House Mouse Populations Research Articles

Estimating The Abundance Of Mouse Populations Of Known Size: Promises And Pitfalls Of New Methods

Knowledge of animal abundance is fundamental to many ecological studies. Frequently, researchers cannot determine true abundance, and so must estimate it using a method such as mark-recapture or distance sampling. Recent advances in abundance estimation allow one to model heterogeneity with individual covariates or mixture distributions and to derive multimodel abundance estimators that explicitly address uncertainty about which model parameterization best represents truth. Further, it is possible to borrow information on detection probability across several populations when data are sparse. While promising, these methods have not been evaluated using mark-recapture data from populations of known abundance, and thus far have largely been overlooked by ecologists. In this paper, we explored the utility of newly developed mark-recapture methods for estimating the abundance of 12 captive populations of wild house mice (Mus musculus). We found that mark-recapture methods employing individual covariates yielded satisfactory abundance estimates for most populations. In contrast, model sets with heterogeneity formulations consisting solely of mixture distributions did not perform well for several of the populations. We show through simulation that a higher number of trapping occasions would have been necessary to achieve good estimator performance in this case. Finally, we show that simultaneous analysis of data from low abundance populations can yield viable abundance estimates.

Self‐regulation within outbreak populations of feral house mice: a test of alternative models

1. Outbreaks of feral house mice, Mus domesticus, in Australia represent a fundamental failure of the behavioural control mechanisms of population density, as proposed in the hypothesis of self-regulation. 2. Mice have the potential to keep numbers in check via a suite of spacing behaviours; however, the self-regulation hypothesis implies that some social change occurs that permits the population to erupt. It also suggests that at different phases of an outbreak, distinct patterns of social activity are evident. 3. We compare predictions from two models encapsulating the self-regulation hypothesis as applied to feral house mice in south-eastern Australia. Each model may be distinguished by the timing of aggressiveness between mice that leads to a closed social system. We compare individual turnover, residency and territoriality in each sex and age cohort during the increase, peak and low phases of a population outbreak that peaked in 2001. 4. The activity of 438 mice was monitored via intensive mark-recapture trapping and an automated event recording system that detected the activity of 300 marked individuals at burrow entrances. 5. Our findings support the second model, which suggests that mice switch from an almost asocial structure at low densities to a territorial system as abundance increases. Adult females appear more likely than males or juveniles to make the significant social shift. The trigger for this change remains unclear and several alternative mechanisms are proposed.

Short- and long-term demographic changes in house mouse populations after control in dryland farming systems in Australia

In Australia, outbreaks of house mice (Mus domesticus) cause significant damage to agricultural crops. Rodenticides are used to reduce damage to crops, but the demographic consequences of applying rodenticides are poorly understood. Furthermore, it is not known whether the reduction induced by rodenticides would be similar to that of a natural crash in abundance at the end of mouse outbreaks. I compared the demographic responses of populations of mice to broad-scale field application of fast-acting, acute rodenticides (strychnine and zinc phosphide) in three grain-growing regions of Australia on baited and unbaited sites through live-trapping of mouse populations before baiting and up to four months after baiting. The reductions in population density in each region immediately after baiting were <40%, 92% and 98%. There were few consistent changes in demographic responses across the three regions for bodyweight (no change, increased or decreased), proportion of juveniles (increased or decreased), sex ratio (no change or bias towards females), survival (no change or decreased) and relative body condition (no change or increased). The differences in demographic responses appeared to be related to differences in the efficacy of the rodenticide. A natural crash in densities occurred over a 2–4-week period after baiting and induced a >85% decline in population densities across all regions on baited and unbaited sites. The natural crash caused increases and decreases in bodyweights, a reduction in the proportion of juveniles, male bias, poor survival and poor relative body condition. Poor survival was the only demographic parameter that was consistent for baiting and the natural crash. Five of seven demographic responses for mice during the natural crash were similar to those found in the literature for the decline phase of cyclic vole and lemming populations in the Northern Hemisphere. These results raise the question of whether mouse populations should be baited if a natural crash would occur anyway, but the timing of the natural crash is always uncertain and rodenticides are inexpensive.

