Microtus Montanus Research Articles

PLANT–HERBIVORE–HYDROPERIOD INTERACTIONS: EFFECTS OF NATIVE MAMMALS ON FLOODPLAIN TREE RECRUITMENT

Floodplain plant–herbivore–hydroperiod interactions have received little attention despite their potential as determinants of floodplain structure and functioning. We used five types of exclosures to differentially exclude small-, medium-, and large-sized mammals from accessing Fremont cottonwood (Populus deltoides Marshall subsp. wizlizenii (Watson) Eckenwalder) seedlings and saplings growing naturally on four landform types at an alluvial reach on each of two rivers, the Green and Yampa, in Colorado and Utah. The two study reaches differed primarily as a result of flow regulation on the Green River, which began in 1962. Landforms were a rarely flooded portion of the alluvial plain, geomorphically active slow- and fast-water channel margin sites on the Yampa reach, and an aggrading side channel on the Green. Small-mammal live-trapping and observational data indicated that, with minor exceptions, the kinds of mammals eating cottonwood within each reach were identical. We monitored condition and fates of individual cottonwood plants from October 1993 through the 1997 growing season. Differences in survival and growth were noted both within and between reaches, and both due to, and independent of, mammalian herbivory. Comparisons of cottonwood growth and survivorship among exclosures and between exclosures and controls indicated that a small mammal, Microtus montanus, reduced seedling and sapling survivorship at the Green River reach, but to a lesser extent (seedlings) or not at all (saplings) on the Yampa reach. In contrast, reductions in sapling height increment attributable to medium- and large-sized herbivores were detected only at the Yampa site. We suggest that these differences are a result of (1) flow regulation allowing Microtus populations to escape the mortality normally accompanying the large, snowmelt-driven spring flood, as well as regulation promoting a herbaceous understory favorable to voles, and (2) greater browsing pressure from overwintering deer and elk at the Yampa reach, unrelated to flow regulation. Within areas used by foraging beaver, the probability of a sapling being cut by beaver was similar on the two reaches. This study suggests that changes in riparian plant–herbivore relationships due to shifts in river hydrology may be a common and important consequence of river regulation.

Gut-Associated Lymphoid Tissue in Microtus montanus from Natural Populations

Impaired immunocompetence may contribute to the multiannual fluctuations ("cycles") in population density of arvicoline rodents (Mihok et al. 1985, Pinter 1988, Lochmiller et al. 1993). In a variety of species environmental variables influence at least one component of the immune system, the gut-associated lymphoid system, GALT (Lopez et al. 1985, Li et al. 1995). Preliminary laboratory observations suggest that the development of GALT in Microtus montanus can be influenced by such factors as age and diet (Smith and Pinter 2000). However, these observations are from captive-bred voles, maintained on a synthetic laboratory diet, under strictly controlled laboratory conditions. The complexity and abundance of variables under natural conditions must influence the development and function of GALT- yet, nothing is known about it from Microtus montanus under natural conditions. The purpose of this study was essentially twofold: (1) to describe the morphometries of GALT in wild-caught M. montanus, and (2) to characterize the morphometric changes that occur in GALT of these animals as a function of their age, sex, and the season of the year.

Climatic Factors, Reproductive Success and Population Dynamics in the Montane Vole, Microtus montanus

Multiannual fluctuations in population density ("cycles") of small rodents have been known since antiquity (Elton 1942). Numerous hypotheses have been proposed to explain this phenomenon (for reviews see Finerty 1980, Taitt and Krebs 1985). However, none of these hypotheses, alone or in combination, has been able to explain the causality of cycles, although recently removal of parasites was shown to prevent population cycles in the red grouse, Lagopus lagopus scoticus (Hudson et al. 1998). The objectives of this long-term study are to determine whether environmental variables, possibly acting through reproductive responses, contribute to the multiannual fluctuations of the montane vole, Microtus montanus.

