Female Deer Mice Research Articles

Differential impact of pegfilgrastim, a recombinant human granulocyte colony stimulating factor, on the neutrophil count of male and female deer mice (Peromyscus maniculatus bairdii)

BackgroundAn increasing body of research implicates inflammatory processes, including alterations in the neutrophil-lymphocyte ratio (NLR), in the pathophysiology of psychiatric illness. The deer mouse (Peromyscus maniculatus bairdii) is commonly studied for its naturalistic expression of compulsive-like behaviour. Towards future efforts to gain an understanding of how innate and adaptive immune processes might be involved in this model, we aimed to study the effects of pegfilgrastim, a pegylated recombinant human granulocyte colony-stimulating factor (g-CSF) analogue, on the NLR of both male and female deer mice.MethodsBriefly, 54 deer mice (equally distributed between sexes) were exposed to a single injection with either control or pegfilgrastim (0.1 or 1 mg/kg) (n = 18 per group). Six mice of each group (three per sex) were euthanized on days two, four and seven post-administration, their blood collected and the NLR calculated. Data were analysed by means of ordinary three-way ANOVA, followed by Bonferroni post-hoc testing.ResultsIrrespective of dose, pegfilgrastim resulted in higher NLR values in mice of both sexes at days four and seven of testing. However, female mice exposed to the higher dose, presented with significantly higher NLR values irrespective of time, compared to male mice exposed to the same.ConclusionThe data generated from this work highlight important dose- and sex-specific aspects of pegfilgrastim with female mice showing heighted elevation of the NLR in response to high-dose pegfilgrastim administration only. Since the innate immune components of male and female deer mice is differentially sensitive to g-CSF stimulation, our results provide a useful basis for further study of sex-specific immunological processes in deer mice.

Physiological links with behavior and fitness: The acute adrenocortical response predicts trappability but not survival in male and female deermice

The “Cort-Fitness” hypothesis predicts a negative relationship between baseline glucocorticoids (GCs) and fitness, although evidence for this hypothesis remains mixed. Such ambiguity could partially exist because blood GCs, typically used in field studies, can fluctuate too rapidly to measure accurately, while the relationship between GCs and trappability is often neglected. Here, by addressing these factors, we examined relationships between GC measures and survival of North American deermice (Peromyscus maniculatus; hereafter deermice) as a model system. To do this, we used more stable GC measures, including the integrated measures of baseline and stress response fecal corticosterone metabolites (FCMs), and downstream measures of neutrophil/lymphocyte ratio (N/L ratio), and body condition score (BCS), to characterize their relationships with survival and trappability. Over two years, deermice were live-trapped monthly, evaluated for BCS, and sampled for feces and blood. Stress response FCMs were evaluated only at first capture. Mark-recapture models, with GC measures as predictors of either survival or trappability, were compared to identify top models. We found that stress response FCMs negatively predicted trappability, and weaker evidence that BCS positively predicted survival. Although the latter provides some support for the “Cort-Fitness” hypothesis, there was no support when using integrated measures. Instead, our findings suggest that deermice with a lower adrenocortical response (i.e. stress response FCMs) were more likely to be captured. Therefore, GC-trappability relationships must be investigated in field studies to avoid linking the wrong GC profile to fitness, and physiological measures other than blood GCs may be useful for detecting GC-fitness patterns.

Odor-based mate choice copying in deer mice is not affected by familiarity or kinship.

Individuals pay attention to the social and mate decisions of others and use these to determine their own choices, displaying mate choice copying. The present study with deer mice, Peromyscus maniculatus, showed that females copied the odor preferences and appetitive components of the mate choice of other females. It was found that an association between male and female odors, which is indicative of the apparent interest expressed by a female in a male, enhanced the preference of another female for the odors of that male. This socially learned odor preference lasted for at least 24h and extended to a preference for the actual male that was the odor source. Neither kinship nor prior familiarity with the female whose odor was presented had a significant influence on the degree of odor-based mate choice copying displayed. These findings show that female deer mice can engage in mate choice copying using the odor-based social interest and mate choice of other females.

