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Most small rodent populations in the world have fascinating population dynamics. In the northern hemisphere, voles and lemmings tend to show population cycles with regular fluctuations in numbers. In the southern hemisphere, small rodents tend to have large amplitude outbreaks with less regular intervals. In the light of vast research and debate over almost a century, we here discuss the driving forces of these different rodent population dynamics. We highlight ten questions directly related to the various characteristics of relevant populations and ecosystems that still need to be answered. This overview is not intended as a complete list of questions but rather focuses on the most important issues that are essential for understanding the generality of small rodent population dynamics.

Chemical communication plays an important role in mammalian life history decisions. Animals send and receive information based on body odour secretions. Odour cues provide important social information on identity, kinship, sex, group membership or genetic quality. Recent findings show, that rodents alarm their conspecifics with danger-dependent body odours after encountering a predator. In this study, we aim to identify the chemistry of alarm pheromones (AP) in the bank vole, a common boreal rodent. Furthermore, the vole foraging efficiency under perceived fear was measured in a set of field experiments in large outdoor enclosures. During the analysis of bank vole odour by gas chromatography–mass spectrometry, we identified that 1-octanol, 2-octanone, and one unknown compound as the most likely candidates to function as alarm signals. These compounds were independent of the vole’s sex. In a field experiment, voles were foraging less, i.e. they were more afraid in the AP odour foraging trays during the first day, as the odour was fresh, than in the second day. This verified the short lasting effect of volatile APs. Our results clarified the chemistry of alarming body odour compounds in mammals, and enhanced our understanding of the ecological role of AP and chemical communication in mammals. Supplementary Information The online version contains supplementary material available at 10.1007/s00442-021-04977-w.

Abstract In the predator–prey arms race, survival-enhancing adaptive behaviors are essential. Prey can perceive predator presence directly from visual, auditory, or chemical cues. Non-lethal encounters with a predator may trigger prey to produce special body odors, alarm pheromones, informing conspecifics about predation risks. Recent studies suggest that parental exposure to predation risk during reproduction affects offspring behavior cross-generationally. We compared behaviors of bank vole (Myodes glareolus) pups produced by parents exposed to one of three treatments: predator scent from the least weasel (Mustela nivalis nivalis); scent from weasel-exposed voles, i.e., alarm pheromones; or a control treatment without added scents. Parents were treated in semi-natural field enclosures, but pups were born in the lab and assayed in an open-field arena. Before each behavioral test, one of the three scent treatments was spread throughout the test arena. The tests followed a full factorial design (3 parental treatments × 3 area treatments). Regardless of the parents’ treatment, pups exposed to predator odor in the arena moved more. Additionally, pups spend more time in the center of the arena when presented with predator odor or alarm pheromone compared with the control. Pups from predator odor–exposed parents avoided the center of the arena under control conditions, but they spent more time in the center when either predator odor or alarm pheromone was present. Our experiment shows that cross-generational effects are context-sensitive, depending on the perceived risk. Future studies should examine cross-generational behavioral effects in ecologically meaningful environments instead of only neutral ones. Significance statement We exposed bank voles to odors signaling predation risk to assess the effects parental predation exposure on the behavior of their offspring. Besides predator odor, we also assessed the role of a conspecific alarm cue as a novel way of spreading the predation risk information. Pup behaviors were assessed in the open-field arena, a standard way of assessing animal behavior in a wide range of contexts. We found that also alarm pheromone increased the time pups spend in the center of the arena similarly to predator odor. While previous studies suggested that offspring would be more fearful, our results indicate that the cross-generational effects are very context-dependent; i.e., they differ significantly depending on which scent cue is presented in the open-field arena. This shows the need for better tools or measurements to translate laboratory results into ecologically meaningful frameworks.

Most small rodent populations in the world have fascinating population dynamics. In the northern hemisphere, voles and lemmings tend to show population cycles with regular fluctuations in numbers. In the southern hemisphere, small rodents tend to have large amplitude outbreaks with less regular intervals. In the light of vast research and debate over almost a century, we here discuss the driving forces of these different rodent population dynamics. We highlight ten questions directly related to the various characteristics of relevant populations and ecosystems that still need to be answered. This overview is not intended as a complete list of questions but rather focuses on the most important issues that are essential for understanding the generality of small rodent population dynamics.