MOVEMENT PATTERNS OF RIPARIAN SMALL MAMMALS DURING PREDICTABLE FLOODPLAIN INUNDATION

Abstract We monitored movements of small mammals resident on floodplains susceptible to spring floods to assess whether and how these animals respond to habitat inundation. The 2 floodplains were associated with 6th order river segments in a semiarid landscape; each was predictably inundated each year as snowmelt progressed in headwater areas of the Rocky Mountains. Data from live trapping, radiotelemetry, and microtopographic surveys indicated that Peromyscus maniculatus, Microtus montanus, and Dipodomys ordii showed different responses to inundation, but all reflected a common tendency to remain in the original home range until “forced” to leave. The reluctance of Dipodomys ordii to abandon the home burrow often resulted in death in situ, whereas individual P. maniculatus and M. montanus moved to nearby higher ground but not necessarily toward upland. This behavior could lead to occupancy of an island that disappeared as floodwaters rose. Peromyscus maniculatus climbed into sapling cottonwood, but the q...

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.

Increased affiliative response to vasopressin in mice expressing the V1a receptor from a monogamous vole.

Arginine vasopressin influences male reproductive and social behaviours in several vertebrate taxa through its actions at the V1a receptor in the brain. The neuroanatomical distribution of vasopressin V1a receptors varies greatly between species with different forms of social organization. Here we show that centrally administered arginine vasopressin increases affiliative behaviour in the highly social, monogamous prairie vole, but not in the relatively asocial, promiscuous montane vole. Molecular analyses indicate that gene duplication and/or changes in promoter structure of the prairie vole receptor gene may contribute to the species differences in vasopressin-receptor expression. We further show that mice that are transgenic for the prairie vole receptor gene have a neuroanatomical pattern of receptor binding that is similar to that of the prairie vole, and exhibit increased affiliative behaviour after injection with arginine vasopressin. These data indicate that the pattern of V1a-receptor gene expression in the brain may be functionally associated with species-typical social behaviours in male vertebrates.

Climatic Factors, Reproductive Success and Population Dynamics in the Montane Vole, Microtus montanus

Multiannual fluctuations in population density ("cycles") of small rodents have been known since antiquity (Elton 1942). Numerous hypotheses have been proposed to explain this phenomenon (for reviews see Finerty 1980, Taitt and Krebs 1985). However, none of these hypotheses, alone or in combination, has been able to explain the causality of cycles, although recently removal of parasites was shown to prevent population cycles in the red grouse, Lagopus lagopus scoticus (Hudson et al. 1998). The objectives of this long-term study are to determine whether environmental variables, possibly acting through reproductive responses, contribute to the multiannual fluctuations of the montane vole, Microtus montanus.

Population dynamics of small mammals in relation to vegetation management in orchard agroecosystems: compensatory responses in abundance and biomass

This study was designed to test the hypothesis that habitat alteration with intensive herbicide (glyphosate) treatment would adversely affect the small mammal community in apple orchards. Multiple applications of herbicide were applied to the total orchard floor in two experimental orchards, and this same herbicide regime was applied in a 2-m-wide strip within tree rows only in a third orchard. Intensive sampling of montane vole ( Microtus montanus), deer mouse ( Peromyscus maniculatus), and northwestern chipmunk ( Eutamias amoenus) populations was conducted in replicate control and treatment blocks of apple orchards at Summerland, British Columbia, Canada from 1983 to 1986. Average abundance of voles declined by 53% to 73% on treatment compared to control blocks after the first herbicide application. Vole populations were consistently reduced in response to the herbicide treatment, with average abundance ranging from 2.8 to 28.0 times higher on control than treatment blocks. Voles declined to, or near, extirpation in all orchards during the winter of 1985–1986. Either deer mouse and northwestern chipmunk populations were significantly higher on treatment than control blocks or there was no difference in abundance after the start of the herbicide applications. The average abundance of deer mice ranged from 1.3 to 11.1 times higher, and that of chipmunks ranged from 1.8 to 13.3 times higher, on treatment than control blocks. The high numbers of deer mice and chipmunks on treatment blocks were composed mainly of resident animals. There were no significant differences in biomass of small mammals between control and treatment populations in summer and winter periods in two out of three orchards. There appears to be a ‘compensatory response’ in this small mammal community whereby deer mice and northwestern chipmunks have essentially ‘replaced’ the montane vole on treatment blocks in an orchard agroecosystem. The quality of the altered habitats seemed sufficiently high to support these populations at comparable or higher levels than on controls. The species diversity of plants and animals in orchard agroecosystems should be investigated to determine their role as part of a mosaic of natural and agrarian habitats.