Small Mammals in Saltcedar (Tamarix ramosissima)–Invaded and Native Riparian Habitats of the Western Great Basin

AbstractInvasive saltcedar species have replaced native riparian trees on numerous river systems throughout the western United States, raising concerns about how this habitat conversion may affect wildlife. For periods ranging from 2 to 11 yr, I used live-trapping to monitor small mammal populations in paired saltcedar and native riparian woodlands at four sites in western Nevada and eastern California. Heteromyid rodents, such as Merriam's and Ord's kangaroo rats, were more likely to occur in saltcedar habitats, but other rodent species, particularly the montane vole and western harvest mouse, occurred more often in native habitats, and this balanced species richness in habitat comparisons. The most common species at all sites, the deer mouse, did not show any consistent differences in abundance or in mean body mass between the two habitat types. However, the ratio of captured male to female deer mice was higher in saltcedar than native habitats at two sites. Deer mice as well as Ord's kangaroo rats also had higher rates of being recaptured following initial capture in native habitats, which may have been due to fewer transient individuals occurring in these habitats. By contrast, Merriam's kangaroo rats may have been more transient in native habitats because they were more likely to be recaptured in saltcedar. Individuals of two species, pinyon mouse and white-tailed antelope ground squirrel, had greater mean body mass in native habitats than they did in saltcedar, implying that they may have maintained superior condition in native habitats.

Long‐Term Patterns of Immune Investment by Wild Deer Mice Infected with Sin Nombre Virus

Immunocompetence of animals fluctuates seasonally, However, there is little consensus on the cause of these fluctuations. Some studies have suggested that these patterns are influenced by changes in reproductive condition, whereas others have suggested that differences result from seasonal variations in energy expenditures. The objective of our study was to examine these contrasting views of immunity by evaluating seasonal patterns of immune response and reproduction in wild populations of deer mice Peromyscus maniculatus exposed to Sin Nombre virus (SNV). Over three consecutive fall (September, October, November) and three consecutive spring (March, April, May) sampling periods, we used titration enzyme-linked immunosorbent assay (ELISA) to quantify virus-specific antibody production in 48 deer mice infected with SNV. Levels of reproductive hormones were quantified using ELISA. SNV antibody titers reached their lowest level during November (geometric mean titer [GMT] = 420) and their highest levels during September (GMT = 5,545) and May (GMT = 3,582), suggesting that the immune response of deer mice to SNV has seasonal patterns. The seeming decrease in antibody titer over winter coupled with the consistency in body masses suggests that during winter, immunocompetence may be compromised to offset the energetic costs of maintenance functions, including those associated with maintaining body mass. Deer mice showed distinct sex-based differences in SNV antibody production, with males producing higher antibody titers (GMT = 3,333) than females (GMT = 1,477). Levels of reproductive hormones do not appear to influence antibody production in either males or females, as there was no correlation between estradiol concentrations and SNV antibody titer in female deer mice (r² = 0.26), nor was there a significant relationship between levels of testosterone and SNV antibody titers in males (r² = 0.28). Collectively, this study demonstrates that immunocompetence of wild deer mice is seasonally variable; however, reproduction is not the primary stressor responsible for this variation. Rather, the data suggest that deer mice may compromise immunocompetence during winter to offset other maintenance costs during this period.

