drive the increase phase of population fluctuations; and second (H2), that density-dependent depression of juvenile survival will be reflected in poor early juvenile survival during high populations. Populations on 2–3 grids of 1 ha were livetrapped at 3- to 8-week intervals in summer and winter except in 5 years of very low populations. Densities ranged from 10/ha to 250/ha. Peak densities occurred in 6 years and an extended low phase occurred from 1999 to 2003. Fluctuations of 3–4 years appeared in our populations but were not always present. Breeding occurred both in summer and winter, and the best predictor of the population growth rate was the fraction of adult females lactating in summer or winter, thereby supporting H1. Juvenile production (number of juveniles/lactating female) varied greatly among years with the mean being over two times higher in low (2.41) than high (1.08) years, thereby supporting H2. There was no clear correlation between population changes and either seasonal temperatures or rainfall, or any combination of these two variables, and no obvious cause of the prolonged low from 1999 to 2003. Thus, both female reproduction and juvenile production drive montane vole dynamics demographically, similar to what is found in other vole species. However, the ultimate cause of these changes remains to be tackled experimentally.

Grazing and wildfire effects on small mammals inhabiting montane meadows

ABSTRACTLivestock grazing and fire can intensively modify montane meadows. Understanding how these factors affect habitat, species richness, and diversity of small mammals can inform management decisions. Few studies have investigated the independent and synergistic effects of grazing and wildfire on vegetation and small‐mammal communities, and none have focused on montane meadows in the southwestern United States. In 2012 and 2013, we captured small mammals at 105 sites to contrast occupancy, species richness, and diversity among livestock grazing levels (present, absent), wildfire severity (unburned, low, or moderate), and meadow classifications (small or large, wet or dry) in Arizona, USA. During 13,741 trap nights, we captured 1,885 rodents of 8 species. Two species represented 88% of captures: deer mouse (Peromyscus maniculatus) and Arizona montane vole (Microtus montanus arizonensis). Deer mice, Navajo Mogollon voles (Microtus mogollonensis navaho), and thirteen‐lined ground squirrels (Ictidomys tridecemlineatus monticola; a subspecies endemic to the White Mountains, AZ) had higher occupancy in large, ungrazed meadows compared to small, grazed meadows. Species richness was greater in unburned than burned sites and small meadows than large. However, higher diversity occurred in ungrazed and dry compared to grazed and wet meadows. Three species demonstrated weak relationships between wildfire and occupancy, suggesting short‐term (<2 yrs) effects of low to moderate burn severity for these species or their habitat. Livestock grazing had a greater effect than wildfire on the small‐mammal community by altering vegetation or other habitat elements and thus decreasing population sizes. Reducing livestock grazing would benefit small‐mammal species and increase diversity and abundance of the small‐mammal community in montane meadows. © 2019 The Wildlife Society.

Relationship Between Sequence Homology, Genome Architecture, and Meiotic Behavior of the Sex Chromosomes in North American Voles.

In most mammals, the X and Y chromosomes synapse and recombine along a conserved region of homology known as the pseudoautosomal region (PAR). These homology-driven interactions are required for meiotic progression and are essential for male fertility. Although the PAR fulfills key meiotic functions in most mammals, several exceptional species lack PAR-mediated sex chromosome associations at meiosis. Here, we leveraged the natural variation in meiotic sex chromosome programs present in North American voles (Microtus) to investigate the relationship between meiotic sex chromosome dynamics and X/Y sequence homology. To this end, we developed a novel, reference-blind computational method to analyze sparse sequencing data from flow-sorted X and Y chromosomes isolated from vole species with sex chromosomes that always (Microtus montanus), never (Microtus mogollonensis), and occasionally synapse (Microtus ochrogaster) at meiosis. Unexpectedly, we find more shared X/Y homology in the two vole species with no and sporadic X/Y synapsis compared to the species with obligate synapsis. Sex chromosome homology in the asynaptic and occasionally synaptic species is interspersed along chromosomes and largely restricted to low-complexity sequences, including a striking enrichment for the telomeric repeat sequence, TTAGGG. In contrast, homology is concentrated in high complexity, and presumably euchromatic, sequence on the X and Y chromosomes of the synaptic vole species, M. montanus Taken together, our findings suggest key conditions required to sustain the standard program of X/Y synapsis at meiosis and reveal an intriguing connection between heterochromatic repeat architecture and noncanonical, asynaptic mechanisms of sex chromosome segregation in voles.