Predation of beech seed by mice: effects of numerical and functional responses

SummaryThe functional response of post‐dispersal seed predators (house mouse,Mus musculus) to absolute densities of southern beech seed (Nothofagus solandrivar.cliffortioides) was studied in laboratory and field trials. House mice showed a Type II (hyperbolic) functional response to seed availability and this was not modified by the presence of an alternative food source.Maximum daily intake rate of beech seeds during field trials averaged 1042 seeds mouse−1. This is sufficient to provide house mice with both the energy and protein required for growth and reproduction.We explicitly incorporated the functional response into the numerical response of house mice to beech seed, measured for field populations monitored in a New Zealand beech forest. House mice showed a strong numerical response to beech seed intake rate that was modified by some density‐dependent mechanism(s).We developed a model that simulated seedfall, house mouse population growth and seed reserve depletion over one year. We found that the previously reported decline in house‐mouse populations in beech forests during spring and summer is likely to be related to spring beech seed germination that renders seed no longer available as a food source for house mice.From our simulation model it does not appear that house‐mouse populations can completely eat‐out beech seed reserves prior to germination in a year of large seedfall. ‘Masting’ behaviour in New Zealand native beech trees is therefore sufficient to satiate an eruptive population of an exotic mammalian omnivore, despite the lack of a long co‐evolutionary interaction.

Kin interactions and changing social structure during a population outbreak of feral house mice

Populations of feral house mice (Mus domesticus L.) in Australia undergo multiannual fluctuations in density, and these outbreaks may be partly driven by some change in behavioural self-regulation. In other vertebrate populations with multiannual fluctuations, changes in kin structure have been proposed as a causal mechanism for changes in spacing behaviour, which consequently result in density fluctuations. We tested the predictions of two alternative conceptual models based on kin selection in a population of house mice during such an outbreak. Both published models (Charnov & Finerty 1980; Lambin & Krebs 1991) propose that the level of relatedness between interacting individuals affects their behavioural response and that this changes with population density, though the nature of this relationship differs between the two models. Neither of the models was consistent with all observed changes in relatedness between interacting female mice; however, our results suggested that changes in kin structure still have potential for explaining why mouse outbreaks begin. Therefore, we have developed a variant of one of these conceptual models suggesting that the maintenance of female kin groups through the preceding winter significantly improves recruitment during the subsequent breeding season, and is therefore necessary for mouse outbreaks. We provide six testable predictions to falsify this hypothesis.

Transmission of two Australian strains of murine cytomegalovirus (MCMV) in enclosure populations of house mice (Mus domesticus)

To control plagues of free-living mice (Mus domesticus) in Australia, a recombinant murine cytomegalovirus (MCMV) expressing fertility proteins is being developed as an immunocontraceptive agent. Real-time quantitative PCR was used to monitor the transmission of two genetically variable field strains of MCMV through mouse populations after 25% of founding mice were infected with the N1 strain, followed by the G4 strain 6 weeks later. Pathogen-free wild-derived mice were released into outdoor enclosures located in northwestern Victoria (Australia). Of those mice not originally inoculated with virus, N1 DNA was detected in more than 80% of founder mice and a third of their offspring and similarly, G4 DNA was detected in 13% of founder mice and in 3% of their offspring. Thus, prior immunity to N1 did not prevent transmission of G4. This result is promising for successful transmission of an immunocontraceptive vaccine through Australian mouse populations where MCMV infection is endemic.

Molecular and quantitative genetic divergence among populations of house mice with known evolutionary histories

Evolutionary biologists have long been interested in the processes influencing population differentiation, but separating the effects of neutral and adaptive evolution has been an obstacle for studies of population subdivision. A recently developed method allows tests of whether disruptive (ie, spatially variable) or stabilizing (ie, spatially uniform) selection is influencing phenotypic differentiation among subpopulations. This method, referred to as the F(ST) vs Q(ST) comparison, separates the total additive genetic variance into within- and among-population components and evaluates this level of differentiation against a neutral hypothesis. Thus, levels of neutral molecular (F(ST)) and quantitative genetic (Q(ST)) divergence are compared to evaluate the effects of selection and genetic drift on phenotypic differentiation. Although the utility of such comparisons appears great, its accuracy has not yet been evaluated in populations with known evolutionary histories. In this study, F(ST) vs Q(ST) comparisons were evaluated using laboratory populations of house mice with known evolutionary histories. In this model system, the F(ST) vs Q(ST) comparisons between the selection groups should reveal quantitative trait differentiation consistent with disruptive selection, while the F(ST) vs Q(ST) comparisons among lines within the selection groups should suggest quantitative trait differentiation in agreement with drift. We find that F(ST) vs Q(ST) comparisons generally produce the correct evolutionary inference at each level in the population hierarchy. Additionally, we demonstrate that when strong selection is applied between populations Q(ST) increases relative to Q(ST) among populations diverging by drift. Finally, we show that the statistical properties of Q(ST), a variance component ratio, need further investigation.