Cold acclimation in Peromyscus: individual variation and sex effects in maximum and daily metabolism, organ mass and body composition

We studied metabolic and organ mass responses to thermal acclimation (7 weeks at 5 degrees C or 23 degrees C) in deer mice, Peromyscus maniculatus. Cold acclimation resulted in significantly higher maximal oxygen consumption in thermogenesis (V(O(2)max)) and daily mean oxygen consumption (V(O(2)mean)), an increase in the mass of most visceral organs, a lower absolute body fat and a marginally significant increase in hematocrit. The mass of digestive organs and body fat content differed significantly between sexes. Acclimation effects on fat content were more pronounced in females. Variation in heart and lung mass was positively correlated with V(O(2)max) and V(O(2)mean), while body fat content was negatively correlated with both traits. Nonetheless, a large fraction of the metabolic difference between cold- and warm-acclimated groups remained unexplained. Associations between traits at lower levels of biological organization measured here and whole-organism energetics remained consistent across acclimation temperatures, except for the correlation between kidney mass and V(O(2)mean), which was positive and significant in cold acclimation and negligible following warm acclimation. We conclude that: (1) V(O(2)max) and V(O(2)mean) share a common physiological basis that remains overall the same across acclimation regimes; (2) changes in these traits are associated primarily with changes in heart mass; and (3) male and female deer mice respond differently to thermal acclimation, possibly due to differences in reproductive allocation.

Differential Resource Allocation in Deer Mice Exposed to Sin Nombre Virus

The resource allocation hypothesis predicts that reproductive activity suppresses immunocompetence; however, this has never been tested in an endemic disease system with free-ranging mammals. We tested the resource allocation hypothesis in wild deer mice (Peromyscus maniculatus) with natural exposure to Sin Nombre Virus (SNV). Immunocompetence was estimated from the extent of swelling elicited after deer mice were injected with phytohemagglutinin (PHA); swelling is positively correlated with immunocompetence. After livetrapping deer mice, we determined their reproductive state and SNV infection status. Males were more likely to be seropositive for SNV than females (37% vs. 25%) and exhibited 10% less swelling after PHA injection. The swelling response of females differed with both infection status and reproductive condition. There was also a significant infection status by reproductive condition interaction: non-reproductive, seropositive females experienced the least amount of swelling, whereas females in all other categories experienced significantly greater swelling. The swelling response of males differed with both SNV infection status and reproductive condition, but there was no significant infection status by reproductive condition interaction. Seronegative males elicited greater swelling than seropositive males regardless of reproductive status. In contrast to the resource allocation hypothesis, these results do not indicate that reproductive activity suppresses immunocompetence of deer mice but rather suggest that chronic SNV infection reduces immunocompetence. Sex-based differences in swelling indicate that SNV modulates the immune system of female deer mice differently than it does that of males, particularly during reproduction. We propose that differences in resource allocation between males and females could result from inherent sex-based differences in parental investment.

Deer Mouse Movements in Peridomestic and Sylvan Settings in Relation to Sin Nombre Virus Antibody Prevalence

Prevalence of antibody to Sin Nombre virus (SNV) has been found to be nearly twice as high in deer mice (Peromyscus maniculatus) in peridomestic settings as in sylvan settings in two studies in Montana and one in New Mexico. We investigated whether this difference may be related to a difference in deer mouse movements in the two settings. We used radiotelemetry to determine home range size and length of movement for 22 sylvan (1991-1992) and 40 peridomestic deer mice (1995-1999). We also determined the percentage of locations inside versus outside of buildings for peridomestic mice. Though variable, average home range size for female deer mice was significantly smaller for peridomestic deer mice than for sylvan deer mice. The smaller home range in peridomestic settings may concentrate shed SNV, and protection from solar ultraviolet radiation inside buildings may increase environmental persistence of SNV. Both these factors could lead to increased SNV exposure of deer mice within peridomestic populations and result in higher antibody prevalence. Peridomestic deer mice moved between buildings and outside areas, which is evidence that SNV can be transmitted between peridomestic and sylvan populations.