Phenotype and life history traits of an individual are a product of environmental conditions and the genome. Environment can be current or past, which complicates the distinction between environmental and heritable effects on the phenotype in wild animals. We studied genome–environment interactions on phenotype and life history traits by transplanting bank voles (Myodes glareolus) from northern and southern populations, originating from low or high population cycle phases, to common garden conditions in large outdoor enclosures. The first experiment focused on the persistence of body traits in autumn-captured overwintering populations. The second experiment focused on population growth and body traits in spring-captured founder voles and F1 generation. This experiment lasted the breeding season and subsequent winter. We verified phase-dependent differences in body size at capture. In the common environment, adult voles kept their original body size differences both over winter and during the breeding season. In addition, the first generation born in the common environment kept the size distribution of their parent population. The increase phase population maintained a more rapid growth potential, while populations from the decline phase of the cycle grew slower. After winter, the F1 generation of the increasing northern population matured later than the F1 of the southern declining ones. Our results suggest a strong role of heredity or early life conditions, greater than that of current juvenile and adult environmental conditions. Environmental conditions experienced by the parents in their early life can have inter-generational effects that manifest in offspring performance. Electronic supplementary material The online version of this article (10.1007/s00442-019-04410-3) contains supplementary material, which is available to authorized users.

Climate change, habitat loss and fragmentation are major threats for populations and a challenge for individual behavior, interactions and survival. Predator–prey interactions are modified by climate processes. In the northern latitudes, strong seasonality is changing and the main predicted feature is shortening and instability of winter. Vole populations in the boreal Fennoscandia exhibit multiannual cycles. High amplitude peak numbers of voles and dramatic population lows alternate in 3–5‐year cycles shortening from North to South. One key factor, or driver, promoting the population crash and causing extreme extended lows, is suggested to be predation by the least weasel. We review the arms race between prey voles and weasels through the multiannual density fluctuation, affected by climate change, and especially the changes in the duration and stability of snow cover. For ground‐dwelling small mammals, snow provides thermoregulation and shelter for nest sites, and helps them hide from predators. Predicted increases in the instability of winter forms a major challenge for species with coat color change between brown summer camouflage and white winter coat. One of these is the least weasel, Mustela nivalis nivalis. Increased vulnerability of wrong‐colored weasels to predation affects vole populations and may have dramatic effects on vole dynamics. It may have cascading effects on other small rodent–predator interactions and even on plant–animal interactions and forest dynamics. peerReviewed

Prey strategically respond to the risk of predation by varying their behavior while balancing the tradeoffs of food and safety. We present here an experiment that tests the way the same indirect cues of predation risk are interpreted by bank voles, Myodes glareolus, as the game changes through exposure to a caged weasel. Using optimal patch use, we asked wild-caught voles to rank the risk they perceived. We measured their response to olfactory cues in the form of weasel bedding, a sham control in the form of rabbit bedding, and an odor-free control. We repeated the interviews in a chronological order to test the change in response, i.e., the changes in the value of the information. We found that the voles did not differentiate strongly between treatments pre-exposure to the weasel. During the exposure, vole foraging activity was reduced in all treatments, but proportionally increased in the vicinity to the rabbit odor. Post-exposure, the voles focused their foraging in the control, while the value of exposure to the predator explained the majority of variation in response. Our data also suggested a sex bias in interpretation of the cues. Given how the foragers changed their interpretation of the same cues based on external information, we suggest that applying predator olfactory cues as a simulation of predation risk needs further testing. For instance, what are the possible effective compounds and how they change “fear” response over time. The major conclusion is that however effective olfactory cues may be, the presence of live predators overwhelmingly affects the information voles gained from these cues. Significance statement In ecology, “fear” is the strategic response to cues of risk an animal senses in its environment. The cues suggesting the existence of a predator in the vicinity are weighed by an individual against the probability of encounter with the predator and the perceived lethality of an encounter with the predator. The best documented such response is variation in foraging tenacity as measured by a giving-up density. In this paper, we show that an olfactory predator cue and the smell of an interspecific competitor result in different responses based on experience with a live-caged predator. This work provides a cautionary example of the risk in making assumptions regarding olfactory cues devoid of environmental context. Electronic supplementary material The online version of this article (10.1007/s00265-018-2600-9) contains supplementary material, which is available to authorized users.