Creation of bunchgrass, sagebrush, and perennial grassland habitats within a semi-arid agricultural setting: Implications for small mammals

Native habitats of the semi-arid intermontane grasslands and shrub-steppe rangelands of the Pacific Northwest of North America are disappearing owing to agricultural, urban, and recreational development. A major small mammal occupying these habitats is the montane vole (Microtus montanus), along with the deer mouse (Peromyscus maniculatus), northwestern chipmunk (Neotamias amoenus), house mouse (Mus musculus), and two less common species at risk. To help restore these native habitats and small mammals, we tested three hypotheses (H) that non-crop habitats of native bunchgrasses, sagebrush (Artemisia tridentata), and perennial grasslands (alfalfa and mix of forage grasses) would: (H1) establish in an agricultural setting, (H2) conserve small mammal species, and (H3) limit vole damage to tree fruit production in orchards. Linear habitats of these non-crop treatments, within a semi-arid agrarian setting in south-central British Columbia, Canada, were sampled for all vascular plants and small mammals from 2006 to 2010. Mean abundance of bunchgrasses declined after the second year owing to competition from orchard herbs, but sagebrush and pasture grasses became well established, thereby partially supporting H1. Alfalfa established initially but then declined potentially owing to consumption by voles. Conservation of the three major small mammal species, but not less common species, partially supported H2. Alfalfa and pasture grasses provided optimum habitat for montane voles, but their high numbers did not result in feeding damage to tree fruit production, thereby supporting H3. Creation of non-crop habitats within as well as adjacent to agricultural settings should become a major endeavour to restore native habitats, enhance small mammal populations, and maintain biodiversity.

Prairie Voles as a Model for Understanding the Genetic and Epigenetic Regulation of Attachment Behaviors.

Over a lifetime, humans build relationships with family, friends, and partners that are critically important for our mental and physical health. Unlike commonly used laboratory mice and rats, Microtine rodents provide a unique model to study the neurobiology underlying pair bonding and the selective attachments that form between adults. Comparisons between monogamous prairie voles and the closely related but nonmonogamous meadow and montane voles have revealed that brain-region-specific neuropeptide receptor patterning modulates social behavior between and within species. In particular, diversity in vasopressin 1a receptor (V1aR) distribution has been linked to individual and species differences in monogamy-related behaviors such as partner preference, mate guarding, and space use. Given the importance of differential receptor expression for regulating social behavior, a critical question has emerged: What are the genetic and epigenetic mechanisms that underlie brain-region-specific receptor patterns? This review will summarize what is known about how the vasopressin (AVP)-V1aR axis regulates social behaviors via signaling in discrete brain regions. From this work, we propose that brain-region-specific regulatory mechanisms facilitate robust evolvability of V1aR expression to generate diverse sociobehavioral traits. Translationally, we provide a perspective on how these studies have contributed to our understanding of human social behaviors and how brain-region-specific regulatory mechanisms might be harnessed for targeted therapies to treat social deficits in psychiatric disorders such as depression, complicated grief, and autism spectrum disorder.

