Disease dynamics in cyclic populations of field voles (Microtus agrestis): cowpox virus and vole tuberculosis (Mycobacterium microti)

The possible role of pathogens in rodent population cycles has been largely neglected since Elton's 'epidemic hypothesis' of 1931. To revisit this question, 12 adjacent, cyclic but out-of-phase populations of field voles (Microtus agrestis) in North East England were studied and the initial results are presented here. The prevalences of antibodies to cowpox virus and of clinical signs of Mycobacterium microti infection (vole tuberculosis) showed delayed (not direct) density dependence (with a lag of three to six months). This did not result from changes in population structure, even though there were such changes associated with the different phases of the cycle. The prevalences rose as vole numbers rose, and peaked as numbers declined. The apparent lag in the numerical response of infection prevalence to changes in host abundance is consistent with the hypothesis that diseases, singly or in combination, play a hitherto underestimated role in the dynamics of cyclic populations.     

Survival of cohorts in a fluctuating population of the vole Microtus townsendii

Survival patterns of cohorts are described during a population cycle of the vole Microtus townsendii near Vancouver, British Columbia, Canada. A two–year live–trapping study on both enclosed and unfenced populations showed that cohorts during the increase phase of growth lived longest and had the best survival. Smaller voles in the peak density spring cohort had poor survival, but survival increased during the peak density summer. Survival of cohorts in the decline phase breeding season was very poor. The suggestions are made that changes in spacing behaviour may cause changes in cohort survival and that the causes of rapid changes in survival need to be determined.     

Functional features of microbial communities in the digestive tract of field voles (Microtus rossiaemeridionalis and Clethrionomys glareolus)

The nitrogen-fixating and cellobiohydrolase activity, the nitrogen (N) and carbon (C) contents, and the number of microorganisms in the prestomach, cecum, and colon of two vole species were studied: the southern vole (Microtus rossiaemeridionalis) and the bank vole (Clethrionomys glareolus), which is characterized by a mixed type of diet. The nitrogen-fixating activity in the cecum was found to be the highest in the voles compared with the mammals studied earlier. The seasonal dynamics of both nitrogenase and cellobiohydrolase activities was registered in the southern vole. The structure of the microbial complex in the southern vole is more varied and includes microorganisms associated with plant substrates.     

野外围栏条件下根田鼠肠道寄生物的感染率

寄生物是调节小哺乳动物种群波动的因子之一。许多生物和非生物因子可影响寄生物的感染。本研究以根田鼠为对象,在野外围栏条件下测定了根田鼠肠道寄生物种类及其感染率,并分析了根田鼠建群者数量、性别、年龄及时间对寄生物感染率的影响。结果表明,根田鼠肠道寄生物有球虫、线虫和绦虫,球虫是主要寄生物, Eimeria wenrichi 为球虫中的优势种;其感染率随根田鼠建群者数量增加而增加,且随时间而显著变化。在根田鼠幼体中,高密度处理组E. wenrichi 的感染率显著高于低密度处理组,而根田鼠幼体的E. ochrogasteri 感染率显著高于成体。本研究未发现根田鼠性别间的球虫感染率差异。上述结果说明,在根田鼠种群中,球虫感染率有时间变化,且受宿主密度的制约,高密度种群引起的生前应激与当前密度制约性感染可能对球虫感染率具有耦合效应。     

Dispersal tendency and duration of life of littermates during population fluctuations of the vole Microtus townsendii

Summary In increasing and peak populations of the vole Microtus townsendii, littermates, whether they did or did not disperse, tended to have similar lifespans. Identifiable families tended to survive or disperse as a unit. Tendency to disperse from peak populations was non-randomly distributed among litters, and littermates dispersed at the same age.     

Smaller Microtus vole species competitively superior in the absence of predators

Interspecific competition is assumed to generate negative effects on coexisting species, possibly including slower population growth and lower survival. The field vole ( Microtus agrestis ) and the sibling vole ( M. rossiaemeridionalis ) are sympatric close relatives which compete for similar resources. Previous non-experimental studies suggest that the smaller sibling vole is a superior competitor, yet more vulnerable to predation than the larger field vole. We studied the effects of coexistence on population densities, reproductive parameters, and survival in these two species by means of experimentation in large, predator-free outdoor enclosures. While populations of both species reached higher densities in the absence of the other, field voles appeared to suffer more from interspecific competition than sibling voles. The proportion of young individuals in the population was higher in the sibling vole than in the field vole at the end of the experiment. The presence of a coexisting species reduced the survival of field voles. Sibling voles, on the other hand, appeared to suffer more from intraspecific competition than interspecific competition. On a population level, the sibling vole seems to be a superior competitor in the absence of predators due to better survival and possibly a higher reproductive capacity. However, predation probably has a profound influence on the interspecific dynamics of these two species indicating that in natural surroundings apparent competition (i.e. competition via shared predators) is stronger than direct competition.     