Mating with close kin may cause inbreeding depression with negative consequences to offspring and local populations. There exist mechanisms like kin-recognition or sex-specific dispersal to avoid mating with kin. In fluctuating population densities, like in many small mammals, both very low and very high densities provide conditions for inbreeding, if kin males are prone to stay in their natal area. Females are choosy and male dominance is thought to be the key feature when selecting mating partners. The aim of this study was to test the possible discrepancy in mate choice and negative fitness effects of inbreeding in two experiments, one in the laboratory and one in field enclosures. We asked (1) how the quality of the potential mating partners affects female choice regardless of relatedness and (2) how inbreeding affects the field populations created either from inbred or outbred individuals. Our results show that primiparous females in post-partum oestrus mated preferably with a dominant male, measured with their urine-marking behaviour, regardless if the selected male was brother or not. Only if the two males offered were of same dominance rank, the female mated with the non-kin male. However, the field experiment verified a negative effect of inbreeding in the bank vole. Thus, there seems to be a mismatch between female mate choice when selecting for dominance among resident males and population viability through long-term inbreeding depression. The study suggests the high importance of sex-biased dispersal as a mechanism to avoid kin individuals to meet in mate choice situations.

Many boreal rodents are territorial during the breeding season but during winter become social and aggregate for more energy efficient thermoregulation. Communal winter nesting and social interactions are considered to play an important role for the winter survival of these species, yet the topic is relatively little explored. Females are suggested to be the initiators of winter aggregations and sometimes reported to survive better than males. This could be due to the higher social tolerance observed in overwintering females than males. Hormonal status could also affect winter behavior and survival. For instance, chronic stress can have a negative effect on survival, whereas high gonadal hormone levels, such as testosterone, often induce aggressive behavior. To test if the winter survival of females in a boreal rodent is better than that of males, and to assess the role of females in the winter aggregations, we generated bank vole (Myodes glareolus) populations of three different sex ratios (male-biased, female-biased and even density) under semi-natural conditions. We monitored survival, spatial behavior and hormonal status (stress and testosterone) during two winter months. We observed no significant differences in survival between the sexes or among populations with differing sex-ratios. The degree of movement area overlap was used as an indicator of social tolerance and potential communal nesting. Individuals in male biased populations showed a tendency to be solitary, whereas in female biased populations there was an indication of winter aggregation. Females living in male-biased populations had higher stress levels than the females from the other populations. The female-biased sex-ratio induced winter breeding and elevated testosterone levels in males. Thus, our results suggest that the sex-ratio of the overwintering population can lead to divergent overwintering strategies in bank voles. peerReviewed

Habitat fragmentation is known to cause population declines but the mechanisms leading to the decline are not fully understood. Fragmentation is likely to lead to changes in predation risk, which may cause behavioral responses with possible population level consequences. It has recently been shown that the awareness of predator presence, resulting in a fear response, strongly affects behavior and physiology of the prey individuals. Costs arising from fear may be as important for the prey population size as the direct killing of prey. We tested how predation risk in the form of scent of a specialist predator, the least weasel (Mustela nivalis nivalis), affects bank vole (Myodes glareolus) behavior in fragmented enclosures consisting of either non-fragmented (one patch) or fragmented (four patches) habitats of the same total area. Vole movement areas tended to be larger in the non-fragmented habitat. Fear decreased vole activity and tended to increase the use of the open matrix area. No interactions between fragmentation and fear treatments or differences in breeding related behaviors or fitness were found in our short-term experiment. However, behavioral mechanisms such as decreased activity and change of movements to the risky matrix could cause negative effects and population decline in the long run. Fragmentation is a serious issue in all human exploited habitats causing animals to face more risks compared to more uniform and sheltering environments. This should be especially taken into account in conservation of habitat and the reintroduction of captive reared animals where both intact sheltering habitats and food providing have habitats become rarer.