Allozymic Polymorphism Among 14 Populations of the House Mouse, Mus musculus domesticus, From Greece

Nineteen loci from 239 individuals of the house mouse Mus musculus domesticus (Rodentia, Muridae) were analyzed by means of thin layer electrophoresis. The mice were collected from 14 localities of Greece mainly confined to the area of NW Peloponnese, where a Robertsonian (Rb) system is observed. The individuals were chromosomally characterized by nine diploid numbers, the 2n = 24, 26, 27, 28, 29, 30, 31, 32, and 40. The statistic elaboration revealed that all 14 populations studied were not characterized by cohesive demic structure and high inbreed levels while the gene flow among them has resulted in low levels of genetic differentiation. The resulting values for Nei's genetic distance corresponded to distances known for the level of geographical populations of M. musculus. Wagner's cladogram for the phylogenetic relations between the populations studied implied that it is the diploid number, rather than the geographical factor, that characterizes or dominates each population, which mainly influences the phylogenetic relationships.

Residues of brodifacoum and other anticoagulant pesticides in target and non‐target species, Nelson Lakes National Park, New Zealand

Cereal‐based bait containing 20 ppm brodifacoum was used in bait stations continuously from December 1997 to August 2000 in the Rotoiti Nature Recovery Project area, for controlling brushtail possum (Trichosurus vulpecula), ship rat (Rattus rattus), and house mouse (Mus musculus) populations. Concurrently (and before and after), baits containing brodifacoum, bromadiolone, flocoumafen, coumatetralyl, or warfarin were also used in St Arnaud village and on farms immediately adjacent to the project area. Brodifacoum residues were detected in the livers of 234 mammals from eight species, and two birds from two species captured alive, and a further seven birds from five species found dead in the project area (cause of death unknown). The highest concentration of brodifacoum residues in mammalian livers was recorded during the period brodifacoum was used in the project area. However, residues were present in the livers of some individuals at least 24 months after brodifacoum use in the project area stopped. These residues may have persisted in animals surviving brodifacoum use in the project area, and/or been transported into the area by animals moving to and from the adjacent village and farms, where brodifacoum use continued. Residues of flocoumafen, coumatetralyl, or warfarin, used only in the village and on farms, were also detected in the livers of animals captured up to at least 8 km from the nearest source. The results concur with other studies cautioning against indiscriminate or prolonged use of persistent anticoagulants for vertebrate pest control. However, the risks from such pesticides must be balanced against the demonstrable benefits of reducing pest abundance.

Agonistic onset during development differentiates wild house mouse males (Mus domesticus)

Wild house mouse populations have been suggested to locally adapt to varying dispersal regimes by expressing divergent aggressivity phenotypes. This conjecture implies, first, genetic polymorphism for dispersive strategies which is supported by the finding of heritable variation for male dispersal tendency in feral house mice. Secondly, aggressivity is assumed to translate into dispersal rates. This speculation is reinforced by experimental evidence showing that non-agonistic males display lower dispersal propensity than same-aged males that have established agonistic dominance. However, the actual ontogenetic behavioural pattern and its variability among populations remain unknown. Hence, in this study the timing of agonistic onset is quantified within laboratory-reared fraternal pairs, and compared between descendants from two different feral populations. Males from the two populations (G and Z) differed strongly in agonistic development, as Z fraternal pairs had a 50% risk of agonistic onset before 23.5+/-2.7 days of age, while this took 57.3+/-5.4 days in males from population G. This difference coincided with significant genetic differentiation between the males of the two populations as determined by 11 polymorphic microsatellite markers. Furthermore, in population G, males from agonistic and amicable fraternal pairs exhibited significant genetic differentiation. These results corroborate the supposition of genetic variability for dispersive strategies in house mice, and identify the ontogenetic timing of agonistic phenotype development as the potential basis for genetic differentiation. This opens a unique opportunity to study the genetic determination of a complex mammalian behavioural syndrome in a life history context, using a simple laboratory paradigm.