Field Vole Microtus agrestis abundance and Hen Harrier Circus cyaneus diet and breeding in Scotland

In many parts of the global range, voles form an important part of the diet of Hen Harriers Circus cyaneus , and breeding numbers are correlated with the abundance of these small mammals. In Scotland, however, little information is available on harrier diet in the spring and our understanding of causes of variation in harrier breeding density is complicated by human interference. In this paper we explore the relationship between Field Vole Microtus agrestis abundance and harrier spring diet, density and productivity in southern Scotland. Over three years, voles occurred on average in 67% of pellets, and 79% in years of high and intermediate vole abundance. From 1992, the number of breeding harriers increased following protection from illegal persecution. After accounting for this trend, harrier numbers correlated strongly with vole abundance. Harrier clutch size was also correlated with vole abundance. Although fledging success tended to be greater in years of vole increase than in years of vole decline, fledging success was not significantly correlated with the relative abundance of voles, or with the abundance of Meadow Pipits or Red Grouse chicks.     

Monitoring of tundra vole (Microtus oeconomus Pall.) population inhabiting areas with increased radiation background

The data of complex examinations of morphophysiological state of Tundra vole (Microtus oeconomus Pall.), the lipid peroxidation (LPO) processes and the energy exchange in their organs, the cytogenetic analysis of marrow cells and the process of multiplication in populations inhabiting areas with increased natural radiation background more 50 years are generalized. The data obtained support the existence of qualitative differences of Tundra vole subpopulations inhabiting control and radioactive nuclide contaminating areas. They allow to suppose that the process of adaptation to chronic low intensity radiation effect of Tundra vole in natural conditions involves the transition of the LPO and another regulatory systems to different level of function and is directed on population survival and homeostasis maintenance in changing radioecological conditions.     

The best in all possible worlds? A quantitative genetic study of geographic variation in the meadow vole,Microtus pennsylvanicus

The meadow vole, Microtus pennsylvanicus , is the most widely distributed Microtus species in North America. Across its range, it shows marked demographic differences, experiences a large range of climatic conditions, and varies considerably in body size and life-history characteristics. To study the genetic basis of the geographic variation in size and life history of this species, we subjected three populations, one from central Canada and two from eastern Canada, to quantitative genetic analysis in the lab. We studied the variance and covariance of several size and growth variables as well as age and size at maturity by means of population crosses, full-sib analysis, and parent-offspring regressions. We found that the phenotypic differences among these populations are almost entirely due to environmental effects. However, within populations, additive genetic and maternal effects explain most of the variation. We discuss possible explanations for the lack of genetic differences among the populations and speculate that a similar reaction norm is maintained in all populations through heterogeneity in the temporal or spatial environment that the populations experience. The heterogeneity may be mediated through population density fluctuations, climatic variation, or variation in site productivity. Thus, we hypothesize that M. pennsylvanicus has evolved to be the best in all possible worlds rather than in one actual world. This study highlights the crucial importance of maternal and environmental effects on the size, growth, and life history of small rodents.     

Home range size and overlap in female root voles: effects of season and density

In small mammals living in highly seasonal environments, observationalstudies show that female home range size and exclusiveness aresmaller in the nonbreeding winter season than in the breedingsummer season. This has led to the notion that nonbreeding femalesare more social and decrease territorial behavior during winter.However, because territoriality decreases with increasing populationdensity, and density normally increases during the breedingseason, the effects of density and season on social structureare usually confounded. To find out which of the 2 factors explainsspace use, we experimentally established 3 high-density and3 low-density root vole (Microtus oeconomus) populations inlate spring and monitored the populations into the nonbreedingwinter season. Population sizes were controlled throughout thebreeding period to minimize seasonal variation in density. Homerange sizes were larger in founder females than in field-bornfemales but did not change with season or density. Area exclusivelyused by individual females was lower in winter than summer,and founder females decreased exclusiveness as density increased.We argue that this seasonal pattern of space use might be causedby variation in benefits of group living, whereas founder femalesalso responded to density-dependent competition by reducingarea exclusively used.     

Cowpox virus infection in natural field vole Microtus agrestis populations: significant negative impacts on survival

1. Cowpox virus is an endemic virus circulating in populations of wild rodents. It has been implicated as a potential cause of population cycles in field voles Microtus agrestis L., in Britain, owing to a delayed density-dependent pattern in prevalence, but its impact on field vole demographic parameters is unknown. This study tests the hypothesis that wild field voles infected with cowpox virus have a lower probability of survival than uninfected individuals. 2. The effect of cowpox virus infection on the probability of an individual surviving to the next month was investigated using longitudinal data collected over 2 years from four grassland sites in Kielder Forest, UK. This effect was also investigated at the population level, by examining whether infection prevalence explained temporal variation in survival rates, once other factors influencing survival had been controlled for. 3. Individuals with a probability of infection, P(I), of 1 at a time when base survival rate was at median levels had a 22.4% lower estimated probability of survival than uninfected individuals, whereas those with a P(I) of 0.5 had a 10.4% lower survival. 4. At the population level, survival rates also decreased with increasing cowpox prevalence, with lower survival rates in months of higher cowpox prevalence. 5. Simple matrix projection models with 28 day time steps and two stages, with 71% of voles experiencing cowpox infection in their second month of life (the average observed seroprevalence at the end of the breeding season) predict a reduction in 28-day population growth rate during the breeding season from lambda = 1.62 to 1.53 for populations with no cowpox infection compared with infected populations. 6. This negative correlation between cowpox virus infection and field vole survival, with its potentially significant effect on population growth rate, is the first for an endemic pathogen in a cyclic population of wild rodents.     