Habitat fragmentation affects individual movements between favorable resource patches. In many small mammal species, an important intrinsic factor affecting recruitment of young is infanticide, committed especially by males. We predict that habitat fragmentation hinders movements of males between patches due to predation risk in the open areas. Thus, fragmentation reduces the number of males to which young litters are exposed to and decreases risk of infanticide in isolated habitat patches. Nonfragmented habitat provides not only breeding possibilities for more females but also safe movements to mate or commit infanticide for males. In a replicated enclosure experiment, we tested how infanticidal status of male bank voles (Myodes glareolus) affects their movements in fragmented vs. nonfragmented habitats with same total area and how this affects female spacing and offspring recruitment into population. We found no difference in the number of offspring recruited per female between infanticidal and fragmentation treatments. Females in the fragmented enclosures had smaller movement areas and stayed closer to their nests, suggesting better protection of pups against intruders. Infanticidal males moved more in general but especially in fragmented enclosures, whilst noninfanticidal males were moving more in nonfragmented enclosures. Our results suggest that behavior of females is affected by the habitat fragmentation, as we expected, but males searching for mates move similarly in safe and risky habitats. Thus, the threat of infanticide was not reflected in the recruitment of young into the population as probably the male–female interactions and effective nest protection by the mothers remained similar in both habitats.

In seasonal environments, the optimal onset of reproduction plays a major role in defining the reproductive success of an individual. Environmental cues, like day length, weather conditions, and food, regulate the initiation and termination of the breeding season. Besides the interspecific variation in response to environmental cues, it has been suggested that due to different selection pressures, females and males can have different responses to environmental stimuli. However, this phenomenon has gained relatively little consideration, and the physiological mechanism behind these differences is not well known. Here, we report how two different environmental cues, variability of temperature and nutritional conditions in spring, affect the onset of breeding in a boreal small rodent, the bank vole, Myodes glareolus. We exposed wild-trapped individuals to four different treatments manipulating temperature (stable vs. variable) and food quality (high vs. low protein content) over five weeks in the laboratory. We monitored body-mass development, food consumption, and initiation of breeding. We found sex-specific responses to temperature variability, as males achieved their breeding condition faster in variable temperature treatments, whereas female maturation was delayed. Food quality had no effect on the onset of breeding. To test for possible reproductive trade-offs caused by reproductive decisions made in early spring, the experiment was continued in large outdoor enclosures. There seemed to be no significant long-term effects on reproduction, but early summer survival was affected by climate conditions experienced in spring. Our results show clear sex differences in the response to environmental cues regulating the onset of reproduction in bank voles. Hence, our study suggests that when an organism experiences rapid environmental changes, as are occurring on a global scale, divergent cues may lead to a reproductive mismatch between the sexes. This could noticeably decrease the fitness of many seasonally breeding species.

Infanticide, the killing of conspecific young, occurs in most mammal species, like in our study species, the bank vole (Myodes glareolus). Infanticide by adult males is regarded as a strong factor affecting recruitment of young into population. It is considered as an adaptive behaviour, which may increase male fitness via resource gain or an increased access to mates. When an intruder is approaching the nest, the mother should not be present, as her nest guarding is very aggressive and successful. Pups use ultrasonic vocalisation to call their mother when mother leaves nest for foraging but it is not know which cues do infanticidal males use to find the nest with vulnerable pups to commit infanticide? We studied whether the pups' sounds or the olfactory cues of the nest guide the males of known infanticidal behavioural trait towards the nest with vulnerable pups. Four nest boxes in a large indoor arena offered different nesting cues: nest odour, pup vocalisation, both odour and sound or control with no cue. The result showed that infanticidal males were more active in their searching behaviour than non-infanticidal males and seemed to target the nest providing only acoustic cues. Four of the males, all infanticidal, intruded the nest box. Infanticidal males seem to actively search for nests with vulnerable pups by eavesdropping pup begging calls for absent mother. However, under natural conditions, mother presence and aggressive nest protection may be an effective counter strategy against strange male infanticide. When trapping study voles from the wild, we monitored occurrence of male infanticide across the breeding season from early to late summer. Proportion of infanticidal males was between 25 and 29% of all males tested along the breeding season. Our results suggest that male infanticide seems to cause a stable threat for pup mortality in increasing breeding season density. peerReviewed

