Social organisation of female field volesMicrotus agrestis in a population in Southern England

The social organisation of a population ofMicrotus agrestis (Linnaeus, 1761) was studied by live trapping and radiotracking in southern England. Radiotracking revealed significant differences in both female home ranges and core areas with breeding condition. Lactating females had the smallest home ranges and core areas, and pregnant females the largest. There were no significant differences in female home range and core areas during the breeding season or between years despite different densities. Females were only territorial whilst lactating during which time their core areas were small. There were some overlaps of core areas among females at the beginning of the breeding season which are thought to be due to familiarity or kin associations. Associations between females were short-lived, lasting approximately one breeding event. As the breeding season progressed core areas became distinct suggesting a change to a territorial system although densities were low and most females were lactating. Patterns of social organisation among females were similar in both 1995 and 1996. Mean nearest neighbour distances tended to increase as the breeding season progressed and were uncorrelated with density.     

Natal dispersal in relation to population density and sex ratio in the field vole,Microtus agrestis

Summary In a sample of 240 juvenile field voles 8% of the males and 22% of the females reached sexual maturity within their natal home range. Among individuals retrapped as adults, 58% of males and 23% of females had dispersed, i.e. had moved more than one home range diameter. The mean distance moved for males (58.5 m) exceeded that for females (28.6 m). Male movement distances were negatively associated with total density, and with density of adult females, but not with male density. Female movements were not related to population density. There were no relation between sex ratio and distance moved. The distribution of distances moved for both males and females fit a geometrical distribution, suggesting the importance of competitive processes.     

Development ofTrichogramma australicum (Hym.: Trichogrammatidae) at low and high population density in artificial diet

We compared the development ofTrichogramma australicum Girault in artificial-diet filled artificial “eggs” containing 10–15 (low density) or 35–50 (high density) immature parasitoids. The size of low- and high-densityT. australicum eggs at 1, 15, 30 and 36 h was not significantly different. Also, incubation time of low- and high-density eggs in artificial diet was similar. Appearance of urate bodies in larvae developing low and high density occurred at the same time (100–108 h). Size of immatureT. australicum in low density significantly increased after the urate bodies had appeared. Black deposits appeared on the surface ofT. australicum prepupae soon after all diet had been consumed (at 108–120 and 144–168 h in low and high density, respectively). We concluded that the appearance of the black deposits was an indication of the onset of the prepupal stage. The pupal size was significantly larger at low density than high density. Mortality of prepupae and pupae was significantly higher at low density than at high density. Proportion of larvae at high density developed to abnormal and normal wasps. Wasps did not emerge from larvae reared at low density.     

Evidence for selection at cytokine loci in a natural population of field voles (Microtus agrestis)

Individuals in natural populations are frequently exposed to a wide range of pathogens. Given the diverse profile of gene products involved in responses to different types of pathogen, this potentially results in complex pathogen-specific selection pressures acting on a broad spectrum of immune system genes in wild animals. Thus far, studies into the evolution of immune genes in natural populations have focused almost exclusively on the Major Histocompatibility Complex (MHC). However, the MHC represents only a fraction of the immune system and there is a need to broaden research in wild species to include other immune genes. Here, we examine the evidence for natural selection in a range of non-MHC genes in a natural population of field voles (Microtus agrestis). We concentrate primarily on genes encoding cytokines, signalling molecules critical in eliciting and mediating immune responses and identify signatures of natural selection acting on several of these genes. In particular, genetic diversity within Interleukin 1 beta and Interleukin 2 appears to have been maintained through balancing selection. Taken together with previous findings that polymorphism within these genes is associated with variation in resistance to multiple pathogens, this suggests that pathogen-mediated selection may be an important force driving genetic diversity at cytokine loci in voles and other natural populations. These results also suggest that, along with the MHC, preservation of genetic variation within cytokine genes should be a priority for the conservation genetics of threatened wildlife populations.     

