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The relative impacts of intrinsic factors (e.g., density dependence) and ex- trinsic factors (e.g., climate) on winter demography may be critical for the generation of different population dynamic patterns (including cyclicity) in northern vole and lemming populations. However, little is known about winter demography because studies with tem- poral and spatial replication at the population level and an adequate sample of individuals with known fates within each population are rare. In this study, we monitored the winter demography of 48 local tundra vole populations introduced to experimentally enclosed plots the preceding spring for four years in Norway. The rate of population change over the winter (November-May) was density dependent due to recruitment. However, the large variation in the rate of change between the different winters was due to a density-independent, and most likely a climatically driven, variation in survival rate. In particular, mild weather that led to the formation of ice on the ground seemed to be detrimental for winter survival. We predict that if increased frequency of such events arose, due to climate change, normal cyclic dynamics of northern small rodent populations would be disrupted. We found support for the hypothesis that voles adjusted their body mass toward a certain mean during the winter so as to maximize winter survival. The survival rate of males was lower than that of females, possibly due to their larger body mass, and this resulted in female-biased population sex ratios in the spring. This result suggests a link between sexual selection (responsible for the sexual size dimorphism) and natural selection (operating though size-dependent winter survival) with implications for the demographic structure of the population.

We studied dispersal movements in 12 enclosed, patchy populations of root voles (Microtus oeconomus) during the breeding season. The habitat was manipulated experimentally so that there were four replicates of three types of habitat patch configuration: two large patches, six small isolated patches, and six small patches of which two patch triplets were connected by corridors. The total habitat area (1350 m2) and the distance to nearest neighboring patch (15 m) were the same in the three different configurations. The matrix area between the patches was kept open and uninhabitable for voles by weekly mowing. Dispersal was defined as shifts between patches. Movements across the open matrix habitat to traps located along the edge of the enclosures were also recorded. The frequency of shifts between patches was higher among small than among large patches. Corridors channeled dispersal between corridor-connected patches but did not enhance the frequency of shifts between patches at the population level. Disper...

It has been proposed that habitat corridors enhance exchange of individuals (transfer) and interbreeding between otherwise isolated demes in fragmented populations. However, due to the paucity of experimental studies this proposition has become contro- versial. We tested these proposed effects of corridors in 12 experimentally fragmented root vole populations. A 50-m long habitat corridor connected two large habitat patches in each of six treatment populations, while equivalent habitat patches in six control populations had no corridor connection. Each of the two patches in each population was initially colonized by founder demes that were monomorphic with respect to two different marker alleles of a single locus. The rate of transfer of individuals and the resultant degree of interbreeding (heterozygosity) between the two demes were monitored by capture-recapture methodology and genetic analyses over the reproductive season. Females were predominantly philopatric in these populations; more than four-fifths of the founder deme females settled and reproduced in their original patch. However, corridors significantly facilitated the transfer of females. The transfer rate of males was generally very high; approximately four-fifths, independent of the experimental factor (absence or presence of corridors). The sex-specific transfer pattern gave rise to very high rates of interbreeding (heterozygosity). The rate of interbreeding was enhanced by the presence of corridors, and more so than expected from the transfer rate. The lack of concordance between expected and observed heterozygosity in the corridor populations was probably due to short-term mating excursions facilitated by the presence of corridors.

Current methods for evaluating habitat selection from animal space-use ob- servations ignore possible interactions between time allocation patterns relative to different resources, their relative abundance, and their spatial arrangements. Habitat selection may occur in situations in which animals experience a trade-off, e.g., between time used foraging in areas with abundant forage but poor protective cover, and time used for resting in areas with good protective cover but low forage abundance. We show how functional responses in habitat use (i.e., change in preference with availability of one of two main habitat types) may be tested. Given radio-telemetry data for a sample of individuals, binomial logit models can be used to regress proportionate use of a habitat type P(u) against the proportion of that habitat available, P(a). Given an appropriate fit to the data by a linear predictor on a logit scale, functional response will be indicated by a estimated slope parameter ? 1, while a slope = 0 will indicate a consistent use as availability changes. Habitat preference is inferred from the logit regression parameters when the fitted value of the proportion of use at a specified proportion of availability, is significantly greater than the proportional availability.