Body and organ mass in agouti and non-agouti deer mice ( Peromyscus maniculatus)

Body, adrenal, brain, heart, liver, kidney, spleen and testis masses were determined for agouti and non-agouti deer mice ( Peromyscus maniculatus gracilis) of both sexes. Body mass was highest for non-agouti females and lowest for agouti females; and sex differences in body mass were significant for agouti, but not non-agouti, deer mice. Adrenal, brain and liver masses were similar between color morphs; heart mass was greater in agouti males; and kidney, spleen and testis masses were all significantly greater for non-agouti deer mice. Splenomegaly in non-agouti deer mice was prominent, as spleens of non-agouti deer mice were 50% larger than those of agouti animals. Sex differences varied across organs and color morphs. For both color morphs, males had heavier adrenals and brains, whereas females had heavier livers and spleens. Kidney and heart mass was greater for female non-agouti deer mice, but for agouti animals, heart mass was greater in males and kidney mass differed little between the sexes. For both color morphs, testes and spleen mass was altered by photoperiod in 72 deer mice housed under short- or long-day conditions and the effect was stronger in non-agouti animals. This is the first report of splenomegaly and sex-specific body mass differences associated with the non-agouti allele.

Thyroxine levels in agouti and non-agouti deer mice ( Peromyscus maniculatus)

Total thyroxine was assessed by radioimmunoassay for 58 female deer mice of two subspecies ( Peromyscus maniculatus bairdii and P. m. gracilis) and two color morphs (agouti and non-agouti). P. m. bairdii of both color-morphs had significantly higher mean thyroxine levels than P. m. gracilis. Non-agouti deer mice of both subspecies had significantly higher mean thyroxine levels than agouti deer mice. This is the first report of thyroid hormone differences associated with the non-agouti allele.

Sex differences in conditioned taste aversion and in the effects of exposure to a specific pulsed magnetic field in deer mice Peromyscus maniculatus

Although conditioned taste aversion (CTA) has been investigated and described in laboratory rodents and domestic animals, less is known regarding wild rodents. Here, we describe CTA in males and females of a “wild” species of rodent, the deer mouse (Peromyscus maniculatus). In addition, as CTA has often been induced by exposure to intense electromagnetic, X or γ, radiation, in a second study, we also investigated the effects of a specifically designed, pulsed extremely low-frequency and low-intensity magnetic field on the flavor preferences of male and female deer mice. The results of these investigations showed that: (i) deer mice quickly developed a marked CTA for a novel flavor after a single pairing with LiCl; (ii) although the intensity of the CTA was the same in males and females, there was a sex difference in the duration of the flavor aversion, with males displaying it for a longer period (4 days) than females (3 days); (iii) both males and females showed a rapid and complete extinction of the aversion, in contrast to what has been reported for laboratory rodents; (iv) there was no recovery of CTA on re-test 10 days after extinction; (v) neither male or female deer mice developed a taste aversion as a consequence of exposure to a weak electromagnetic field; and (vi) there was a sex difference in response to the magnetic field, with exposure to the magnetic field significantly enhancing novel taste preference in male but not in female deer mice. Overall, our results show that there are several sex differences in the behavior of deer mice, both in the characteristics of the CTA and in the response to magnetic field exposure. The sex differences are discussed in terms of a sexually dimorphic sensitivity to experimental manipulation and the induction of stress and/or anxiety.

Responses to lactation and cold exposure by deer mice (Peromyscus maniculatus).

Recently, much interest has been expressed in understanding how animals use phenotypic plasticity of tissue size and function to meet increased metabolic demands. We set out to learn (i) whether female deer mice (Peromyscus maniculatus) given lactation (two to seven pups per litter), cold (5 degrees C), or cold plus lactation as energy demands display phenotypic plasticity in organ size and function; (ii) whether that plasticity is similar to laboratory mice given the same demands; and (iii) whether lactational performance in deer mice is derived from limits on central or peripheral organs. We found that deer mice responded to lactation by increasing digestible food intake and increasing the masses of the stomach, small intestine, cecum and liver, and the length of the small intestine. Heart mass was lower in lactating than in nonlactating mice. Cold exposure also caused increases in digestible food intake and increases in the masses of the small intestine, kidney, and heart. We conclude that deer mice display organ tissue plasticity in response to both lactation and cold exposure in a similar manner to laboratory mice. We also conclude that deer mice are not limited by central processing organs because they are able to increase digestive organ size continuously with increased energetic demands.

