Small Mustelids Research Articles

Does risk of predation by mammalian predators affect the spacing behaviour of rodents? Two large-scale experiments.

Predator-prey interactions between small mammals and their avian and mammalian predators have attracted much attention. However, large-scale field experiments examining small-mammal antipredatory responses under the risk of predation by mammals are rare. As recently pointed out, the scale of experiments may cause misleading results in studies of decision-making under predation risk. We studied the effect of small mustelid predators on the spacing behaviour of the gray-tailed vole (Microtus canicaudus) and the bank vole (Clethrionomys glareolus) in two separate field enclosure experiments. The experiments were conducted during the breeding season in North America and northern Europe, where small mustelids have been suggested to be important mammalian predators of voles. As in most of the earlier laboratory studies, predation risk was simulated using fresh mustelid faeces and urine. This made it possible to compare the results from experiments at different spatial scales. We did not find any effect of increased predation risk on spacing behaviour (mean and/or maximum distance moved and home range size) or trappability in either vole species. Simulated predation risk did not affect the breeding of females in gray-tailed voles, as has previously been shown in bank voles. The results disagree with most of the studies conducted in laboratory conditions with small mammals. We discuss whether this discrepancy could be caused by differences in the scale of the experiments.

Population Fluctuations of Voles in North Fennoscandian Tundra: Contrasting Dynamics in Adjacent Areas with Different Habitat Composition

During 1991-1996. we studied population fluctuations of microtine rodents (primarily Clethrionomys rufocanus). of their winter food plants, and of their predators in a low arctic habitat complex, dominated by unproductive lichen dwarf-birch tundra. More productive habitats occurred patchwise throughout the landscape. On a south-facing slope, productive scrubland habitats prevailed, and luxuriant habitats were locally abundant. Our main method was live-trapping on 14 grids, representing typical lowland tundra (5 replicates), the productive slope (4 replicates) and barren high-altitude tundra (5 replicates). Within the slope vole populations were cyclic. In the lowland tundra. vole fluctuations were primarily seasonal, but the vole erash on the productive slope coincided with a phase of relatively low vole densities in the lowland. The highland was characterised by low vole densities. During the phase of rapid population growth, long-range dispersal occurred within the slope and from the slope to surrounding areas. Moreover, small mustelids which had initially been present only on the slope, started to move elsewhere, along natural dispersal corridors. Shoot mortalities of the main winter food plant. Vaccinium myrtillus. remained low. The observed scenario is consistent with the hypothesis that vole cycles represent a mustelid-microtine limit cycle, because cycles created by this mechanism should disappear when the productive habitats, capable of supporting resident predators, become fragmented and embedded in a vast unsuitable area.

EXPERIMENTAL REDUCTION OF PREDATORS REVERSES THE CRASH PHASE OF SMALL-RODENT CYCLES

The mechanisms driving short-term (3–5 yr) cyclic fluctuations in densities of boreal small rodents, and especially, those causing a crash in numbers, have remained a puzzle, although food shortage and predation have been proposed as the main factors causing these fluctuations. In the first large-scale vertebrate predator manipulation experiment with sufficient replication, densities of small mustelids (the least weasel Mustela nivalis and the stoat M. erminea) and avian predators (mainly the Eurasian Kestrel Falco tinnunculus and Tengmalm’s Owl Aegolius funereus) were reduced in six different areas, 2–3 km2 each, in two crash phases (1992 and 1995) of the 3-yr cycle of voles (field vole Microtus agrestis, sibling vole M. rossiaemeridionalis, and bank vole Clethrionomys glareolus). The reduction of all main predators reversed the decline in density of small rodents in the subsequent summer, whereas in areas with least weasel reduction and in control areas without predator manipulation, small rodent densities continued to decline. That only reduction of all main predators was sufficient to prevent this summer crash was apparently because least weasels represent <40% of vole-eating predators in western Finland. These results provide novel evidence for the hypothesis that specialist predators drive a summer decline of cyclic rodent populations in northern Europe.

Breeding Suppression in Voles under Predation Risk of Small Mustelids: Laboratory or Methodological Artifact?