Climate instability strongly affects overwintering conditions in organisms living in a strongly seasonal environment and consequently their survival and population dynamics. Food, predation and density effects are also strong during winter, but the effect of fragmentation of ground vegetation on ground-dwelling small mammals is unknown. Here, we report the results of a winter experiment on the effects of habitat fragmentation and food on experimental overwintering populations of bank voles Myodes glareolus. The study was conducted in large outdoor enclosures containing one large, two medium-sized or four small habitat patches or the total enclosure area covered with protective tall-grass habitat. During the stable snow cover of midwinter, only food affected the overwintering success, body condition, trappability and earlier onset of breeding in bank voles. However, after the snow thaw in spring, habitat fragmentation gained importance again, and breeding activities increased the movements of voles in the most fragmented habitat. The use of an open, risky matrix area increased along the habitat fragmentation. Our experiment showed that long-lasting stable snow cover protects overwintering individuals in otherwise exposed and risky ground habitats. We conclude that a stable winter climate and snow cover should even out habitat fragmentation effects on small mammals. However, along prolonged snow-free early winter and in an earlier spring thaw, this means loss of protection by snow cover both in terms of thermoregulation and predation. Thus, habitat cover is important for the survival of small ground-dwelling boreal mammals also during the non-breeding season.

A predator has a large impact on its prey. Besides a direct lethal effect, mere predator presence can increase stress and vigilance reflecting on prey behaviour and physiology. Such an effect should be stronger in more vulnerable prey individuals, depending e.g. on sex and reproductive status. We studied the short-term physiological response, the change in CO 2 production as a measure of metabolism, and the behavioural response in non-pregnant and pregnant bank vole Myodes glareolus females. The bank vole is a common small mammal in boreal forest environments and one of the major prey species for predators, especially small mustelids. The least weasel Mustela nivalis nivalis is a vole specialist and the only predator which can enter the tunnels of voles and their nests. Furthermore, weasels, like all mustelids, have strong scents used in social communication, and weasel odour may be used as a cue of increased risk by prey individuals. We simulated an increase in predation risk by the injection of least weasel odour into an open-flow respirometry system housing either a pregnant or non-pregnant female bank vole. We focused on immediate and post-stress response in CO 2 production and changes in female behaviour. We found that injections of pure air ("disturbance") or of weasel odour ("threat") similarly increased CO 2 production. Surprisingly, non-pregnant females responded to stress more strongly than pregnant ones, i.e. the increase in CO 2 was higher. Weasel odour increased female activity more than the injection of air. According to our prediction, after the disappearance of weasel odour there were no differences in physiological or behavioural parameters between pregnant and non-pregnant females. Contrary to our prediction, our results showed quite similar response to harmless air disturbance and the risky weasel cue. However, it may be that in a situation of sudden change in environment, regardless of its nature, a short-term vigilance response is a better response than more costly antipredatory behaviours.

Habitat fragmentation causes negative population trends or even local extinction in many species. Understanding the role of fragmentation on behavior and space use of animals is an essential part of revealing the mechanism behind observed population declines. We studied experimentally the effects of small-scale habitat fragmentation on the distribution and movement of bank voles (Myodes glareolus) in seminatural enclosures. We predicted that besides habitat structure, availability of two resources, food for both sexes, and receptive females for males, determine individual distribution. We manipulated female density (either 0, 4, or 12 per enclosure) and followed movements of radio-collared males. We also studied the effect of unequal food distribution between the fragments on vole distribution. The habitat of 0.25-ha enclosures was manipulated by forming either one large, two medium-sized, or four small habitat patches surrounded by an inhospitable matrix. Female density and distribution clearly affected male spacing behavior. Males had larger home ranges in the medium female density enclosures. Furthermore, the use of the inhospitable and risky matrix area increased with habitat fragmentation. Food supplementation improved individual condition measured as body mass but did not affect breeding success. Our experiments demonstrated that both small-scale habitat fragmentation and resource distribution affect the behavior and condition of individuals. Increased fragmentation led to increased risk taking in both mating and foraging behavior. This should have direct survival and fitness consequences, and therefore our results may be extrapolated to population-level consequences of habitat fragmentation.

