Microtine Rodents Research Articles

Mortality factors in a cyclic vole population.

The causes of cyclic fluctuations in microtine rodent populations are still a bone of contention. In particular, the actual causes of mortality in the different phases of the 3-4-year vole are an enigma. We present results from studies of radio-collared voles (Microtus agrestis, M. rossiaemeridionalis and Clethrionomys glareolus), which show that small mustelid predation was the major mortality factor of voles in the decline phase, but had less importance in the increase phase of the 3-year population cycle. After the initial decline in the non-breeding season (winter), vole-kill rate from predators increased to a point where mortality substantially exceeded the reproductive capacity of microtine prey. Our results suggest that predators may alone cause a decline in the density of these vertebrate-prey populations.

Microtine Rodent Dynamics in Northern Europe: Parameterized Models for the Predator‐Prey Interaction

We added seasonality to a general predator—prey model to make it a more realistic description of the interaction between microtine rodents (Microtus) and their mustelid predators (Mustela nivalis) in northern Europe. The model parameters were estimated from field data, with the exception of three parameters, for which a range of plausible values was selected. Most parameter combinations generated high—amplitude, chaotic oscillations with a distinct periodic component; the median parameter values predicted dynamics closely resembling the observed 3—5 yr rodent oscillations in northern Fennoscandia. The results were not sensitive to a number of structural changes in the model, suggested by empirical observations: stochastic variation in the prey—carrying capacity, predator refuge at low density, predation on mustelids by large generalist predators in the years when rodents decline, and an alternative assumption about winter breeding in rodents. The model—predicted period and amplitude of oscillations agreed most closely with observations when the model included a low—density refuge for the mustelids. Another predator—prey model with ratio—dependent functional response did not predict dynamics resembling the observed dynamics. The present results provide further support to the hypothesis that the rodent oscillations are maintained by delayed density dependence imposed by specialist predators. Our results also suggest that the dynamics may be chaotic, with a periodic component, rather than a limit cycle with environmental stochasticity.

Recovery of evergreen clonal dwarf shrubVaccinium vitis-idaea after simulated microtine herbivory in a boreal forest

The evergreen dwarf shrub lingonberry (Vaccinium vitis-idaea L.) was subjected to microtine herbivory simulations to investigate its recovery in a boreal forest. Beside the control (A), the study included three levels of herbivory: clipping of half the annual branches from 50% of ramets (B), removal of 50% of ramets (C), and removal of 100% of ramets (D). Density, growth, and fecundity of the ramets were monitored during the study. New ramets emerged rapidly from dormant basal buds at the base of the removed ramets in groups C and D. After three growth seasons, 60–117% and 42–112% of density and biomass, respectively, had been regained in the groups damaged relative to the control, the difference being insignificant between the treated groups apparently because of the small sample size. Survival, fecundity and net growth of ramets were not affected by the treatment. Mean dry weight of annual branches in new ramets was lower in the damaged groups than the control. The recovery of the lingonberry was unexpectedly good. The results suggest that, since natural herbivory seldom is as heavy as in this work, the time between population density peaks of microtine rodents is usually long enough for the lingonberry to recover in boreal ecosystems.

Sources of scent used by prairie voles, Microtus ochrogaster, to convey sexual identity to conspecifics

The presence or absence of specific odor-producing tissues has been used to suggest phylogenetic relationships among microtine rodents, but has not been related to patterns of social organization. We examined the sources of sex-specific scents in prairie voles, Microtus ochrogaster, using a preference task. Prairie voles have eight sources of sex-specific scents. Four scents, namely those from the urine, feces, anogenital area, and mouth were more attractive to opposite- than same-sex conspecifics. Three scents were attractive to one sex but not the other. Scent from the back of females was attractive to males and scents from the chest and head–neck–ears of males were attractive to females. Scent from the male posterolateral region was attractive to both females and males. We then compared these eight sources of scent with the known sources of scent from meadow voles, Microtus pennsylvanicus, a species whose social system is unlike that of prairie voles. Prairie voles have more sources of sex-specific scent than meadow voles. This difference supports the hypothesis that the number of sources of sex-specific scent is greater in a social species (prairie voles) than in an asocial species (meadow voles).