Many prey animals have life-history strategies that seem to have evolved to avoid predation from specialist predators. During the past few years. the hypothesis of breeding suppression (BSH) of voles as an adaptation to avoid small mustelid predation has prompted several empirical and theoretical studies. However, the validity of empirical work as well as justification of the conclusions in these studies have been criticised. Here we report results of an experiment in which we studied the breeding suppression of bank voles, Clethrionomvs glareolus. in four replicated enclosures. We found no effects of simulated least weasel, Mustela nivalis nivalis, predation risk on the reproductive output of female voles. In contrast to earlier laboratory studies, the weights of females did not differ between treatment groups after the experiment. We reanalysed results of our earlier laboratory studies in light of these results and criticism of the BSH. At present there is no direct evidence for brecding suppression of voles in field populations. Furthermore, the underlying assumptions of the BSH have not been tested experimentally. Thus the most parsimonious explanation for breeding suppression is that it may be a methodological or laboratory artifact.

Small Mustelid Predation Slows Population Growth of Microtus Voles: A Predator Reduction Experiment

1. The effects of predator reduction on the breeding performance and sex ratio of Microtus voles were studied in a cyclically fluctuating vole assemblage in western Finland, where the field vole (Microtus agrestis) and the sibling vole (M. rossiaemeridionalis) are the main prey of small mustelids. 2. The densities of small mustelids [the least weasel (Mustela nivalis nivalis) and the stoat (M. erminea)] were reduced experimentally in three large (2-3 km 2 ) unfenced areas in 1992, during a crash phase of the vole cycle, and in three different areas during the following crash phase in 1995. The reproductive performance of Microtus voles was compared between control and manipulation areas before and after reducing predators. 3. The reduction of predators increased the productivity of female voles, mainly due to an increased proportion of pregnancies, whereas the body condition of voles was not affected by the manipulation. This suggests that high predation risk in control areas suppressed the breeding of free-living voles. 4. The sex ratio of trapped voles before the manipulation did not differ between reduction and control areas, but after predators were reduced the sex ratio was more male-biased in the control areas. This indicates that small mustelids selectively killed female voles. 5. Our results suggest two possible mechanisms which may have operated concurrently. An adaptive explanation is that voles traded their current reproductive investment against future breeding under high predation risk. A simpler explanation is that small mustelids selectively preyed on pregnant female voles. Both suppressed breeding and selective killing would lead to a lower proportion of reproducing individuals in the vole population. Therefore, our results show that the presence of small mustelid predators slowed the population growth of Microtus voles.

Dynamics of voles and small mustelids in the taiga landscape of northern Fennoscandia in relation to habitat quality

Dynamics of voles and small mustelids in the taiga landscape of northern Fennoscandia in relation to habitat quality

Dynamics of voles and small mustelids in the taiga landscape of northern Fennoscandia in relation to habitat quality

In the forests of northern Fennoscandia during the 1980's, the dynamics of microtine rodents changed from multiannual high amplitude fluctuations (cycles) to, depending on species, fluctuations with a strong seasonal component or fluctuations with smaller amplitude and lower frequency Microtine and predator data from the Pallasjarvi area, Finnish Lapland, suggest that this transition took place at different rates in different parts of the taiga landscape Generally, densities m forest habitats have been primarily seasonal since 1985‐86 In mesic spruce taiga and m drier forest habitats microtmes had a prolonged peak in 1981‐83 and a crash in 1984–85 At the timberline, however microtme populations dropped from peak to low densities already in 1982‐83 but the final crash did not occur until spring 1985 The synchronous decrease in microtmes densities m all habitat types in 1984‐85 coincided with increase in weasel activity Activity of other carnivores was consistently high in mesic lowland habitats The data support following three conjectures 1) Periodic abundance of least weasels is crucial for sustained vole cycles 2) Predominance of stoats and other generalist predators lead to less regular fluctuations with a strong seasonal component where density declines occur in autumn and early winter 3) In barren tundra areas, the vegetation cannot sustain high densities of microtmes and, consequently, predation is not a necessary condition for population crashes

Microhabitat use and behavior of voles under weasel and raptor predation risk: predator facilitation?