Origin of the House Mice (Superspecies Complex Mus musculus sensu lato) from the Transcaucasia Region: A New Look at Dispersal Routes and Evolution

We analyzed our results and literature evidence on variability of nuclear protein genes in 39 populations of eight synanthropic and wild species of house mice (superspecies complexes Mus musculus and M. spicilegus) from Transcaucasia, Eastern and Western Europe, Near and Middle East, Central, South, and East Asia, and Cuba. These data were for the first time ever combined into a single database by unification of nomenclature of 21 loci examined by different authors in 39 populations. Analysis of geographical allele distribution have shown that populations of domestic Transcaucasian mice are close to Indo-Pakistani populations of form oriental of the species M. castaneus, which preserved a high level of ancestral polymorphism. We concluded that a very heterogeneous, rich gene pool of house mice from Transcaucasia could not develop only by secondary contacts of differentiated M. musculus s. str. and M. domesticus forms, since it is similar to the ancestral gene pool of the superspecies complexes M. musculus and M. spicilegus. In this context, unique characteristics of some Central Asian populations were examined; these populations may have served as a "transit station" in the dispersal of synanthropic house mice forms. We suggest that the Transcaucasian populations are genealogically closely related to an early Near East form of M. musculus, which, as M. domesticus and M. castaneus, split from the common ancestor and preserved nondifferentiated pool of ancestral alleles of protein genes. This hypothesis admits the involvement of differentiated species M. musculus s. str. and M. domesticus in the ultimate formation of the gene pool of Transcaucasian house mice. Apparently, these populations resulted from alternation and (or) "overlapping" of different evolutionary processes. A scenario suggesting that hybrid events having occurred in Transcaucasia at different times, were "superposed" on the gene pool of the ancient autochtonous population of house mice from this region seems most plausible. Analysis of allozyme variability in the modern Transcaucasian Mus populations could not always distinguish between ancestral polymorphism and hybridization consequences.

Adaptations of animals to commensal habitats: population dynamics of house mice Mus musculus domesticus on farms

Summary Commensal mammals live in habitats that appear to provide both benefits and costs compared with natural and semi‐natural (non‐commensal) habitats. These commensal habitats offer potentially rich food resources but are also characterized by instability in time and space. We expected to demonstrate that animals in these habitats have high reproductive rates to counter high mortality rates and show flexibility in spatial organization. House mice (Mus musculus domesticus) are unusual because they are able to persist entirely in both commensal and non‐commensal habitats, and so can provide a test of the distinctiveness of commensal populations. We studied populations of commensal house mice on two neighbouring farms in North Yorkshire, UK, for 2 years by capture–mark–recapture using the robust design. A total of 568 house mice were captured, with a total of 1053 recaptures. Population size varied from nine to 93 individuals, estimated with closed population models. Apparent survival was surprisingly low (0·54 per month) and was best modelled as constant across age, sex and time. In situ reproductive recruitment occurred throughout the study and was numerically more important than immigration. Immigration was important during only two intervals and was probably from untrappable areas within the study site. Breeding throughout the year allowed the population to persist despite low survival rates. The results suggested that births and deaths had more influence on the overall population dynamics than movement. The population was divided into subgroups to represent the territorial, demic structure known to be present in commensal house mouse populations. Dispersal between subgroups within the population was limited, representing only 6·6% of recaptures. The low rates of dispersal suggested that house mice responded to their environment as consisting of aggregated patches of suitable habitat. In comparison, non‐commensal house mice have lower mortality, seasonal breeding, and individuals move more frequently and further than commensal house mice. These differences illustrate the responses of house mice to the specific opportunities and demands of commensal habitats and demonstrate the importance of a flexible life‐history strategy for animals exploiting these habitats.

Habitat structure mediates the non‐lethal effects of predation on enclosed populations of house mice

SummaryPrey behavioural changes in response to predation risk can result in significant effects on prey body growth rates and reduced reproductive output, with resultant impacts on prey population dynamics. This paper examines the influence of habitat structure on these non‐lethal impacts of predation using a model, field‐based experimental system, with house mice as prey.Three treatments were employed in eight 50 × 50 m pens that contained mice, but allowed access to a suite of free‐living vertebrate predators, which included feral foxes, feral cats and native raptors: a treatment where the natural grassland vegetation in the pens was maintained at a height < 10 cm; a treatment where small, felled cypress pine trees covered with wire netting were added to low grassland vegetation to create refuge areas covering 10–15% of the area in a pen; and a treatment where predators were excluded from a 25 × 25 m section of some pens with an underlying grassland structure. A 5 × 5 grid of felled trees was added to grassland and predator‐exclusion pens to allow assessment of mouse behaviour.Mice in grassland pens avoided open areas, had lower body growth rates, and began breeding later in spring than mice in both predator‐exclusion areas, where they foraged more readily in the open, and in refuge pens, where mice avoided open areas but had safe access to supplementary food located within the refuge. These results occurred despite mouse population densities being much lower in grassland pens, and presumably competition for food being much less, compared with under the other treatments.The results indicate predators can have significant non‐lethal impacts on prey, and these effects can be mediated by habitat structure.