The maintenance ofBdellovibrio at low prey density

A mathematical model for the interaction ofBdellovibrio and its prey predicted that a relatively high prey density (7×10⁵ cells ml^–1) would be required for the establishment of an equilibrium in a mixed population [8]. The present report shows thatBdellovibrio can be maintained in a continuous culture when the prey cell density is much lower (2–5×10⁴ cells ml^–1), and closer to that of naturally occurring bacterial populations in sea waters.     

Individual growth rates in natural field vole, Microtus agrestis, populations exhibiting cyclic population dynamics

Rodents that have multi-annual cycles of density are known to have flexible growth strategies, and the “Chitty effect”, whereby adults in the high-density phase of the cycle exhibit larger average body mass than during the low phase, is a well-documented feature of cyclic populations. Despite this, there have been no studies that have repeatedly monitored individual vole growth over time from all phases of a density cycle, in order to evaluate whether such variation in body size is due to differences in juvenile growth rates, differences in growth periods, or differential survival of particularly large or small voles. This study compares growth trajectories from voles during the peak, increase and crash phases of the cycle in order to evaluate whether voles are exhibiting fast or slow growth strategies. We found that although voles reach highest asymptotic weights in the peak phase and lowest asymptotes during the crash, initial growth rates were not significantly different. This suggests that voles attain larger body size during the peak phase as a result of growing for longer.     

Cowpox virus infection in natural field vole Microtus agrestis populations: delayed density dependence and individual risk

1. Little is known about the dynamics of pathogen (microparasite) infection in wildlife populations, and less still about sources of variation in the risk of infection. Here we present the first detailed analysis of such variation. 2. Cowpox virus is an endemic sublethal pathogen circulating in populations of wild rodents. Cowpox prevalence was monitored longitudinally for 2 years, in populations of field voles exhibiting multiannual cycles of density in Kielder Forest, UK. 3. The probability that available susceptible animals seroconverted in a given trap session was significantly positively related to host density with a 3-month time lag. 4. Males were significantly more likely to seroconvert than females. 5. Despite most infection being found in young animals (because transmission rates were generally high) mature individuals were more likely to seroconvert than immature ones, suggesting that behavioural or physiological changes associated with maturity contribute to variation in infection risk. 6. Hence, these analyses confirm that there is a delayed numerical response of cowpox infection to vole density, supporting the hypothesis that endemic pathogens may play some part in shaping vole cycles.     

The Muslim population of Great Britain

Following the growth of Islamic consciousness in Great Britain, expressed particularly in the protests over the publication of Salman Rushdie's novel, The Satanic Verses, but also expressed in the foundation of the Islamic Party of Britain, there has been interest in the size of the Islamic community living there. Estimates have varied between one and two millions. However, using a variety of sources, the present article argues that a figure of between 550,000 and 750,000 is more likely.     

The effect of increased deposition of atmospheric nitrogen on Calluna vulgaris in upland Britain

Regular (monthly) additions of NH₄NO₃ (4–12 g N m⁻² yr⁻¹) were made over a period of 8 yr (1989–98) to areas of moorland in North Wales dominated by the ericaceous shrub Calluna vulgaris . Results from the early stages of the experiment (1990–94) have shown marked and dose-related increases in shoot extension and canopy height in response to the nitrogen treatments, with significantly higher shoot nitrogen contents. The nitrogen-related stimulation in the growth of the C. vulgaris canopy over this period has resulted in large accumulations of litter on the high-nitrogen-treated plots (6.6 kg m⁻² in plots treated with 12 g N m⁻² yr⁻¹, compared with 3.8 kg m⁻² for the water controls). Litter nitrogen concentrations were also significantly increased at the higher rates of nitrogen addition, leading to a doubling of the total return of nitrogen to the litter layer over the experimental period. These changes in vegetation structure were associated with large reductions in the abundance of the bryophyte and lichen species normally present under the untreated canopy. Results since 1994, however, show little increase in shoot extension in response to the nitrogen treatments, with no clear dose response to increasing levels of addition. These findings are associated with a dose-related increase in the susceptibility of the nitrogen-treated areas of the C. vulgaris canopy to late winter injury, characterized as browning of the shoot tips in early to late spring. These results indicate that deleterious effects are now accumulating as a result of the long-term addition of nitrogen to these moorland plots.     