Climatic Factors Reproductive Success and Population Dynamics in the Montane Vole, Microtus montanus

Multiannual fluctuations in population density ("cycles") of small rodents have been known since antiquity (Elton 1942). Numerous hypotheses have been proposed to explain this phenomenon (for reviews see Finerty 1980, Taitt and Krebs 1985). However, none of these hypotheses, alone or in combination, has been able to explain the causality of cycles, although recently removal of parasites was shown to prevent population cycles in the red grouse, Lagopus lagopus scoticus (Hudson et al. 1998). The objectives of this long-term study are to determine whether environmental variables, possibly acting through reproductive responses, contribute to the multiannual fluctuations of the montane vole, Microtus montanus.

Ontogeny of oxytocin and vasopressin receptor binding in the lateral septum in prairie and montane voles

Adult prairie ( Microtus ochrogaster) and montane voles ( M. montanus) differ in the distribution of oxytocin (OT) and vasopressin (AVP) receptor binding in the brain. The present study examined the ontogenetic pattern of these receptor bindings in the lateral septum in both species to determine whether adult differences in the receptor binding are derived from a common pattern in development. In both species, OT and AVP receptor binding in the lateral septum were detected neonatally, increased during development, and reached the adult level at weaning (third week). The progression of OT and AVP receptor differed, as OT receptor binding increased continually until weaning while AVP receptor binding did not change in the first week, increased rapidly in the second week, and was sustained thereafter. For both receptors, the binding increased more rapidly in montane than in prairie voles, resulting in species differences in receptor binding at weaning and in adulthood. Together, these data indicate that OT and AVP could affect the brain during development in a peptide- and species-specific manner in voles.

Recognition of Kin and Avoidance of Inbreeding in the Montane Vole, Microtus montanus

Young females of Microtus montanas were housed with an adult male for 9 days postweaning and their sexual maturation noted. Males caused sexual maturation (seen from increased uterine weight) in strange females, but not in daughters. When litters were cross-fostered at birth, males caused sexual maturation in biological daughters that were separated from them at birth, but not in foster daughters. This shows that this species has a mechanism for avoiding inbreeding, with recognition of kin based on familiarity.

Social Environment and Steroid Hormones Affect Species and Sex Differences in Immune Function among Voles

Testosterone has bipotential effects on male fitness; that is, it both suppresses immune function and maintains characteristics important for reproductive success. Presumably, these effects of testosterone may be more pronounced among polygynous species because testosterone concentrations are generally higher among polygynous than monogamous males. The present study examined sex and species differences in cell-mediated immunity among four arvicoline rodents. The role of mating system and sex steroids in sex differences in immune function was examined in individually housed polygynous meadow (Microtus pennsylvanicus) and montane (M. montanus) voles and monogamous prairie (M. ochrogaster) and pine (M. pinetorum) voles in Experiment 1. No sex differences in splenocyte proliferation were observed among the four species and circulating testosterone concentrations did not correlate with immune function of individuals within each species. The contribution of social isolation to these results was examined in Experiment 2, in which meadow and prairie voles were housed individually, or with same- or opposite-sex conspecifics in either pairs or groups of four per cage for 28 days. Overall, prairie voles exhibited more robust immune responses than meadow voles when housed in pairs or in same-sex groups. Sex differences in immune function were also apparent; male meadow voles had higher immune responses than female conspecifics when housed in pairs, whereas female prairie voles had higher responses than male conspecifics when housed in same-sex pairs. Circulating sex steroid hormones and corticosterone appear to mediate some, but not all, of the changes in immune function evoked by differential housing conditions. Taken together, these results suggest that social factors have significant effects on immunity and should be considered in studies of sex differences in immunity at both proximate and ultimate levels.