We radiotracked 184 reproductive active root vole Microtus oeconomus in- dividuals in 12 experimentally fragmented populations to test predictions regarding re- sponses in spatiosocial organization to habitat fragmentation and connectivity. We used two genetically distinct strains with different intrinsic characteristics to test explicitly for interactions between spacing behavior and the degree of habitat fragmentation and con- nectivity. The experimental habitat configurations were created by mowing enclosed mead- ow plots so that habitat (meadow) fragments were imbedded in a hostile, barren matrix of nonhabitat. We used three types of habitat patch configurations to contrast two levels of fragmentation (large vs. small habitat fragments) and two levels of connectivity (small isolated vs. small corridor connected habitat fragments), while keeping the total area of habitat and distances between fragments constant. The least fragmented systems contained two habitat fragments large enough to encompass several female home ranges (large frag- ment plots). Both of the two other habitat configurations contained six small habitat frag- ments, each of the size corresponding to the home range of a single female. The two small fragment configurations differed with respect to connectivity in that one configuration had 0.5 m wide corridors connecting two triplets of patches (corridor plots), while the other small patch configuration had no corridors (small fragment plots). Four population replicates (two of each root vole strain) of each configuration were employed over two years. Home range area estimates showed rather complex responses to habitat fragmentation depending on strain and sex. This was expected as the two strains and males and females had been previously shown to exhibit different space requirements. Core areas were, how- ever, unaffected by fragmentation pattern at the spatial scale employed. The most pro- nounced and consistent responses to habitat fragmentation concerned movement rates for both sexes and the degree of space sharing between matrilineally related and unrelated females. The frequency of home range overlaps (irrespective of matrilineal relatedness and strain) was highest in the large-fragment plots and lowest in the small-fragment plots. The degree of overlap between matrilineally related females showed a reverse trend, however, indicating that space sharing was more directed towards kin in the most fragmented systems. The rate of interfragment movements increased with habitat fragmentation. Corridors in- duced more individuals to move between the smallest fragments. Males generally moved more frequently between fragments than did females. Our results match the hypothesis predicting fusion and fission responses based on intrinsic social mechanisms in patchy populations.

I studied growth rate and reproduction in two geographically distinct strains of root voles, Microtus oeconomus, in the laboratory to unravel the mechanisms underlying the observed increase in body size from south to north among European microtine rodents. In the field, adult northern root voles are 51% heavier than southern. Many proximate and ultimate mechanisms underlying this geographic pattern have been suggested, but none has been adequately tested. I measured offspring growth rates in one northern and one southern strain over three consecutive laboratory generations, employing two separate experimental protocols (cross-mating and cross-fostering) to control for differential maternal investment. Paternal investment in the young was evaluated in separate tests. Northern root voles were larger at birth and grew faster than southern voles. Litter size and litter mass at birth were largest in the southern strain, when maternal body mass was adjusted for, indicating a larger prenatal reproductive expenditure in southern females. Cross-fostering and cross-breeding experiments demonstrated that the north-south difference in body mass is of genetic origin. Postnatal growth rate of cross-bred litters was intermediate between the two purebred strains, thus being consistent with an additive genetic mechanism. In contrast, neonatal mass of offspring from cross-bred pairs was similar to that of the southern strain, consistent with southern genetic dominance. Cross-fostering between the two strains showed that parental investment had no discernible effects on postnatal growth rate, although southern fathers showed more parental care than did northern fathers. Together, the cross-fostering and the cross-breeding experiments suggest that the growth rate of southern root voles was con- strained by their own genotype and not by the characteristics of their mothers. Low en- vironmental predictability in southern habitats and high productivity of northern habitats probably result in different selection pressures on growth rates and reproductive effort.

When reproduction competes with the amount of resources available for survival during an unpredictable nonbreeding season, individuals should adopt a risk-sensitive regulation of their reproductive allocation. We tested this hypothesis on female reindeer (Rangifer tarandus), which face a trade-off between reproduction and acquisition of body reserves during spring and summer, with autumn body mass functioning as insurance against stochastic winter climatic severity. The study was conducted in a population consisting of two herds: one that received supplementary winter feeding for four years while the other utilized natural pastures. The females receiving additional forage allocated more to their calves. Experimental translocation of females between the herds was conducted to simulate two contrasting rapid alterations of winter conditions. When females receiving supplementary feeding were moved to natural pastures, they promptly reduced their reproductive allocation the following summer. However, when winter conditions were improved, females were reluctant to increase their reproductive allocation. This asymmetric response to improved vs. reduced winter conditions is consistent with a risk-averse adjustment in reproductive allocation. The ability of individuals to track their environment and the concordant risk-sensitive adjustment of reproductive allocation may render subarctic reindeer more resilient to climate change than previously supposed.