Damped oscillation of population density at equilibrium

It has been theoretically assumed that the population density at the equilibrium oscillates with damping from generation to generation. In the adult population of the southern cowpea weevil, Callosobruchus maculatus, it was exemplified. But, it was not so clear in the adult population of the azuki bean weevil, C. chinensis as seen in that of C. maculatus. This difference seems to be due to the scramble type of competition that occurs in larval stage in C. maculatus, instead of in the egg stage as C. chinensis. Comparing with the oscillation from generation to generation obtained in the present experiment to that of Lucilia population found byNicholson , the oscillation in Lucilia population is composed of the cycle in a generation and the descending phase of each cycle of it is not regulated density-dependently. The present result seems to be more appropriate for the demonstration of the theory of self-adjustment of population.     

Adult philopatry and dispersal in the field vol Microtus agrestis

Summary Using mark-recapture data, we related the movements of adult field voles to population density, sex ratio and population growth. Dispersal movements (defined as distances larger than 1 home range diameter) were few in both sexes; 4 out of 197 (2.0%) in males and 8 of 316 (2.5%) in females. The distance moved between sequential trapping periods was similar for males and females; the mean being 10.2 m and 9.0 m respectively. Both males and females moved larger distances during the breeding season than during the nonbreeding period. The distance moved between sequential trapping periods showed a strong negative relation to density, i.e. both sexes moved shorter distances at higher densities, but there were no differences between periods of increasing and declining population densities. These results contradict the dispersal predictions of all major hypotheses proposed to explain population fluctuations in small mammals. The dispersal patterns fit a geometric distribution, suggesting that competition is the primary factor determining the dispersal characteristics of this population.     

Photoperiod regulates leptin sensitivity in field voles, Microtus agrestis

We have previously shown that cold-acclimated (8°C) male field voles (Microtus agrestis) transferred from short (SD, 8:16 h L:D) to long photoperiod (LD, 16:8 h L:D) exhibit increases in body mass, adiposity and food intake. To assess whether these increases were associated with decreased leptin sensitivity, we infused LD and SD voles with physiological doses of murine leptin (or saline) delivered peripherally for 7 days via mini-osmotic pumps. Measurements were made of body mass (weight-reducing effect of leptin), food intake (anorectic effect of leptin) and gene expression of uncoupling protein 1 (UCP1) in brown adipose tissue (BAT) (thermogenic effect of leptin). The SD animals were sensitive to the weight-reducing effects of leptin (mean body mass decrease of 1.2 g over 7 days) and appetite-reducing effect of leptin (mean food intake decrease of 2.5 g over 7 days), whereas LD voles were resistant to the hormone treatment. The switch from a leptin-sensitive to leptin-resistant state appears to act as a desensitisation mechanism that allows voles transferred from SD to LD to ignore elevated leptin levels generated by increased body fat and accumulate adipose tissue without stimulating compensatory changes opposing the weight gain. Neither SD nor LD voles responded to infusion of leptin by changes in BAT UCP1 gene expression, suggesting dissociation of anorectic and thermogenic effects of leptin, possibly related to chronic cold exposure. Our results indicate that cold-acclimated voles show photoperiod-regulated changes in leptin sensitivity and may provide an attractive model for elucidating molecular mechanisms of leptin resistance.     