Influence of living mulches on vole populations and feeding damage to apple trees

Voles of the genus Microtus are considered the major mammalian pest species affecting tree fruit crops in North America and Europe. During winter, voles feed on bark, vascular tissues, and sometimes roots of trees. Rodenticides continue to be the major method used to reduce vole populations in orchards, but an alternative approach could be a living mulch along tree rows that would provide weed management, have a low growth habit, and be repellent to voles. We examined four living mulches installed in young apple orchards: hairy vetch (Vicia villosa Roth), birdsfoot trefoil (Lotus corniculatus L.), sweet woodruff (Galium odoratum (L.) Scop.), and creeping thyme (Thymus serpyllum L.). Forage grasses were used as a “control” in two experiments. Efficacy of these mulches was evaluated by their growth and effects on (1) abundance of total orchard herbaceous plants, (2) abundance of montane voles (Microtus montanus Peale), and (3) feeding damage to apple trees in field trials at Summerland, British Columbia, Canada in 2007–2010. A sixth treatment, glyphosate herbicide, was added in Experiment 2. Hairy vetch and birdsfoot trefoil did not suppress herbaceous vegetation along tree rows, but birdsfoot trefoil had fewer voles than either hairy vetch or forage grasses. Sweet woodruff and creeping thyme both reduced abundance of orchard herbs compared with the control forage grasses and this effect was maintained for two of three growing seasons. Forage grasses and sweet woodruff had increasing proportions of mulch dominating the growing sites, whereas creeping thyme did not. Both sweet woodruff and creeping thyme seemed to repel voles with reduced damage to trees in two winters, but not summer 2010, compared with the forage grasses sites. Vole numbers were, on average, 1.4 to 4.9 times higher in the forage grasses than the other two sites. However, the relatively high degree of feeding damage (>45% mortality) to young apple trees (except for 16% mortality for creeping thyme in one winter) in all three seasons was a major limitation. These tree mortality levels are unacceptable economically and suggest strongly that living mulches may not be a practical solution for weed management in organic or conventional orchards. Orchard-scale experiments that emulate real-world farming practices might yield living mulch habitats that are not as desirable for voles as those examined in our study. Herbicide sites had the lowest abundance of herbaceous vegetation, voles, and provided complete protection of apple trees compared to than any of the other treatment sites.

Comparative studies need to rely both on sound natural history data and on excellent statistical analysis.

We certainly agree with Schradin [1] that comparative studies need to use the best information available—as well as clear definitions, up-to-date statistical techniques and common sense. We have divided our response into two sections: the first dealing with Schradin's critique of our 2017 paper and the classification of the social organization of shrews; the second dealing with three general issues that his commentary raises. The paper that Schradin is commenting on [2] asks whether cooperative breeders (which we define as species where non-breeding helpers assist breeders to raise their young) in mammals are more commonly found in arid environments than monogamous species (defined as species where breeding pairs of males and females remain together for more than one season) from which cooperative breeders appear to be derived [3]. It shows that, as in birds, cooperative breeders tend to live in relatively arid habitats compared to monogamous species. As we describe below, these conclusions are unaffected by differences between our classification of the social organization of shrews and the classification suggested by Schradin. Schradin's first criticism is that the categorizations of shrews that we have used in this analysis were based on one paper on a single species. This is incorrect, for our categorization of shrews was based on more than 70 separate sources (see electronic supplementary material, appendix S1). When we published our dataset [4], we were required to reduce the associated reference list by the editors of the journal and, to comply, we removed around 75% of our 2000+ sources, often listing a single reference per species or group of species, and frequently using generic reviews for particular groups where these were available: for example, for many rodent species, we simply listed Wolff and Sherman's Rodent Societies [5], and …

Ectoparasites of Microtus californicus and Possible Emergence of an Exotic Ixodes Species Tick in California.