An example of predator facilitation is that a microhabitat shift in a prey species induced by one predator increases the probability of the prey falling victim to other predators. Least weasels (Mustela nivalis) hunt in dense plant cover, whereas kestrels (Falco tinnunculus) hunt in habitats with sparse plant cover. Field voles (Microtus agrestis), the main food of weasels and kestrels, prefer open country with a high grass layer. We simulated a multipredator environment in an aviary (3.0 X 4.8 X 2.2 m) to find out whether predator facilitation plays a role in the interactions between voles, small mustelids, and raptors. In each replicate, we placed a field vole in a pen including sides of high and low grass layers (cover and open). In a predator-free situation, voles preferred cover but shifted to open when a weasel was introduced to cover. In the presence of a kestrel, voles occupied cover and decreased their mobility. In the presence of a weasel plus a kestrel, voles behaved as under the kestrel risk alone. Therefore, in these aviary circumstances, voles perceived the kestrel risk as greater than the weasel risk. Predator facilitation in the assemblage of predators subsisting on rodent prey may contribute to the crash of the four-year vole cycle: microhabitat shift due to an avoidance of weasel jaws may drive voles to raptor talons. Key mords: antipredatory behavior, field vole, four-year vole cycle, kestrel, least weasel, microhabitat shift, predator facilitation. [Behav Ecol 7:30-34 (1996)]

Social organization in European small mustelids

Abstract A review of the social organization in the weasel ( Mustela nivalis ) and stoat ( M. erminea ) is made, based mainly on my earlier mark-recapture studies on weasels and stoats and radio-tracking studies on stoats. During the non-breeding season, from September to April, the small mustelids are organized in a territorial pattern; individuals of the same sex exploit exclusive areas. The extensive male territories often include one or several female ranges. The territories are probably delineated mainly by scent marking. During the breeding season the males increase their movement activity and extend their ranges. The territorial systems brake down and male ranges generally overlap considerably. Depending on their social status, males adopt different strategies, i.c. roaming or staying, in their attempts to obtain as many matings as possible. Prey abundance and distribution are considered to be the decisive factors determining female spacing behaviour throughout the year. The great amount of food required by females when rearing young makes it rewarding to defend and use restricted areas exclusively. In males there is a seasonal change in social organization resulting from a shift in decisive resources; i.e. priority is placed on procuring receptive females during the breeding and food during the non-breeding season. Riassunto Organizzazione sociale nei piccoli mustelidi europei - L'autore presenta un'analisi dell'organizzazione sociale della donnola ( Mustela nivalis ) e dell'ermellino ( M. erminea ), basata soprattutto sui suoi studi effettuati mediante i metodi di marcamento-ricattura per entrambe le specie e di radio-telemetria per l'ermellino. Durante la stagione non riproduttiva, da settembre ad aprile, i piccoli mustelidi sono organizzati secondo un modello territoriale: individui dello stesso sesso utilizzano aree esclusive. Gli ampi territori dei maschi spesso includono quello di una o piu femmine. Probabilmente i territori sono delimitati principalmente con marcamenti odorosi (feci). Durante la stagione riproduttiva i maschi incrementano i loro movimenti e estendono le loro aree vitali. Il sistema territoriale non viene piu mantenuto e le aree vitali dei maschi generalmente si sovrappongono notevolmente. In relazione al loro stato sociale, i maschi adottano differenti strategie: cioe, vagano o restano nella loro area nel tentativo di avere piu accoppiamenti possibile. La distribuzione e l'abbondanza delle prede sono considerati i fattori prioritari che determinano il comportamento spaziale delle femmine nell'arco dell'anno. L'elevata richiesta di cibo da parte delle femmine durante l'allevamento della prole si traduce nella difesa ed uso esclusivo di aree limitate. L'organizzazione sociale dei maschi varia stagionalmente in relazione al cambiamento delle risorse di vitale importanza: durante la stagione riproduttiva diventa prioritario procurarsi femmine recettive, mentre durante quella non riproduttiva diventa fondamentale il cibo.