In prey communities with shared predators, variation in prey vulnerability is a key factor in shaping community dynamics. Conversely, the hunting efficiency of a predator depends on the prey community structure, preferences of the predator and antipredatory behavioural traits of the prey. We studied experimentally, under seminatural field conditions, the preferences of a predator and the antipredatory responses of prey in a system consisting of two Myodes species of voles, the grey-sided vole (M. rufocanus Sund.) and the bank vole (M. glareolus Schreb.), and their specialist predator, the least weasel (Mustela nivalis nivalis L.). To quantify the preference of the weasels, we developed a new modelling framework that can be used for unbalanced data. The two vole species were hypothesised to have different habitat-dependent vulnerabilities. We created two habitats, open and forest, to provide different escape possibilities for the voles. We found a weak general preference of the weasels for the grey-sided voles over the bank voles, and a somewhat stronger preference specifically in open habitats. The weasels clearly preferred male grey-sided voles over females, whereas in bank voles, there was no difference. The activity of voles changed over time, so that voles increased their movements immediately after weasel introduction, but later adjusted their movements to times of lowered predation risk. Females that were more active had an elevated mortality risk, whereas in the case of males, the result was the opposite. We conclude that, in vulnerability to predation, the species- or habitat-specific characteristics of these prey species are playing a minor role compared to sex-specific characteristics.

Due to global climate change, the winter conditions in the North are predicted to change, as the time with an intact insulating snow cover gets shorter or disappears altogether. For small mammals, this could cause exposure to strong temperature fluctuations and increased predation risk, inducing severe stress and leading to alterations in the physical condition and behavior. To test this, we exposed bank voles (Myodes glareolus) to different temperature regimes and cues of predator threat under laboratory conditions. The test animals experienced either a stable but cool temperature regime resembling the stable conditions under snow cover, or an unstable temperature regime with cold nights and warmer days simulating the climate change scenario with unstable winter. After 3 weeks, the animals were additionally exposed to owl calls or sounds of non-predatory bird species. Stress levels, activity, food consumption, and body mass were monitored. We observed that the voles exposed to unstable temperatures adjusted their normal, mostly nocturnal, and circadian activity pattern towards a more diurnal rhythm without any significant responses in their stress level. Introducing the sound manipulation elevated the stress levels in females but not in males. The sound-induced stress levels did not differ between the temperature treatments. However, the temperature regime tended to affect anti-predator behavior as individuals experiencing unstable temperatures and a threatening sound decreased their overall activity, unlike individuals under stable temperature treatments. It seems that behavioral plasticity in bank voles may diminish the risk of accumulation of stress in cases of multiple simultaneous stressors. The future course of many ecosystems in the northern hemisphere has raised many questions as climate change is predicted to have notable consequences especially in these regions due to alterations in winter conditions. Our results suggest that these environmental changes could alter the circadian activity pattern and possibly, even the anti-predator behavior of bank voles. However, the adaptability and behavioral plasticity in this species may diminish the risk of pathological accumulation of stress effects.

Parental care often produces a trade-off between meeting nutritional demands of offspring and the duties of offspring protection, especially in altricial species. Parents have to leave their young unattended for foraging trips, during which nestlings are exposed to predators. We investigated how rodent mothers of altricial young respond to risk of nest predation in their foraging decisions. We studied foraging behavior of lactating bank voles (Myodes glareolus) exposed to a nest predator, the common shrew (Sorex araneus). We conducted the experiment in summer (high resource provisioning for both species) and autumn (less food available) in 12 replicates with fully crossed factors “shrew presence” and “season.” We monitored use of feeding stations near and far from the nest as measurement of foraging activity and strategic foraging behavior. Vole mothers adapted their strategies to shrew presence and optimized their foraging behavior according to seasonal constraints, resulting in an interaction of treatment and season. In summer, shrew presence reduced food intake from feeding stations, while it enhanced intake in autumn. Shrew presence decreased the number of visited feeding stations in autumn and concentrated mother’s foraging efforts to fewer stations. Independent of shrew presence or season, mothers foraged more in patches further away from the nest than near the nest. Results indicate that females are not investing in nest guarding but try to avoid the accumulation of olfactory cues near the nest leading a predator to the young. Additionally, our study shows how foraging strategies and nest attendance are influenced by seasonal food provision., Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe, 857