The effects of dietary protein content on growth and maturation in deer mice

Growth and female maturation appear to be limited by the availability of dietary protein in natural populations of deer mice (Peromyscus maniculatus borealis) in the Kananaskis Valley, Alberta. We examined the effects of dietary protein content on nestling growth rates and sexual maturation of female deer mice in two laboratory experiments. In the first, mice whose mothers were fed a low-protein mixture of sunflower seeds and oats (14% protein) exhibited slow growth prior to weaning and those fed high-protein cat food (30% protein) postweaning showed compensatory growth. Preweaning but not postweaning diet quality affected the proportion of females who were sexually mature at 42 days of age. Therefore, while deficient nestling growth can be compensated for, the effects of a low-quality maternal diet during lactation may have lasting effects on the maturation of female offspring. In the second experiment, mice raised on isocaloric diets of 14, 20, and 30% protein did not differ in growth as nestlings or juveniles. Differences among the three diets in the proportion of mature females at 42 days did not correspond to dietary protein levels as predicted. Dietary protein content from 14 to 30% appear to be sufficient for juvenile mice raised in captivity.

Modulation of endocrine pathways by 4,4′-DDE in the deer mouse Peromyscus maniculatus

4,4′-DDT and 4,4′-DDE are widespread environmental contaminants that cause eggshell thinning in birds, altered sex ratios in the American alligator, and changes in the anal–genital distance in rodents. These contaminants are known to cause some of their toxicity by altering steroid receptor-mediated mechanisms. However, chemical-specific alterations in the expression of hormone-metabolizing enzymes may also be a mechanism for endocrine disruption, by altering the half-life of hormones in critical tissues. Previously, we showed that 4,4′-DDE causes a dose-dependent increase in ethoxyresorufin- O-deethylase (EROD) activity, but not pentoxyresorufin- O-dealkylase (PROD) activity, in the deer mouse. In this study, we demonstrated that 4,4′-DDE elicited a corresponding increase in CYP1A protein expression but not CYP2B using Western blotting and immunoprecipitation. 4,4′-DDE-mediated changes in phase II conjugating enzymes; UDP-glucuronosyltransferase (UGT) and phenolsulfotransferase (ST), were also investigated for the first time. Prepubescent female deer mice were dosed with 4,4′-DDE by gavage on days 1 and 2, then euthanized on day 4. As anticipated, dose-dependent increases in hepatic EROD and MROD activities, but not PROD or BROD, were observed. UGT activity was monitored by incubating liver microsomes and 14C-UDP-GA with potential substrates and measuring incorporation of radioactivity into TLC-resolved glucuronides. Dose-dependent increases in conjugation were observed with p-nitrophenol (a general UGT substrate) but not testosterone. Interestingly, a biphasic dose–response curve was observed for ST activity, with a peak at the 3 mg/kg dose. Dose-dependent increases in CYP1A1 and UGT-specific immunoreactive proteins were observed, suggesting de novo synthesis as a consequence of 4,4′-DDE exposure. We also measured Phase I and II enzymes in deer mouse platelets. Preliminary results indicate that the 4,4′-DDE-induced changes in liver Phase I and II enzyme activity were similar, but not identical, to those found in platelets. These results indicate that environmentally-relevant levels of 4,4′-DDE modulate the activity and expression of CYP1A1 and phase II enzymes in the deer mouse and that certain changes may be measured non-lethally.

Relationships of deer mouse movement, vegetative structure, and prevalence of infection with Sin Nombre virus.