Does Predation Risk Constrain Maturation in Cyclic Vole Populations?

Animals may delay reproduction under temporarily high predation risk. If temporarily high predation risk has a suppressive effect on prey reproduction on a large scale, there should be a negative association between the predation risk level and the proportion of breeding individuals in prey populations. We used long-term trapping data on two vole species and data on predator densities to find out whether there was a relationship between the predation risk level and the proportion of breeding individuals in a small mammal assemblage with cyclic fluctuations. In spring and, in the field vole (Microtus agrestis), in autumn, current predation risk from small mustelids could explain observed variation in the proportion of mature voles better than their own density, current or previous. The association between the proportion of breeding field voles and the current predation risk from small mustelids was negative in autumn, but not in summer when the current reproductive value is high relative to the residual value. For bank voles (Clethrionomys glareolus), which have a lower predation risk than field voles, there was a negative relationship between predation risk from mustelids and proportion of breeding individuals in early summer, but not in late summer or autumn. In late summer, the proportion of breeding year-born bank voles was negatively related to the vole density in spring. Our results indicate that the indirect effects of predators on vole populations may be of great importance on a large scale, and that these effects should be taken into consideration in studies on factors regulating vole populations

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.

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.

Long‐term microtine dynamics in north Fennoscandian tundra: the vole cycle and the lemming chaos

Densities of microtine rodents in two habitat complexes in the tundra of Finnmarks‐vidda, Norwegian Lapland, were studied during 1977‐89 by means of snap trapping (Small Quadrat Method) Predator populations were studied by mapping breeding raptors and by snow‐tracking small mustelids During 1977‐85, snow‐trackmg was conducted only during peak and decline years, whereas during 1986‐89, snow‐tracking was conducted every winter (November‐December) and live‐trapping (in August) was used as an additional methodLowland vole populations had regular density fluctuations with peaks in 1978‐79. 1982‐84 and 1987‐88 Highland vole populations fluctuated less regularly and at lower over‐all densities Highland lemming populations had two outbreaks, in 1978 and 1988, ending in abrupt winter crashes In the lowland, outbreak levels were reached only in 1978 All microtine declines in relatively productive lowland habitats were accompanied by intense activity of small mustelids. whereas avian predators were common only in 1983 Lowland declines also showed clear between‐habitat asynchrony they started in areas with an exceptional abundance of productive habitats and then spread to more barren areas These lowland data are consistent with the hypothesis of a mustelid‐microtine limit cycle, although also several other hypotheses remain unrefuted The highland lemming data suggest a simple exploiter‐victim interaction between lemmings and the vegetation

Habitat use of small mustelids in north Fennoscandian tundra: a test of the hypothesis of patchy exploitation ecosystems

The habitat use of small mustelids in a tundra area in Norwegian Lapland was studied chiefly by means of snow‐tracking 1986‐89 Stoats showed strong peference to a habitat complex immediately beneath the thrust line of the Scandes, with exceptional abundance of luxuriant habitats, whereas weasel activity was more evenly spread over the lowland tundra Mustelid activity on the high tundra above the thrust line was consistently low Within each subarea. stoat activity was concentrated to the most luxuriant habitats Similar preferences were shown by weasels on the lowland but not in the vicinity of the thurst cliff Daily movements of both species varied from local (c 200 m) to extensive (up to 4 km), no consistent interspecific differences in travel distances could be observedThe results largely conform to the hypothesis of patchy exploitation ecosystems (T Oksanen 1990a), according to which predator activity tends to “spill over’ from luxuriant habitats, capable of supporting predator populations, to adjacent barren ones, due to despotic behavior within and between species and due to opportunistic predation by transient predators However, predator activity in barren habitats during the crash winter could not be explained by these mechanisms alone A third mechanism ‐ breakdown of habitat preferences of predators during crash phases of a cyclic prey population ‐ was thus introduced

Responses of stoats and least weasels to fluctuating food abundances: is the low phase of the vole cycle due to mustelid predation?