A First Report of HSRs in Chromosome 1 of Mus Musculus Domesticus from South America

Despite its ubiquitous geographic distribution, the domestic mouse has not received attention from the cytogenetic point of view in South America, possibly because of its apparently conservative karyotype. In this paper we report, for the first time outside the Old World, a polymorphism for a Homogeneously Staining Region (HSR) between G-bands C5 and D of chromosome 1 in commensal populations of house mice from Misiones and Corrientes provinces (Argentina). The HSR is of the Mus musculus domesticus type as has been previously described for European populations. The comparison at the cytogenetic and molecular levels of chromosome polymorphisms such as the one described in this paper, between Old and New World house mice can help clarify the origin and antiquity of New World populations.

Can farm-management practices reduce the impact of house mouse populations on crops in an irrigated farming system?

The impacts of a range of farm-management practices on house mouse (Mus domesticus) populations were tested in a large replicated field study in a complex irrigated farming system in southern New South Wales, Australia. An advisory panel, made up of farmers, extension officers, industry representatives and scientists developed a series of best-practice farm-management actions to minimise the impact of mice. Twelve experimental sites were split into six treated sites, where farmers were encouraged to conduct the recommended practices, and six untreated sites, where farmers conducted their normal farming practices. Mouse abundance was generally low to moderate for the 4-year project (5–60% adjusted trap success). We found significant reductions in population abundance of mice on treated sites when densities were moderate, but no differences when densities were low. Biomass of weeds and grasses around the perimeter of crops were significantly lower on treated sites because of applications of herbicide sprays and grazing by sheep. We could not detect any significant difference in mouse damage to crops between treated and untreated sites; however, levels of damage were low (<5%). Yields of winter cereals and rice crops were significantly higher on treated sites by up to 40%. An analysis of benefits and costs of conducting farming practices on treated sites compared with untreated sites showed a 2 : 1 benefit to cost ratio for winter cereals, 9 : 1 for rice and 4 : 1 for soybeans.

Pathogens of house mice on arid Boullanger Island and subantarctic Macquarie Island, Australia.

Studies on island populations of house mice (Mus domesticus) and their viruses reveal insights into viral persistence in isolated communities. We surveyed the ectoparasites, endoparasites, and antiviral antibodies for 11 murine viruses and two bacteria of house mice inhabiting two islands off Australia. House mice on Boullanger Island were seropositive to two viruses, murine cytomegalovirus and epizootic diarrhea of infant mice. On subantarctic Macquarie Island, house mice were seropositive for five viruses: murine cytomegalovirus, lymphocytic choriomeningitis virus, mouse parvovirus, epizootic diarrhea of infant mice, and Theiler's murine encephalomyelitis virus. The diversity of antiviral antibodies was lower among populations of house mice on islands than those inhabiting mainland Australia. The decreased diversity of viruses in island populations of house mice may be a function of which agent the founder mice transfer to the island and related to the low densities which the host population may periodically reach over time.

Motivational and Heritable Determinants of Dispersal Latency in Wild Male House Mice (Mus musculus musculus)

AbstractDivergence of dispersal regimens has been suggested to be the selective basis for the evolutionary differentiation of agonistic phenotypes found in natural populations of house mice. Dispersal propensity may, therefore, be expected to exhibit heritable variation in wild house mice, ultimately related to motivational differences causing observable differences in agonistic behaviour. To test for heritable components in dispersal propensity in wild house mice, father–offspring regressions of dispersal latencies from residential social groups were determined in standardized seminatural social settings. To evaluate potential motivational causes of phenotypic variation in dispersal behaviour, all test animals (fathers, sons, and daughters) were scored prior to the dispersal experiment in a standardized behavioural test, at 60 d of age. Activities were monitored in a 1 m2 square test arena during 10‐min observation periods. Test arenas exhibited four equidistant openings leading to cages containing fresh, own, sibling, or foreign bedding material. The apparatus allowed for scoring anxiety, exploratory activity, and kin preference. Subsequently, test animals were exposed to a resident population in a semi‐natural enclosure providing a dispersal opportunity. Father–son regressions of dispersal latencies were significantly positive, but no significant relationship was found for daughters. Dispersal latency decreased with increasing exploratory activity scores in males, but increased in females. Anxiety as well as kin preferences did not affect dispersal propensity. Hence sex‐linked, motivational components reflect heritable social behaviour variation in male house mice that may ultimately be caused by diverging dispersal regimens.