Intracellular trafficking of pigeon beta-very low density lipoprotein and low density lipoprotein at low and high concentrations in pigeon macrophages

Foam cell formation occurs in vitro at lipoprotein concentrations above 50 microgram/ml in pigeon macrophages. Hypothetically, intracellular trafficking of lipoproteins at higher concentrations may differ from uptake of lipoproteins associated with low concentrations, revealing a separate atherogenic endocytic pathway. Macrophage intracellular trafficking of pigeon beta-very low density lipoprotein (beta-VLDL) and low density lipoprotein (LDL) at low concentrations (12 microgram/ml) near the saturation of high affinity binding sites and high lipoprotein concentrations (50-150 microgram/ml) used to induce foam cell formation were examined. Pigeon beta-VLDL and LDL, differentially labeled with colloidal gold, were added simultaneously to contrast trafficking of beta-VLDL, which causes in vitro foam cell formation, with LDL, which does not. The binding of lipoproteins to cell surface structures, distribution of lipoproteins in endocytic organelles, and the extent of colabeling in the endocytic organelles were determined by thin-section transmission electron microscopy.At low concentrations, the intracellular trafficking of pigeon LDL and beta-VLDL was identical. At high concentrations, LDL was removed more rapidly from the plasma membrane and reached lysosomes more quickly than beta-VLDL. No separate endocytic route was present at high concentrations of beta-VLDL; rather, an increased residence on the plasma membrane, association with nonmicrovillar portions of the plasma membrane, and slower trafficking in organelles of coated-pit endocytosis reflected a more atherogenic trafficking pattern.     

Summer acclimatization in the short-tailed field vole,Microtus agrestis

We investigated the changes that occurred in basal and noradrenaline-induced metabolic rate, body temperature and body mass in short-tailed field voles,Microtus agrestis, during exposure to naturally increasing photoperiod and ambient temperature. These parameters were first measured in winter-acclimatized voles (n=8) and then in the same voles which had been allowed to seasonally acclimatize to photoperiod and ambient temperature (6 months later). Noradrenaline induced metabolic rate, basal metabolic rate and nonshivering thermogenesis were significantly higher in winter-acclimatized compared to summer-acclimatized voles. There was a significant positive relationship between basal metabolic rate and noradrenaline-induced metabolic rate. Body mass was significantly higher in summer-acclimatized compared to winter-acclimatized voles. There was a significant positive relationship between body mass and noradrenaline-induced metabolic rate in both winter-acclimalized and summer-acclimatized voles; however, there was no relationship between basal metabolic rate and body mass in either seasonal group of voles. Body temperature after measurements of basal metabolic rate was not significantly different in the seasonal cohorts of voles. However, body temperature was significantly higher in winter-acclimatized compared to summer-acclimatized voles after injection of noradrenaline. Previously we have found that a long photoperiod was not a sufficient stimulus to reduce thermogenic capacity in winter-acclimatized voles during cold exposure, since basal metabolic rate increased to compensate for a reduction in regulatory nonshivering thermogenesis. Here we found that a combination of increased ambient temperature and photoperiod did significantly reduce thermogenic capacity in winter-acclimatized voles. This provided evidence that the two aspects of non-shivering thermogenesis, obligatory and regulatory, are stimulated by different exogenous cues. Summer acclimatization in the shorttailed field vole is manifest as a significant decrease in both basal and noradrenaline-induced metabolic rate, combined with a significant increase in body mass.Abbreviations ANCOV A analysis of covariance - BAT brown adipose tissue - BM body mass - BMR basal metabolic rate - NST non-shivering thermogenesis - NA noradrenaline - V the maximum V recorded following mass specific injection of noradrenaline - V the maximum V recorded following mass specific injection of saline - T _a ambient temperature - T _b rectal body temperature - T _1c lower critical temperature - UCP uncoupling protein - V oxygen consumption     