California voles (Microtus californicus Peale) harbor fleas and ticks, may be infected with vector-borne pathogens, and could themselves suffer from disease and serve as a source of infection for people and other animals. Here we summarize publications, museum archives, and recent records of ticks and fleas from California voles. There have been 18 flea species reported on California voles with geographic locations reported for 13. During recent statewide surveys, we found six flea species, with the highest species richness in Humboldt County. We found three of five previously reported tick species as well as a tick resembling the eastern North American tick Ixodes minor Neumann (which we here designate Ixodes "Mojave morphotype") on isolated Amargosa voles and Owens Valley voles (Microtus californicus vallicola Bailey) in Inyo County in 2012 and 2014. Additional incidental observations of this Mojave morphotype tick were on a western harvest mouse (Reithrodontomys megalotis Baird) at the Mojave site and a montane vole (Microtus montanus Peale) in the Owens Valley, both in March, 2014. We cannot rule out that this tick species has been present in remote areas of California but gone unrecognized, but these data are consistent with recent introduction of this tick, possibly from migrating birds. Changes in the ectoparasite fauna suggest changing ecologies of vectors and vector-borne pathogens that could influence animals and people as well.

Wild cyclic voles maintain high neutral and MHC diversity without strong evidence for parasite-mediated selection

The major histocompatibility complex (MHC) is an important component of vertebrate immune defense involved with self/nonself recognition and disease susceptibility. The high variability of genes of the MHC is thought to arise from both parasite-mediated and sexual selection. An outstanding question involves the degree to which balancing selection can oppose genetic drift to maintain high MHC diversity in the face of population bottlenecks. To address this question we examined genetic diversity and population structure at neutral (microsatellite) and MHC genes in montane voles [Microtus montanus (Peale, 1848)] subject to high amplitude population fluctuations, and compared these to measures of infection by common gastrointestinal parasites. We found high neutral and MHC allelic variability, indicating low impacts of genetic drift despite large fluctuations in population size. Greater MHC diversity did not predict lower parasite richness or infection by the two most common endoparasites (cestodes and coccidian protozoa), as might be expected if genotypic composition confers resistance to infection. One specific MHC allele predicted lower cestode intensity, but we found no other associations between MHC and infection measures. Neutral heterozygosity was positively associated with total parasite richness, possibly owing to greater parasite tolerance among heterozygous relative to more inbred hosts. Overall, these results suggest that factors beyond the parasites examined here, such as high inter-patch migration, mate choice, gene conversion or other infectious agents, are likely maintaining the high levels of MHC diversity observed in wild montane voles.

Long-term responses in population dynamics and diversity of small mammals in riparian and upland habitats within an agricultural landscape

Riparian zones in agricultural landscapes provide linear non-crop habitats for a variety of plant and mammal species, and hence are an important component of biodiversity. To date, variable responses of abundance, species richness, and species diversity of small mammals have been recorded in riparian and upland habitats. To address this variability, we provide a detailed analysis of seasonal changes in abundance and diversity of terrestrial small-mammal communities over a 7-year period within an agricultural landscape in south-central British Columbia, Canada. We tested the hypotheses (H) that abundance, species richness, and species diversity of communities of small mammals (H1), and demographic parameters of reproduction, recruitment, and survival of the major species: deer mouse (Peromyscus maniculatus) and montane vole (Microtus montanus) (H2), would be higher in riparian than upland habitats. Mean total abundance of small mammals was higher in summer and winter, and species richness higher in summer, in riparian than hedgerow habitats. Winter population data supported the total and species abundance patterns for small mammals, but species richness was similar, and diversity lower, in riparian than hedgerow sites during winter periods. Deer mice were the dominant species in terms of abundance and reproductive output for pregnancies and recruitment, but not survival, in riparian sites. Montane voles were similar in abundance and demographic parameters in the two habitats. House mice (Mus musculus) preferred hedgerows and wandering shrews (Sorex vagrans) riparian sites. Demographic parameters for deer mice and montane voles indicated that both riparian and hedgerow sites were “source” rather than “sink” habitats, and likely contribute to maintenance of mammal diversity in agricultural landscapes.