We studied responses of stoats and least weasels to fluctuating vole abundances during seven winters in western Finland. Density indices of mustelids were derived from snow-tracking, diet composition from scat samples, and vole abundances from snap-trapping. Predation rate was estimated by the ratio of voles to mustelids and by the vole kill rate by predators (density of predator x percentage of voles in the diet). We tested the following four predictions of the hypothesis that small mustelids cause the low phase of the microtine cycle. (1) The densities of predators should lag well behind the prey abundances, as time lags tend to have destabilizing effects. The densities of stoats fluctuated in accordance with the vole abundances, whereas the spring densities of least weasels tracked the vole abundances with a half-year lag and the autumn densities with a 1-year lag. (2) Predators should not shift to alternative prey with declining vole densities. The yearly proportion of Microtus voles (the staple prey) in the diet of stoats varied widely (range 16-82%) and was positively correlated with the winter abundance of these voles. In contrast, the same proportion in the food of least weasels was independent of the vole abundance. (3) The ratio of voles to small mustelids should be smallest in poor vole years and largest in good ones. This was also observed. (4) Vole densities from autumn to spring should decrease more in those winters when vole kill rates are high than when they are low. The data on least weasels agreed with this prediction. Our results from least weasels were consistent with the predictions of the hypothesis, but stoats behaved like "semi-generalist" predators. Accordingly, declines and lows in the microtine cycle may be due to least weasel predation, but other extrinsic factors may also contribute to crashes.

Avian Predation on Mustelids in Europe 1: Occurrence and Effects on Body Size Variation and Life Traits

mustelids have been recorded in the diet of seven Fennoscandian raptors and nine owls in the breeding season, averaging 0.8% of the prey items identified. The percentage of mustelids in the food was positively correlated with the body weight of birds of prey. The largest avian predators were able to take both medium-sized and small mustelids, and the smallest species only least weasels. There was no difference in the percentage of mustelid prey between Fennoscandian, and central and western European birds of prey. Breeding common buzzards, kestrels, eagle owls, Ural owls and short-eared owls in western Finland took small mustelids most frequently in the decline phase of the vole cycle. The estimated consumption of least weasels by breeding kestrels and shorteared owls in an arable farmland (area 50 km2) was 40 individuals in the increase phase of the vole cycle (1985), 510 in the decline phase (1986), and zero in the low phase (1984 and 1987). The estimated impact of avian predation on least weasels was ca. 20% in 1985 and 80% in 1986, suggesting that in the decline phase avian predation was an important factor limiting the densities of least weasels. As mustelids are very mobile, they are vulnerable to avian predators. Large size helps to reduce the predation risk. This may partly explain the size variation of stoats and weasels in Europe (opposite to that suggested by Bergmann's rule) and promote the larger size of males in comparison with females.

Avian Predation on Mustelids in Europe 2: Impact on Small Mustelid and Microtine Dynamics: A Hypothesis

Avian Predation on Mustelids in Europe 2: Impact on Small Mustelid and Microtine Dynamics: A Hypothesis

Extrafloral Nectaries: A Defense against Ant-Homoptera Mutualisms?