Determinants of parental effort: a behavioural study in the Eurasian kestrel, Falco tinnunculus

We recorded behaviour of kestrels (Falco tinnunculus) in western Finland during the courtship (1988–1992), incubation (1989–1991), early nestling (age of young 1–2 weeks, 1989–1992) and late nestling stages (3–4 weeks, 1989–1991) to examine determinants of their parental effort (PE). In males, PE was estimated as the hunting effort (the proportion of budget time spent in flight-hunting) and in females as the food provisioning rate (number of prey items delivered to the nest per hour). The following predictions derived from the parental investment theory were examined. (1) Parents rearing large clutches and broods should invest more in breeding than do parents rearing small clutches and broods. The hunting effort of parents did not increase with clutch or brood size, but males tending large broods had a higher prey delivery rate than males tending small broods (Figs 1–2). (2) PE of parents should increase in the course of the breeding season. In males, this was true only between the incubation and early nestling phases (Fig. 3). (3) The early pairs should invest more in breeding than late ones. This tended to be true during the early (for males) and late nestling phases (for females) (Fig. 4). (4) There should be a negative correlation between PE of mates within pairs, but no evidence for such adjustment was found (Fig. 5). (5) Females mated with bright-coloured attractive males should show higher PE than females mated with dull-coloured males but our results were inconsistent with this prediction. We conclude that PE decisions of kestrels are mainly based on cost-benefit estimates of residual reproductive value, rather than on current investment indicators, like clutch or brood size. This might be beneficial in environments with highly variable survival prospects of offspring caused by pronounced among-year variation in abundance of the main food (microtine rodents). The results also show that hypotheses explaining variation in PE in the short term are not necessarily valid for long-term PE, e.g. tending clutches or broods, which also reflects the demands of female and young.

Breeding suppression in the bank vole as antipredatory adaptation in a predictable environment

In northern Fennoscandia, microtine rodent populations fluctuate cyclically. The environment of an individual vole can be considered to be predictable when the risks of predation and intra- and interspecific competition change with the cycle, such that both are high during the population highs of voles. The risk of predation is also high during the vole crash. After the crash, the vole population is characterized by low intra- and interspecific competition and low predation pressure. The main predators affecting voles during the crash are the small mustelids, least weasel and stoat. The density of these specialist predators declines drastically during the winter after the vole crash. We studied experimentally the impact of the perceived presence of stoats on the breeding and mating behaviour of voles. In a series of breeding experiments with bank voles,Clethrionomys glareolus, both old and young females suppressed breeding when exposed to the odour of stoats,Mustela erminea. The weights of females decreased in both experimental and control groups, but more among the voles under odour exposition. It seems that females actively avoided copulations under high predation risk and that breeding suppression is mediated by a change in female mating behaviour. There was no change in male behaviour or physical condition between the experimental and control treatments. An alternative mechanism for the observed breeding suppression could be the one caused by decreased feeding in females mediated with low energy intake which does not allow breeding. Regardless of its mechanism, delay of breeding should increase the probability of non-breeding females to survive to the next breeding season. The females surviving the crash should gain a strong selective advantage in a predator-free environment of the subsequent breeding season, which could explain the adaptive function of this antipredatory strategy.

Patterns of brain vasopressin receptor distribution associated with social organization in microtine rodents

Central vasopressin pathways have been implicated in the mediation of paternal behavior, selective aggression, and affiliation in monogamous prairie voles. Here we demonstrate markedly different patterns of brain vasopressin receptor binding in the monogamous prairie vole and the congeneric nonmonogamous (promiscuous) montane vole. Vasopressin binding was assessed with both 3H-vasopressin and 125I-sarc-AVP using receptor autoradiography. The specificity of binding was consistent with a V1a receptor, the saturation kinetics were similar in the two species, and neither species showed evidence of sexual dimorphisms. In the prairie vole, highest specific binding was observed in the accessory olfactory bulb, diagonal band, laterodorsal thalamus, and superior colliculus. In the montane vole, specific binding was observed in the accessory olfactory bulb and superior colliculus as well, but in several other regions with high levels of binding in the prairie vole, binding was low or undetectable in the montane vole. In this nonmonogamous species, specific binding was high in lateral septum. Functional studies demonstrated the induction of phosphoinositol by AVP in the septum of the montane vole but not in the prairie vole. The pattern of 125I-sarc-AVP binding to lateral septum may reflect the social organization of these two species, as similar differences in AVP receptor distribution in the lateral septum were also observed in two related species, pine voles and meadow voles, which are monogamous and nonmonogamous, respectively. These results, along with earlier studies of AVP's effects on pair bonding, suggest the importance of this neuropeptide for the mediation of behaviors related to social organization.