Asymmetrical Competition between Microtus montanus and Microtus longicaudus in the Greater Yellowstone Ecosystem

Stable coexistence of ecologically similar species is often a complicated phenomenon involving a synergy of biotic and abiotic factors. We investigated competitive interactions and small scale division of space in the closely related vole species Microtus montanus and Microtus longicaudus, which co- occur across the Greater Yellowstone Ecosystem (GYE) in the Rocky Mountains. We directly manipulated in situ populations of these species in two habitat types over three consecutive years. We confirmed previously described habitat associations of the study species: M. montanus was most abundant in open grassy meadows; M. longicaudus was most abundant in ecotonal areas with greater woody vegetation cover. Additionally, removal experiments conducted in both habitat types revealed a habitat-dependent competitive interaction between species: M. montanus excluded M. longicaudus from resource-rich meadows; M. longicaudus was limited to patchy ecotonal habitats that M. montanus was not observed to colonize. We did not observe an effect of the experimental treatment on either species' body mass. The outcome of the asymmetric competition between these species is influenced by local environmental conditions and enables their coexistence in the GYE.

The effects of vitamin K1-rich plant foods on the efficacy of the anticoagulant rodenticides chlorophacinone and diphacinone, used against Montane Voles (Microtus montanus)

Voles can cause significant losses to agriculture and wood fibre production. California growers typically rely on baits containing chlorophacinone and diphacinone to reduce vole population densities, but the efficacy of those rodenticides has been decreasing. One hypothesis suggests that voles are consuming high levels of an antidote (vitamin K1) to the anticoagulants, contained within green leafy plants. We tested that hypothesis by first feeding Montane Voles (Microtus montanus) diets that were high in vitamin K1, and then providing the animals with either: (1) chlorophacinone-containing bait, (2) diphacinone-containing bait, or (3) a control diet. We found that the chlorophacinone-containing bait remained efficacious (100% mortality), whereas the diphacinone-containing bait had a much lower efficacy (60% mortality). When only the diphacinone-containing bait was presented, the efficacy was somewhat better (80%). We infer that a diet rich in vitamin K1 did not negate the effects of the chlorophacinone for voles, and so we recommend its continued use in California unless anticoagulant resistance is known to have developed in the vole population. We hypothesise that: (1) diphacinone has a relatively low efficacy against Montane Voles when compared to chlorophacinone, and (2) this lower efficacy could be further reduced by a vitamin K1-rich diet.

Duplication and population dynamics shape historic patterns of selection and genetic variation at the major histocompatibility complex in rodents.

Genetic variation at the major histocompatibility complex (MHC) is vitally important for wildlife populations to respond to pathogen threats. As natural populations can fluctuate greatly in size, a key issue concerns how population cycles and bottlenecks that could reduce genetic diversity will influence MHC genes. Using 454 sequencing, we characterized genetic diversity at the DRB Class II locus in montane voles (Microtus montanus), a North American rodent that regularly undergoes high-amplitude fluctuations in population size. We tested for evidence of historic balancing selection, recombination, and gene duplication to identify mechanisms maintaining allelic diversity. Counter to our expectations, we found strong evidence of purifying selection acting on the DRB locus in montane voles. We speculate that the interplay between population fluctuations and gene duplication might be responsible for the weak evidence of historic balancing selection and strong evidence of purifying selection detected. To further explore this idea, we conducted a phylogenetically controlled comparative analysis across 16 rodent species with varying demographic histories and MHC duplication events (based on the maximum number of alleles detected per individual). On the basis of phylogenetic generalized linear model-averaging, we found evidence that the estimated number of duplicated loci was positively related to allelic diversity and, surprisingly, to the strength of purifying selection at the DRB locus. Our analyses also revealed that species that had undergone population bottlenecks had lower allelic richness than stable species. This study highlights the need to consider demographic history and genetic structure alongside patterns of natural selection to understand resulting patterns of genetic variation at the MHC.