The effects of vegetative structure on movement of deer mice (Peromyscus maniculatus) were examined in two distinct vegetation associations, one near Hesperus and the other near Molina in western Colorado (USA) from June-October 1994 to October 1998. We monitored movement by live-trapping small mammals within Gambel's oak/mixed-grass (Hesperus) and sage brush/juniper (Molina) vegetation types. Vegetative structure differed between the sites with Molina having more cover provided by shrubs and Hesperus having more cover provided by forbs. Adult male deer mice moved greater distances at Hesperus than at Molina. Sub-adult males tended to move greater distances than did adult females. Relative abundances of deer mice tended to differ by season, but the average relative abundance of deer mice was greater at Molina. Long-term prevalence of infection with SNV was greater at Hesperus and was greatest in adult males at Hesperus (36.1%). Adult males at Molina exhibited a prevalence of infection with SNV of 25.0%. Infection with SNV was highly associated with scars or wounds for adult male, adult female, and juvenile male deer mice at Hesperus, but only for adult female deer mice at Molina. The presence of scars or wounds tended to be associated with greater age, but male deer mice at Hesperus were more likely to have wounds than female deer mice of the same age class. A similar pattern, excluding juveniles, was observed at Molina. Intraspecific interactions and environmentally elicited long-distance movements of deer mice may play a role in prevalence of infection with SNV in these animals.

Breeding by young-of-the-year female deer mice: Why weight?

Female deer mice (Peromyscus maniculatus) in the Kananaskis Valley, Alberta seldom breed during the summer of their birth despite having a breeding season which appears to be long enough for them to sexually mature. Thirteen years of live-trapping data indicate that only 3.6% of all females recruited into the trappable population bred as young-of-the-year (YY). Exclusion of those YY females who were born too late in the season to have bred and those who may have bred outside the trappable population indicated that only 14% of YY females with the opportunity to breed did so. Most females who bred as YY conceived within days of emerging from their natal nest. Breeding by YY females was associated with longer breeding seasons, which were lengthened through later cessation rather than earlier initiation of breeding, increased nestling growth rates and female-biased litters. The results of this study support the conclusion that nestling growth rates are important for YY breeding in these populations. Further, it appears that nestling growth varies annually, as well as among litters within breeding years, but not among individuals within breeding litters.

Silvicultural Use of Herbicide in Sub-Boreal Spruce Forest: Implications for Small Mammal Population Dynamics

Vegetation management in northern coniferous forest ecosystems may have implications for small mammal communities. This study was designed to test the hypothesis that herbicide-induced habitat alteration would reduce small mammal populations in early-successional stages of sub-boreal spruce forest. A secondary objective was to determine the long-term influence of herbicide treatment on reproduction, survival, and growth attributes of deer mouse (Peromyscus maniculatus) and southern red-backed vole (Clethrionomys gapperi) populations. Small mammal populations were intensively sampled in 4 pairs of control and treatment sites dominated by early-successional vegetation (5-10 yr after clearcut harvesting) near Prince George, British Columbia, Canada. Sampling was conducted during 1987 (pretreatment year) 1988-89 (first and second posttreatment years), and in 1991-92 (fourth and fifth posttreatment years). Higher mean numbers of red-backed voles (P = 0.03) and shrews (Sorex spp.; P = 0.001) were present on control than treatment sites during 1988-92. Mean abundance of meadow voles (Microtus pennsylvanicus; P = 0.69) and deer mice (P = 0.20) were similar on control and treatment sites throughout the study. Short-tailed weasels (Mustela erminea) and long-tailed weasels (Mustela frenata) were commonly captured on both control and treatment sites. There were no consistent differences between control and treatment populations in body mass, total biomass, or proportion of adult male and female red-backed voles and deer mice in breeding condition. However, there was a higher mean number of successful pregnancies in control than treatment populations of red-backed voles (P = 0.02) but not deer mice (P = 0.18) during posttreatment years. Mean Jolly-Seber estimates of survival of red-hacked voles (P = 0.01) but not deer mice (P = 0.70) were higher in treatment than control populations. The magnitude of observed demographic effects from herbicide treatment were well within the mean values of natural fluctuations of these variables. Herbicide treatment of areas dominated by early-successional vegetation in northern coniferous forests should be staggered in time and space to allow recovery (2-3 yr) of vegetation and small mammal species sucl as red-backed voles and shrews.