Gene Targeting Approaches to Neuroendocrinology: Oxytocin, Maternal Behavior, and Affiliation

Transgenic technology affords exciting new opportunities in the field of behavioral neuroendocrinology. We have extended our research into the behavioral function of oxytocin in maternal and social behavior using two transgenic approaches: (i) targeted deletion of the oxytocin gene in mice and (ii) augmented oxytocin receptor expression in the brain. Mice genetically deficient in oxytocin can mate, give birth, and display normal maternal behavior; however, milk ejection and certain aspects of social behavior are affected. Comparative studies of oxytocin receptors have led to the observation that species differences in social organization are associated with differences in receptor distribution. Specifically, monogamous prairie voles and nonmonogamous, asocial montane voles exhibit different patterns of OT receptor expression in the brain. Transgenic mice have been created with a reporter gene driven by the prairie vole oxytocin receptor gene promoter. Analysis of the expression pattern suggests that it should be possible to manipulate receptor expression in the vole brain in order to examine the effects of receptor distribution on behavior.

The Hypothalamic–Pituitary–Adrenal Axis of Prairie Voles (Microtus ochrogaster): Evidence for Target Tissue Glucocorticoid Resistance

Basal plasma corticosterone levels in prairie voles (Microtus ochrogaster) are extremely high, in the absence of any apparent negative consequences of glucocorticoid excess. We tested the hypothesis that prairie voles are a novel rodent model of target tissue resistance to glucocorticoids. Prairie voles had a significantly higher adrenal-to-body weight ratio, 5- to 10-fold greater basal plasma corticosterone, and 2- to 3-fold greater basal plasma ACTH concentrations than montane voles (Microtus montanus) and rats. While plasma corticosterone binding globulin (CBG) was 2-fold higher in prairie voles than in rats, both estimated and directly measured plasma free corticosterone were significantly higher in prairie voles than in rats. Plasma corticosterone levels in prairie voles were responsive to both circadian cues and a stressor, but were resistant to suppression by the synthetic glucocorticoid, dexamethasone (DEX). Western blots of brain and liver protein extracts, using a glucocorticoid receptor (GR) antibody, revealed the presence of a ≈97 kDa immunoreactive band, the expected size for GR. Binding assays revealed significantly lower DEX affinity of corticosteroid receptors (CR) in cytosol of prairie vole brain and liver than that in the same tissues in rats. We conclude that prairie voles are a novel rodent model of glucocorticoid resistance, and that decreased affinity of CR for ligand might be partially responsible for this phenomenon.

Species differences in vasopressin receptor binding are evident early in development: Comparative anatomic studies in prairie and montane voles

Monogamous prairie voles (Microtus ochrogaster) and promiscuous montane voles (Microtus montanus) exhibit remarkable differences in the distribution of vasopressin (AVP) receptors in the adult brain. This difference in receptor distribution is associated with species differences in the behaviors, including pair bond formation and paternal care, found selectively in the monogamous vole. To investigate a potential mechanism for this species difference in AVP receptors, the present study examined the ontogeny of receptor binding in the two species to determine whether the adult maps arose from a shared pattern in development. By using 125I-linear-AVP, which is a selective high-affinity ligand for the V1a receptor, we found early appearance and transient expression of AVP receptor binding during postnatal development in both species. However, the ontogenetic patterns of regional AVP receptor binding were species specific. In the diagonal band, the bed nucleus of the stria terminalis, and the central nucleus of the amygdala, prairie voles had higher AVP receptor binding at birth than montane voles, and this difference persisted with little variation into adulthood. In these areas, therefore, species differences in AVP receptor binding appeared to be determined primarily by genetic or prenatal factors. In the lateral septum, both species had low levels of AVP receptor binding at birth. Thereafter, the binding increased rapidly in montane voles, but it remained unchanged in prairie voles. In the cingulate cortex, AVP receptor binding in prairie voles showed a peak in early development with a subsequent decline and reached the adult level at weaning, whereas the binding in montane voles remained unchanged into adulthood. A similar but opposite pattern was found in the frontoparietal cortex, in which AVP receptor binding showed an early peak in montane voles but did not change significantly in prairie voles. These results demonstrate that 1) species differences in regional AVP receptor binding are evident in the early postnatal period and, in several areas, may be determined by genetic or prenatal factors, and 2) AVP may target brain areas differently in infant and adult prairie and montane voles and, thus, could exert differential effects on the organization of the central nervous system in the two species of voles.