1. In seasonal environments, optimal onset of breeding and survival plays major roles in individual fitness. Many physiological and behavioural factors related to breeding increase the risk of predation; thus, breeding decisions should be based on current risks and benefits. According to life-history theory, if current predation risk is high and breeding itself increases the risk, it may be beneficial to postpone breeding. 2. During winter in northern hemispheres, food availability is limited and is at its lowest just prior to the onset of breeding in spring. Food constraint may lead to poor condition and reduced ability to start breeding. 3. We studied the effects of food and predation risk on winter survival and onset of breeding in a common boreal rodent, the bank vole (Myodes glareolus). In a 2 × 2 factorial experiment, we manipulated food availability (food supplemented or not) and predation risk (presence/absence of predator odour) in 20 large outdoor enclosures in central Finland. 4. Survival probabilities were highest in no predation risk treatments, whereas they were lowest in the predator risk treatment. The same trend was observed in vole densities and the weight change in individuals. Voles with food addition bred earlier than in the other treatments. 5. We conclude that during energy constrained harsh conditions in winter, predation risk causes behavioural changes throughout the winter and has significant negative survival and fitness effects for small mammals, reflected as delay in the start of breeding.

The relationship between predators and prey is thought to change due to habitat loss and fragmentation, but patterns regarding the direction of the effect are lacking. The common prediction is that specialized predators, often more dependent on a certain habitat type, should be more vulnerable to habitat loss compared to generalist predators, but actual fragmentation effects are unknown. If a predator is small and vulnerable to predation by other larger predators through intra-guild predation, habitat fragmentation will similarly affect both the prey and the small predator. In this case, the predator is predicted to behave similarly to the prey and avoid open and risky areas. We studied a specialist predator's, the least weasel, Mustela nivalis nivalis, spacing behavior and hunting efficiency on bank voles, Myodes glareolus, in an experimentally fragmented habitat. The habitat consisted of either one large habitat patch (non-fragmented) or four small habitat patches (fragmented) with the same total area. The study was replicated in summer and autumn during a year with high avian predation risk for both voles and weasels. As predicted, weasels under radio-surveillance killed more voles in the non-fragmented habitat which also provided cover from avian predators during their prey search. However, this was only during autumn, when the killing rate was also generally high due to cold weather. The movement areas were the same for both sexes and both fragmentation treatments, but weasels of both sexes were more prone to take risks in crossing the open matrix in the fragmented treatment. Our results support the hypothesis that habitat fragmentation may increase the persistence of specialist predator and prey populations if predators are limited in the same habitat as their prey and they share the same risk from avian predation.

Indirect exploitative competition, direct interference and predation are important interactions affecting species coexistence. These interaction types may overlap and vary with the season and life-history state of individuals. We studied effects of competition and potential nest predation by common shrews (Sorex araneus) on lactating bank voles (Myodes glareolus) in two seasons. The species coexist and may interact aggressively. Additionally, shrews can prey on nestling voles. We studied bank vole mothers’ spatial and temporal adaptations to shrew presence during summer and autumn. Further, we focused on fitness costs, e.g. decreased offspring survival, which bank voles may experience in the presence of shrews. In summer, interference with shrews decreased the voles’ home ranges and they spent more time outside the nest, but there were no effects on offspring survival. In autumn, we found decreased offspring survival in enclosures with shrews, potentially due to nest predation by shrews or by increased competition between species. Our results indicate a shift between interaction types depending on seasonal constraints. In summer, voles and shrews seem to interact mainly by interference, whereas resource competition and/or nest predation by shrews gain importance in autumn. Different food availability, changing environmental conditions and the energetic constraints in voles and shrews later in the year may be the reasons for the varying combinations of interaction types and their increasing effects on the inclusive fitness of bank voles. Our study provides evidence for the need of studies combining life history with behavioural measurements and seasonal constraints.