Dietary variation in arctic foxes (Alopex lagopus)-an analysis of stable carbon isotopes.

We used stable carbon isotopes to analyse individual variation in arctic fox diet. We extracted collagen from bones (the lower jaw), and measured stable carbon isotopes. The foxes came from three different localities: Iceland, where both microtines and reindeer are rare; west Greenland, where microtines are absent; and Sweden, where scat analyses showed the primary food to be microtine rodents and reindeer. The Icelandic samples included foxes from both coastal and inland habitats, the Swedish sample came from an inland area, and the Greenland sample from coastal sites. The spatial variation in the isotopic pattern followed a basic division between marine and terrestrial sources of protein. Arctic foxes from inland sites had δ13C values of -21.4 (Iceland) and -20.4‰ (Sweden), showing typical terrestrial values. Coastal foxes from Greenland had typical marine values of -14.9‰, whereas coastal foxes from Iceland had intermediate values of -17.7‰. However, there was individual variation within each sample, probably caused by habitat heterogeneity and territoriality among foxes. The variation on a larger scale was related to the availability of different food items. These results were in accordance with other dietary analyses based on scat analyses. This is the first time that stable isotopes have been used to reveal individual dietary patterns. Our study also indicated that isotopic values can be used on a global scale.

The Effect of Relatedness on Spacing Behavior and Fitness of Female Prairie Voles

Numerous behavioral hypotheses have been suggested to explain population cycles in microtine rodents. Charnov and Finerty (1980) proposed that preferential behavior toward relatives drives population cycles through kin selection. Their hypothesis makes three assumptions: (1) individuals behaviorally discriminate kin from non—kin, (2) kin associate, and (3) kin benefit from this association. These assumptions were tested in female prairie voles (Microtus ochrogaster) in eastern Kansas. An equal number of laboratory—born females and males were released into 12 fenced enclosures; 6 enclosures contained groups of littermate sisters and 6 contained groups of unrelated females. Space use, reproductive success, and survival were monitored during two replications of the experiment (Fall 1989 and Spring 1990). Female survival and reproductive success were not affected by relatedness, although more young were born in the Spring than in the Fall. Home range size and overlap were greater for females in related groups than those in unrelated groups. Home range size also was larger in the Fall than in the Spring. Prairie vole sisters associated spatially, but our measurements showed no fitness benefits conferred by these associations. Although the kin—selection hypothesis could not be demonstrated in this experimental population, relatedness in local neighborhoods may have consequences for small mammal population ecology.

Temporal Variability and Geographical Patterns in the Population Density of Microtine Rodents: A Reply to Xia and Boonstra

Temporal Variability and Geographical Patterns in the Population Density of Microtine Rodents: A Reply to Xia and Boonstra

Reproductive investment under fluctuating predation risk: Microtine rodents and small mustelids

We studied the reproductive investment of microtine rodents (bank vole (Clethrionomys glareolus),Microtus epiroticus andMicrotus agrestis) in western Finland under predation risk from small mustelids. During 1984–1992, the yearly mean litter size of overwintered bank voles was smaller at high least weasel and stoat densities than at low densities (close to 3 versus 4–5). In addition, the annual mean litter size of young bank voles was negatively correlated to the least weasel density. In youngM. agrestis voles, the yearly late summer litter size was negatively associated with the autumn density of small mustelids. In the crash phase of the vole cycle (1989 and 1992), we removed small mustelids (mainly least weasels) from four unfenced areas in late April to late May and studied the reproduction of voles in four removal and comparable control areas (each 2–4 km2). Reduction of small mustelids significantly increased the proportion of pregnant bank vole females, but not that of pregnantMicrotus vole females. We conclude that predation risk apparently reduced reproductive investment of free-living bank vole females; these voles appear to trade their current parental investment against future survival and reproductive prospects. Accordingly, the presence of small mustelids (or their scent) may slow down the reproductive rate of voles. As antipredatory behaviours occurred on a large scale, our results add evidence to the hypothesis that crashes in multiannual vole cycles are driven by small mustelid predators.