Abundance Estimators and Truth: Accounting for Individual Heterogeneity in Wild House Mice

We compared the actual abundance of 12 confined populations of wild house mice (Mus musculus) with closed population estimates based on mark-recapture data. Goodness-of-fit tests consistently detected individual heterogeneity in capture probability. Estimators designed to take such heterogeneity into account were expected to perform best, and generally did. However, of the 9 abundance estimators we considered, only Chao's modified moment estimator had no obvious bias and produced confidence intervals that always included the actual population size. The reliability of the 9 estimators and minimum number known alive (MNA), as indices of abundance, was quantified by (1) linear regression, and (2) calculating a Spearman rank correlation coefficient between the ranking obtained using the index and the true ranking. We found Petersen estimates to provide the most accurate ranking, and MNA also performed well. Since Chao's modified moment estimator was designed for populations with high levels of heterogeneity and low average capture probability, we concluded that this is an accurate description of trap response for confined mice populations. Body-length frequency distributions and sex ratios for mice caught once, more than once, and not at all revealed that body length and sex could explain some of the observed individual heterogeneity. Males were more likely to enter traps than females, and large mice were more likely to be trapped than small mice. Field populations of house mice are monitored regularly throughout southeastern Australia because high densities cause considerable damage to wheat crops and place substantial stress on farming communities. We found that field populations trapped during the breeding season exhibit heterogeneity in capture probability similar to that for confined populations, with the same relationships between capture probability, sex, and body length. However, these relationships did not hold for the same field population later in the season when breeding and recruitment had ceased. Chao's modified moment estimator was applied to field data and found to be stable when populations were trapped for ≥5 nights, but unreliable trapped for <5 nights.

The mode of evolution of molecular markers in populations of house mice under artificial selection for locomotor behavior.

A complete understanding of the mode of evolution of molecular markers is important for making inferences about different population genetic parameters, especially because a number of studies have reported patterns of allelic variation at molecular markers that are not in agreement with neutral evolutionary expectations. In the present study, house mice (Mus domesticus) from the fourteenth generation of a selection experiment for increased voluntary wheel-running activity were used to test how selection on a complex behavior affects the distribution of allelic variation by examining patterns of variation at six microsatellite and four allozyme loci. This population had a hierarchical structure that allowed for simultaneous testing of the effects of selection and genetic drift on the distribution of allelic variation by comparing observed patterns of allele frequencies and estimates of genetic divergence at multiple hierarchical levels to expectations under models of neutral evolution. The levels of genetic divergence among replicate lines and between selection groups, estimated from microsatellite data or pooled microsatellite and allozyme data, were not significantly different from expectations under neutral evolution. Furthermore, the pattern of change of allele frequencies between the base population and generation 14 was largely in agreement with expectations under neutral evolution (although the PGM locus exhibited a pattern of change within populations that was difficult to explain under neutral evolution). Overall the results generally provide support for the neutral evolution of molecular markers.

Matching productivity to resource availability in a small predator, the stoat (Mustela erminea)

Stoats (Mustela erminea), introduced to New Zealand in the late nineteenth century, are common in New Zealand beech (Nothofagus sp.) forests, where populations of feral house mice (Mus musculus) fluctuate between years much as voles do in the northern hemisphere. We present new field evidence and two models demonstrating (i) a strong correlation between density indices for young stoats in summer and for mice in the previous spring, and (ii) a significant linear relationship between productivity per female and spring density of mice up to 25 mice captures per 100 trap-nights. These models confirm that short-lived small mustelid predators dependent on fluctuating populations of prey have evolved means of matching their productivity to the prospects of success across a wide range, from total failure in rodent crash years to &gt;12 independent young per female in rodent peak years. We suggest that the enhanced reproductive success of female stoats when rodents are abundant is due to a combination of critical improvements in both the reproductive physiology and the foraging behaviour of female stoats in rodent peak years. Conversely, a drastic shortage of rodents increases the mortality of embryos and nestlings, while the adult females are able to survive, and even remain relatively fat, on other foods.