Species differences in vasopressin receptor binding are evident early in development: Comparative anatomic studies in prairie and montane voles

Species differences in vasopressin receptor binding are evident early in development: Comparative anatomic studies in prairie and montane voles

Molecular aspects of monogamy.

Comparative studies of monogamous and nonmonogamous voles demonstrate species differences in the regional expression of oxytocin (OT) receptors in the brain. These species differences have not been observed with other neurotransmitter receptors (except vasopressin). Species differences for OT receptor distribution were also observed in other microtine and murine species selected as monogamous or promiscuous. These chemical neuroanatomic differences appear to be functionally relevant, as treatments with selective OT agonists and antagonists influence those behaviors that appear critical to pair bonding in the monogamous prairie vole. To investigate the mechanism controlling tissue-specific expression of OT receptors, we sequenced the OT receptor gene in both prairie voles and montane voles. The findings are inconclusive. Although both species differ markedly from rat and human in their regulatory (but not their coding) sequences, the species show very subtle differences from each other. Ongoing studies are investigating the consequences of these subtle differences between prairie and montane voles. At the same time, several transactivating factors that might influence OT receptor expression need to be explored. NOTE ADDED IN PROOF: The rat oxytocin receptor gene sequence, cited in FIGURES 4 and 5, was based on an error published in ref. 22. The corrected sequence has now been published (Rosen et al. 1996. Proc. Natl. Acad. Sci USA 93: 12501). The correct sequence shows greater homology with the vole oxytocin receptor gene sequences, but the remaining differences support the argument made herein for species differences in regional receptor expression.

Metapopulations of Montane and Long-Tailed Voles (Microtus montanus and M. longicaudus), and Hantavirus and Plague in the Jackson Hole Small Mammal Community

The purpose of the long-term research begun in 1971 on metapopulations of voles is to enumerate patterns in survivorship, reproduction, and morphology across the sections of the metapopulations. The study sites for Microtus montanus were chosen to represent primary and secondary habitat, and proximate and isolated habitat patches. The purpose of the research on hantavirus begun in 1994 is to ascertain the extent of hantavirus among as many species of small mammals as possible, to identify the strain(s), to understand the presence of hantavirus across species and among the metapopulations of voles, and to assess the potential for human contact. The purpose of the research on plague begun in 1995 is to test the long-standing hypothesis that voles are a reservoir in the intervals between plague outbreaks among ground squirrels.

Climatic Factors, Reproductive Success and Population Dynamics in the Montane Vole, Microtus montanus

Multiannual fluctuations ("cycles") in population density of small rodents doubtless result from the interaction of a multitude of factors, as evidenced by the variety of hypotheses proposed to explain the phenomenon (for reviews see Finerty 1980, Taitt and Krebs 1985). However, the inability of these hypotheses - alone or in combination - to explain the causality of cycles rests in no small measure with the fact that long-term studies of the phenomenon are notoriously uncommon. The objectives of this project are to continue a long-term study of the population dynamics of the montane vole, Microtus montanus, in Grand Teton National Park. On the basis of earlier observations (Pinter 1986, 1988) particular emphasis will be placed on how environmental variables, possibly acting through reproductive responses, contribute to the population density cycles of these rodents.

Occurrence of hantavirus within the rodent population of northeastern California and Nevada.

These studies were initiated to determine the prevalence and hosts of hantaviruses within the rodent population of Nevada and northeastern California. A total of 1,867 rodents were collected, sexed, weighed, identified, and tested by enzyme-linked immunosorbent assay for the presence of antibody against hantavirus nucleocapsid. The primary hosts for hantaviruses in this region were found within the family Muridae (Peromyscus maniculatus. Reithrodontomys megalotis. and Microtus montanus). Studies over time of animals within a defined geographic area indicated that animals with hantavirus antibody are not distributed uniformly over the rodent population in a specific area but were found in foci spanning a distance of only several hundred meters. The antibody prevalence in a given geographic area remained relatively constant with repeated sampling of between 0% and 30%. These data support the hypothesis that rodents within the family Muridae are the primary reservoir for hantaviruses, and the primary risk to biologists for exposure to hantavirus is by contact with members of this family.