Synchronous Dynamics of Microtine Rodent Populations on Islands in Lake Inari in Northern Fennoscandia: Evidence for Regulation by Mustelid Predators

We surveyed the densities of small mammals on 17 islands in Lake Inari in Finnish Lapland during 5 yr to estimate the degree of spatial synchrony in the dynamics of microtine rodents (Clethrionomys, Microtus) and shrews (Sorex). Microtine rodents have a 4-5 yr population cycle in Lapland. Most island populations of microtine rodents were in complete synchrony with the mainland populations and each other, and the same-island rodent and shrew populations showed largely synchronous population changes. The few exceptions included small and isolated populations of Microtus oeconomus fluctuating out of synchrony

Differences in recovery between a deciduous and an evergreen ericaceous clonal dwarf shrub after simulated aboveground herbivory and belowground damage

Insect and microtine rodent herbivory (defoliation and branch clipping, respectively) were simulated in stands of the deciduous Vaccinium myrtillus L. and evergreen V. vitis-idaea L. Rhizome connections around the treated plots were either left intact or severed. Recovery in the plots was monitored for two growth seasons, from 1991 to 1992. It was hypothesized that the evergreen V. vitis-idaea would suffer more from the simulated herbivory treatment than V. myrtillus, but that severing the rhizome connections would decrease recovery of both species, because clonal integration is disrupted and wounding may have a harmful influence on growth. After two growth seasons, recovery was complete from simulated herbivory in V. myrtillus, whereas the new dry biomass was reduced slightly (22 – 37%) or significantly (49%) compared with that of the control in V. vitis-idaea. Rhizome severing reduced the growth of the branches and ramets and the aboveground dry biomass by 34% compared with that of the unsevered control in V. myrtillus. In the evergreen V. vitis-idaea, severing slightly reduced growth, the effect being significant only in the clipped treatment. The results suggest that the deciduous V. myrtillus recovers more rapidly from simulated herbivory than the evergreen V. vitis-idaea. Rhizome severing is more detrimental to the growth of V. myrtillus, but less detrimental to recovery from simulated herbivory. Sexual reproduction decreases more in V. vitis-idaea than in V. myrtillus after damage. Key words: compensation, growth form, herbivory, rhizome severing, Vaccinium

Densities of microtime rodents along a pollution gradient from a copper-nickel smelter.

Population densities of microtine rodents were studied along an air pollution gradient in the Kola Peninsula, Russia, by long-term and short-term trapping. The study area is affected by high sulphur dioxide and heavy metal emissions from the Severonikel copper-nickel smelter in Monchegorsk. The density of Clethrionomys rufocanus, the most abundant vole species in the area, was lowest close to the smelter and increased with distance up to the farthest, less polluted trapping sites. Clethrionomys glareolus, C. rutilus and Lemmus lemmus were absent from the most severely damaged area and were also scarce at the moderately polluted area 28 km south of the smelter. Although the population of C. glareolus has previously been cyclic at the moderately polluted area, we were unable to demonstrate any regular cycle. The most likely explanation for the low number of microtine rodents in the damaged and moderately polluted areas in a decrease in the quantity of important food plants: epiphytic lichens for C. glareolus and possibly C. rutilus, mosses for L. lemmus and seed plants, especially Vaccinium myrtillus, for C. rufocanus. Close to the smelter, direct toxic effects of heavy metals may also reduce population densities. The results show that pollutants may change the relative proportions of microtine species.

Microtine ultradian rhythm of activity: an evaluation of different hypotheses on the triggering mechanism

AbstractUltradian activity rhythms are a peculiar behavioural pattern, in which the 24‐h day is divided into several short‐term activity cycles. Such patterns are particularly prominent in microtine rodents. In this paper we review experimental findings and proposed hypotheses on the proximate triggering mechanism of ultradian rhythmicity. Eight different mechanisms (classified as homeostatic, pacemaker and stochastic hypotheses) were analysed within the same theoretical model‐framework, considering feeding, food processing, energy turnover and activity behaviour.Ultradian activity patterns could consistently be generated by each of the analysed models. None of the models, however, resulted in frequency dependencies in respect to body weight, metabolic rate, food quality and food deprivation, unless additional assumptions were introduced.From our model evaluations we concluded that the stochastic model does not represent an independent hypothesis. Hence, only homeostatic and pacemaker hypotheses remained as alternative approaches to the triggering mechanism. Although the pacemaker hypothesis appeared more reasonable, a combination with some feedback mechanism to metabolism seemed even more likely. Some key experiments are suggested, which could give insight into the complex mechanism.