Sex differences in opioid and N-methyl- d-aspartate mediated non-opioid biting fly exposure induced analgesia in deer mice

There is evidence for sex differences in responses to noxious stimuli and in the expression and mediation of analgesia. In particular, results of investigations with swim stress and the more ethologically appropriate stress of predator odor exposure have suggested sex differences in N-methyl- d-aspartate (NMDA) receptor system involvement in the mediation of analgesia. Whether or not this sex difference generalizes to other environmental stressors is, however, not clear. Biting flies are a natural aversive stimuli commonly encountered by wild and domestic animals and humans. The present study examined the opioid and non-opioid mediated nociceptive (50°C hot plate) responses of reproductive male and female deer mice, Peromyscus maniculatus, exposed to biting fly attack. A 30 min exposure to biting flies (stable flies, Stomoxys calcitrans (L.)) elicited a naloxone sensitive, opioid-mediated analgesia that was of a greater magnitude in males than in female deer mice. In contrast, a 5 min exposure to biting flies elicited a `on-opioid' analgesia that was of similar magnitude in both sexes and insensitive to both naloxone and the specific kappa opiate antagonist, nor-binaltorphimine. In male mice this non-opioid analgesia was antagonised by the competitive NMDA antagonist, NPC 1262, while in reproductive females the biting fly-induced analgesia was insensitive to NPC 12626. These results show that there are sex differences in NMDA involvement in the mediation of the non-opioid analgesia arising from brief exposure to the stress of biting fly attack. These data from a common, natural environmental challenge support the presence of basic sex difference in NMDA involvement in the mediation of stress-induced analgesia.

Relationship between deer mouse population parameters and dieldrin contamination in the Rocky Mountain Arsenal National Wildlife Refuge

A small-mammal capture-recapture study was conducted in the Rocky Mountain Arsenal National Wildlife Refuge to quantify the effects of soil contamination with dieldrin on demographic parameters of deer mouse (Peromyscus maniculatus) populations. Increased dieldrin concentrations were significantly associated with larger deer mouse populations, although the size of populations on contaminated sites decreased during the study. The most parsimonious model for estimating survival rates was one in which survival was a decreasing function of dieldrin concentration. A significantly higher proportion of female deer mice in the populations residing on the more highly contaminated sites exhibited signs of reproductive activity. Development of genetic resistance in P. maniculatus to chronic chemical exposure is suggested as a possible mechanism responsible for the species' observed dominance and relatively high densities on contaminated sites. Under the additional stress of unfavorable environmental conditions, however, these populations may suffer disproportionately greater mortality. The design and analytical methods presented offer a rigorous statistical approach to assessing the effects of environmental contamination on small mammals at the population level.

Relationship between deer mouse population parameters and dieldrin contamination in the Rocky Mountain Arsenal National Wildlife Refuge

A small-mammal capture-recapture study was conducted in the Rocky Mountain Arsenal National Wildlife Refuge to quantify the effects of soil contamination with dieldrin on demographic parameters of deer mouse (Peromyscus maniculatus) populations. Increased dieldrin concentrations were significantly associated with larger deer mouse populations, although the size of populations on contaminated sites decreased during the study. The most parsimonious model for estimating survival rates was one in which survival was a decreasing function of dieldrin concentration. A significantly higher proportion of female deer mice in the populations residing on the more highly contaminated sites exhibited signs of reproductive activity. Development of genetic resistance in P. maniculatus to chronic chemical exposure is suggested as a possible mechanism responsible for the species' observed dominance and relatively high densities on contaminated sites. Under the additional stress of unfavorable environmental conditions, however, these populations may suffer disproportionately greater mortality. The design and analytical methods presented offer a rigorous statistical approach to assessing the effects of environmental contamination on small mammals at the population level.