The low density lipoprotein receptor

The study of familial hypercholesterolemia at the molecular level has led to its advancement from a clinical syndrome to a fascinating experimental system. FH was first described 50 years ago by Carl Müller who concluded that the disease produces high plasma cholesterol levels and myocardial infarctions in young people, and is transmitted as an autosomal dominant trait determined by a single gene. The existence of two forms of FH, namely heterozygous and homozygous, was recognized by Khachadurian and Fredrickson and Levy much later. The value of FH as an experimental model system lies in the availability of homozygotes, because mutant genes can be studied without interference from the normal gene. The first and most important breakthrough was the realization that the defect underlying FH could be studied in cultured skin fibroblasts. Rapidly, the LDL receptor pathway was conceptualized and its dysfunction in cells from FH homozygotes was demonstrates. Isolation of the normal LDL receptor protein and studies on the biosynthesis and structure of abnormal receptors in mutant cell lines provided essential groundwork for elucidation of defects at the DNA level. The power of the experimental system, FH, became nowhere more obvious than in work that correlated structural information at the protein level with the elucidation of defined defects in the LDL receptor gene. In addition to revealing important structure-function relationships in the LDL receptor polypeptide and delineating mutational events, studies of FH have established several more general concepts. First, the tight coupling of LDL binding to its internalization suggested that endocytosis was not a non-specific process as suggested from early observations. The key finding was that LDL receptors clustered in coated pits, structures that had been described by Roth and Porter 10 years earlier. These investigators had demonstrated, in electron microscopic studies on the uptake of yolk proteins by mosquito oocytes, that coated pits pinch off from the cell surface and form coated vesicles that transport extracellular fluid into the cell. Studies on the LDL receptor system showed directly that receptor clustering in coated pits is the essential event in this kind of endocytosis, and thus established receptor-mediated endocytosis as a distinct mechanism for the transport of macromolecules across the plasma membrane. Subsequently, many additional systems of receptor-mediated endocytosis have been defined, and variations of the overall pathway have been described.(ABSTRACT TRUNCATED AT 400 WORDS)     

The many faces of population density

Population density, one of the most fundamental demographic attributes, may vary systematically with spatial scale, but this scale-sensitivity is incompletely understood. We used a novel approach—based on fully censused and mapped distributions of eastern grey squirrel (Sciurus carolinensis) dreys, beaver (Castor canadensis) lodges, and moose (Alces alces)—to explore the scale-dependence of population density and its relationship to landscape features. We identified population units at several scales, both objectively, using cluster analysis, and arbitrarily, using artificial bounds centred on high-abundance sites. Densities declined with census area. For dreys, this relationship was stronger in objective versus arbitrary population units. Drey density was inconsistently related to patch area, a relationship that was positive for all patches but negative when non-occupied patches were excluded. Drey density was negatively related to the proportion of green-space and positively related to the density of buildings or roads, relationships that were accentuated at coarser scales. Mean drey densities were more sensitive to scale when calculated as organism-weighted versus area-weighted averages. Greater understanding of these scaling effects is required to facilitate comparisons of population density across studies.     

Dissociation of low density lipoprotein-antibody precipitates at alkaline pH

The effect of alkaline pH on the dissociation of immunoprecipitates of low density lipoproteins (LDL) of the S(f) 0-10 class was studied by immunological and ultracentrifugal methods. The precipitates prepared at the equivalence point were dissolved and centrifuged in sodium chloride solutions of density 1.063 and pH's between 10.25 and 11.5. Analytical centrifugation of the top fraction, which floated at density 1.063, after dialysis against 0.9% sodium chloride of pH 7.4 revealed the presence of LDL and of soluble LDL-antibody complex. The amount of soluble complex was greater for the preparations obtained at lower pH than those obtained at higher pH and was undetectable at pH 11.5. The yield of immunoglobulin from the bottom fractions was maximal when the pH of the centrifugation medium was 11.0. Below pH 11.0, the greatly reduced yield of immunoglobulin was due partly to incomplete dissociation and partly to aggregation of soluble complex, while above pH 11.0 the decreased yield was possibly due to alkaline denaturation of the globulin. The immunoglobulin separated at pH 11.0 and dialyzed to pH 7.4 was reprecipitatable by LDL, and the reactivity did not seem to be appreciably influenced by the alkaline treatment.