Parasite infection and host dynamics in a naturally fluctuating rodent population

Parasites can both influence and be affected by host population dynamics, and a growing number of case studies support a role for parasites in causing or amplifying host population cycles. In this study, we examined individual and population predictors of gastrointestinal parasitism on wild cyclic montane voles ( Microtus montanus (Peale, 1848)) to determine if evidence was consistent with theory implicating parasites in population cycles. We sampled three sites in central Colorado for the duration of a multiannual cycle and recorded the prevalence and intensity of directly transmitted Eimeria Schneider, 1875 and indirectly transmitted cestodes from a total of 267 voles. We found significant associations between host infection status, individual traits (sex, age, and reproductive status) and population variables (site, trapping period, and population density), including a positive association between host density and cestode prevalence, and a negative association between host density and Eimeria prevalence. Both cestode and Eimeria intensity correlated positively with host age, reproductive status, and population density, but neither parasite was associated with poorer host condition. Our findings suggest that parasites are common in this natural host, but determining their potential to influence montane vole cycles requires future experimental studies and long-term monitoring to determine the fitness consequences of infection and the impact of parasite removal on host dynamics.

Seasonal Variation of Coyote Diet in Northwestern Wyoming: Implications for Dietary Overlap with Canada Lynx?

Exploitative competition through resource utilization may occur between coyotes (Canis latrans) and other carnivores. In the southern periphery of Canada lynx (Lynx canadensis) range, there is concern that increased snowmobile activity may enable coyotes to increase their movements into deep snow areas during the winter months, thereby potentially creating heightened resource competition with lynx, mainly for the lynx’s main prey, snowshoe hares (Lepus americanus). We studied the seasonal variation of coyote diets and the dietary overlap between coyotes and lynx in a 512-km2 high-elevation study area in northwestern Wyoming. Dietary shifts by coyotes were documented during the winter, spring, summer, and fall from August 2006 through June 2008. Although lynx are known to primarily prey on snowshoe hares, lynx scats were also collected to assess their diet for comparative purposes. A total of 470 coyote scats and 24 lynx scats were collected, dried and analyzed. Mule deer (Odocoileus hemionus) was the predominant prey item for coyotes by percent occurrence (20.1%) for all 3 years combined, followed by elk (Cervus elaphus, 12.5%), montane vole (Microtus montanus, 12.0%), and snowshoe hare (8.0%). Snowshoe hares were the dominant prey item for lynx during the winter, accounting for 85.2% of all prey occurrences. Coyote use of snowshoe hares peaked in the fall (24.1% of all occurrences). We found little dietary overlap between coyotes and lynx during the winter months when lynx mainly fed on snowshoe hares and coyotes fed mostly on ungulates.

Small Mammals in Saltcedar (Tamarix ramosissima)–Invaded and Native Riparian Habitats of the Western Great Basin

AbstractInvasive saltcedar species have replaced native riparian trees on numerous river systems throughout the western United States, raising concerns about how this habitat conversion may affect wildlife. For periods ranging from 2 to 11 yr, I used live-trapping to monitor small mammal populations in paired saltcedar and native riparian woodlands at four sites in western Nevada and eastern California. Heteromyid rodents, such as Merriam's and Ord's kangaroo rats, were more likely to occur in saltcedar habitats, but other rodent species, particularly the montane vole and western harvest mouse, occurred more often in native habitats, and this balanced species richness in habitat comparisons. The most common species at all sites, the deer mouse, did not show any consistent differences in abundance or in mean body mass between the two habitat types. However, the ratio of captured male to female deer mice was higher in saltcedar than native habitats at two sites. Deer mice as well as Ord's kangaroo rats also had higher rates of being recaptured following initial capture in native habitats, which may have been due to fewer transient individuals occurring in these habitats. By contrast, Merriam's kangaroo rats may have been more transient in native habitats because they were more likely to be recaptured in saltcedar. Individuals of two species, pinyon mouse and white-tailed antelope ground squirrel, had greater mean body mass in native habitats than they did in saltcedar, implying that they may have maintained superior condition in native habitats.