Natural space-use patterns and hippocampal size in kangaroo rats.

The size of the hippocampus, a forebrain structure that processes spatial information, correlates with the need to relocate food caches by passerine birds and with sex-specific patterns of space use in microtine rodents. The influences on hippocampal anatomy of sexual selection within species, and natural selection between species, have not yet been studied in concert, however. Here we report that natural space-use patterns predict hippocampal size within and between two species of kangaroo rats (Dipodomys). Differences in foraging behavior suggest that Merriam's kangaroo rats (D. merriami) require better spatial abilities than bannertail kangaroo rats (D. spectabilis). Sex-specific differences in mating strategy suggest that males of both species require more spatial ability than females. As predicted, hippocampal size (relative to brain size) is larger in Merriam's than in bannertail kangaroo rats, and males have larger hippocampi than females in both species. Males of a third species (D. ordii) also have smaller hippocampi than Merriam's kangaroo rat males, despite being similar to Merriam's in brain and body size. These results suggest that both natural and sexual selection affect the relative size and perhaps function of mammalian hippocampi. They also reassert that measures of functional subunits of the brain reveal more about brain evolution than measures of total brain size.

Grazing by Food-Limited Microtine Rodents on a Productive Experimental Plant Community: Does the "Green Desert" Exist?

In many productive terrestrial biomes there seems to be an abundance of forage which is not exploited by herbivores. This observation has generated several hypotheses. One set of hypotheses suggests that the herbivores are either regulated by predators, social interactions or pathogens, and that the abundant plant biomass is an effect of relatively low herbivore densities and a low grazing pressure. The other set claims that the quality of the bulk of the vegetation is persistently too low to give herbivores positive energy or nutrient balance. The herbivores should be critically dependent on high-quality plants or plant organs, but the bulk of the vegetation will remain untouched. The herbivores should thus live in a «green desert». The first set assumes that all plants are edible and of sufficiently high quality, while the second set assumes that plants are generally toxic or of low quality (...)

Diel Pattern of Predation Risk in Microtine Rodents

By reviewing information about the activity behaviour of potential predators in northern and central Europe (75 species of mammals, raptors, owls, other birds and snakes) it was examined whether predation risk in microtine rodents varies in the course of the day. Relative predation risk was estimated for four different periods of the 24-h day (dawn, day, dusk and night) by means of an index I pr . This index was calculated on the base of the activity pattern, the degree of prey specialization and the relative abundance of all resident predators in a given area. Day-time appeared as slightly more dangerous than twilight or night when only the activity patterns of potential predators were taken into account. A differentiated analysis including the predators' specialization on microtine prey, however, indicated that higher risk during day-time was mainly due to predators which take microtines only by chance

Natural history of small mammals of subtropical montane areas in central Taiwan

Based on samples collected in 1989–90, I report on several aspects of the natural history of species of small mammals from seasonal montane areas in subtropical central Taiwan, including their reproduction, demography, sex ratio and body size. The climatic seasonality in the areas is striking. The wet season lasts from May through September and the dry season from October through April. The phenology of plant life is related to this seasonality: vegetative growth occurs primarily in the wet season, but fruit and seed production is concentrated in the dry season. Variation in the reproduction and demography are analysed between the wet and the dry season for six species, Apodemus semotus, Niviventer culturatus. Eothenomys melanogaster, Microtus Kikuchii, Anourosorex syuamipes and Soriculus fumidus. Two murid rodents. A semotus and N. culturatus. are capable of breeding year‐round, whereas the microtine rodents, E. melanogaster and M. Kikuchii, have discrete breeding seasons. The shrews, A. squamipes and S. fumidus are both seasonal breeders. but they differ in the timing of their breeding cycles. While S. fumidus breeds primarily in the late dry season, A. syuamipes reproduces in the wet season. Finally, seasonal variation in reproduction is discussed in relation to phylogenetic and physio‐ecological characteristics of these six species.

Qualitative analysis of a predator-prey system with limit cycles

Fairly regular multiannual microtine rodent cycles are observed in boreal Fennoscandia. In the southern parts of Fennoscandia these multiannual cycles are not observed. It has been proposed that these cycles may be stabilized by generalist predation in the south.