microtine and shrew populations in western Finland. Oikos 46: 124-127. 1986b. Gradients in population fluctuations of Tengmalm's Owl Aegolius funereus in Europe. Oecologia (Berl.) 69: 195-201. 1989. Breeding performance of Tengmalm's Owl Aegolius funereus: effects of supplementary feeding in a peak vole year. Ibis 131 (in press). and Norrdahl, K. 1987. Low proportion of shrews in the diet of small mustelids in western Finland. Z. Saugetierkunde 52: 257-260. and Norrdahl, K. 1989a. Predation of Tengmalm's Owls: numerical responses, functional responses and dampening impact on population fluctuations of microtines. Oikos 54: 154-164. and Norrdahl, K. 1989b. Avian predation on mustelids in Europe 1: occurrence and effects on body size variation and life traits. Oikos 55: 205-215. and Sulkava, S. 1987. Diet and breeding performance of Ural Owls Strix uralensis under fluctuating food conditions. Ornis Fennica 64: 57-66. Kotler, B. P., Brown, J. S., Smith, R. J. and Wirtz II, W. 0. 1988. The effects of morphology and body size on rates of owl predation on desert rodents. Oikos 53: 145-152. Latham, R. M. 1952. The fox as a factor in the control of weasel populations. J. Wildl. Manage. 16: 516-517. 986a. Predation causing synchronous declin phases in icrotine and shrew populations in western Finland. Liberg, 0. 1984. Food habits and prey impact by feral and house-based Domestic cats in a rural area in southern Sweden. J. Mammal. 65: 424-432. M cLean, S. F. Jr., Fitzgerald, B. M. and Pitelka, F. A. 1974. Population cycles in arctic lemmings: winter reproduction and predation by weasels. Arct. Alp. Res 6: 1-12. Maher, W. J. 1967. Predation by weasels on a winter population of lemmings, Banks Island, Northwest Territories. Can. Field-Nat. 81: 248-250. 1970. The pomarine jaeger as a brown lemming predator in northern Alaska. Wilson Bull. 82: 130-157. Norrdahl, K. 1985. The population fluctuations of small mammals in Suomenselka and southern Ostrobothnia, western Finland, in 1969-84. Suomenselain Linnut 20: 57-68 (In Finnish with summary in English). Pearson, 0. P. 1966. The prey of carnivores during one cycle of mouse abundance. J. Anim. Ecol. 35: 217-233. 1971. Additional measurements of the impact of carnivores on California voles (Microtus californicus). J. Mammal. 52: 41-49.

Sex Biases in Trapped Samples of Mustelidae

Sex ratios significantly different from 1:1 usually are observed in trap captures of mustelids. Although these ratios could be caused by skewed sex ratios in wild populations, trapped samples consistently are skewed toward males. This apparent sampling bias generally has been attributed to sexual dimorphism of home-range sizes in mustelids, postulated to result in greater exposure of males to traps. The magnitude of differences in rates of capture between sexes appears to be body-size dependent because small mustelids exhibit more strongly skewed sex ratios in trap captures than do large ones. We investigated possible causes of sexual differences in rates of capture of mustelids. Mathematical equations were derived to describe the effect of trap spacing in relation to home-range diameter as a cause of these sex biases. Large trap-spacing values in relation to home-range diameter result in these biases, but this “exclusion effect” (the effect of some smaller home ranges having no traps at all) is only one consequence of sexual dimorphism of home-range size. At low trap-spacing intervals, variations in home-range size result in differential numbers of traps in male and female home ranges (the “trap number effect”), although the density of traps is the same. Computer simulations were used to examine the interaction of known and postulated body-size-dependent factors, including home-range size, rate of travel, sensory acuity, and a body-size-independent factor, dimensionality of trap arrangement, as a possible cause of sexual differences in rates of encounter with traps. Rates of encounter with traps that favor capture of males and that produce greater differences in rates of capture between sexes in small species than in large ones were observed if home ranges were sexually dimorphic, traps were arranged in grids, rate of travel was proportional to the 0.25 power of body mass, and males perceived traps at the same distances as did females. Sex-specific behaviors, either resulting from sexual differences in territory packing, or in the way that animals respond to traps that they perceive, are hypothesized to be contributing causes of differences in rates of capture between sexes.

Fates and movements of meadow voles (Microtus pennsylvanicus) following a population decline

The movements of 24 meadow voles were monitored with radiotelemetry during summer 1985 near Pinawa, Manitoba, to ascertain fates following a severe winter population decline. Low breeding season survival of 22% per month resulted in the persistence of low numbers through spring 1986. Despite densities of only a few individuals per hectare, voles were captured in both meadows and forests. Some voles first caught in forests (mostly males) moved over large areas (up to 4 ha), including both meadow and forest habitat. In contrast, voles captured in meadows were sedentary, often confining their movements to small areas of 0.1–0.2 ha. Radiotransmitters were recovered from 18 of 19 voles that died during the study. Five voles (28%) appeared to have died in situ; six voles (33%) may have been taken by an unusually clean predator or scavenger. Fates of the remaining voles were uncertain. No direct evidence of predation by small mustelids was found to support the hypothesis that these predators account for the persistence of low density following population declines.