Pink-eyed dilution, a coat color mutation in the Japanese field vole (Microtus montebelli)

Inheritance of dilute coat color with pink eye in the Japanese field vole (Microtus montebelli) was investigated by mating of the dilute mutant with a normal agouti vole and a white vole. As the results, it was cleared that an autosomal recessive gene p is responsible for the pink-eyed dilution in M. montebelli.     

Comparison of social interaction and neural activation in the main olfactory bulb and the accessory olfactory bulb between Microtus mandarinus and Microtus fortis

To gain insight into the function of AOB and MOB during different social interaction and in different vole species,the behaviors and neural activation of the olfactory bulbs in social interactions of mandarin voles Microtus mandarinus and reed voles Microtus fortis were compared in the present research.Mandarin voles spent significantly more time attacking and sniffing their opponents and sniffing sawdust than reed voles.During same sex encounters,mandarin voles attacked their opponents for a significantly ...     

Islands and the evolution of Microtus arvalis (Microtinae)

The widespread European vole species Microtus arvalis (Pallas) occurs in the British Isles only on Guernsey and various of the Orkney islands. The traditional explanation for this peculiar distribution has been that the island races are the sole relicts of a previously cosmopolitan form. This idea is not supported by the present study, in which population samples are compared with voles from a number of localities from Continental Europe. Using the frequencies of non-metrical skull variants as genetical markers, it emerged that although the Guernsey population ( Microtus arvalis sarnius ) was virtually indistinguishable from typical German specimens, and therefore likely to be a relict from the time when Guernsey was connected to France, the Orkney populations ( Microtus arvalis orcadensis ) formed a distinct group with affinities more to south Europe than elsewhere. It seems probable that arvalis was brought to Orkney by some of the earliest human colonizers about 4000 years ago, and then radiated to the different islands. It is argued that this manner of peripheral differentiation may be an important method of evolutionary change.     

Multiple predators induce risk reduction in coexisting vole species

Large predators may affect the hunting efficiency of smaller ones directly by decreasing their numbers, or indirectly by altering their behaviour. Either way this may have positive effects on the density of shared prey. Using large outdoor enclosures, we experimentally studied whether the presence of the Tengmalm's owl Aegolius funereus affects the hunting efficiency of the smallest member of the vole-eating predator guild, the least weasel Mustela nivalis, as measured by population responses of coexisting prey species, the field vole Microtus agrestis and the sibling vole M. levis . We compared the density and survival probability of vole populations exposed to no predation, weasel predation or combined predation by a weasel and an owl. The combined predation of both owl and weasel did not result in obvious changes in the density of sibling and field vole populations compared to the control populations without predators, while predation by least weasel alone decreased the densities of sibling voles and induced a similar trend in field vole densities. Survival of field voles was not affected by predator treatment while sibling vole survival was lower in predator treated populations than in control populations. Our results suggest that weasels are intimidated by avian predators, but without changing the effects of predators on competitive situations between the two vole species. Non-lethal effects of intraguild predation therefore will not necessarily change competitive interactions between shared prey species.     

Sperm DNA and sex chromosome differences between two geographical populations of the creeping vole, Microtus oregoni

The relative DNA content of the "O" and Y chromosome-bearing sperm is presented for the creeping vole, Microtus oregoni. The animals had been trapped in Oregon and in Washington State. The two populations had very similar autosomal chromosome relationships but differed greatly in the size of their X chromosome (which is not carried by vole sperm) and in their Y chromosome. The greater size and banding differences of the Y chromosome of the Washington State vole compared to the Oregon vole paralleled the greater differences in sperm DNA between the Y-bearing sperm and the sperm carrying no sex chromosome (O). The actual DNA differences between O and Y sperm was 12.5% for the sperm from the Washington State voles and 9.1% for sperm from the Oregon voles. The difference in sperm DNA content (12.5%) for Washington State voles was far greater than the difference shown for other voles or other mammals.     

Intraspecific epigenetic polymorphism of the East European vole (Microtus levis Miller, 1908) in South-eastern Europe and Turkey

This study aims to represent the first report on population variation of 20 non-metric skull characters in East European vole (Microtus levis) from the Balkan (populations from Northern Dobruja; Southern Dobruja; East part of the Danube Plain; North-east Trace; Sofia field; South-east Trace) and Anatolian peninsulas (populations from North-west Anatolia region and Central Anatolia region), on the basis of which to determine its epigenetic variability and to analyse their mutual geographical epigenetic relations through comparison of the epigenetic divergence among them. Estimation of epigenetic variation of the studied populations of M. levis showed similar pattern of variation, but it is mostly higher than the other rodent species with a similar range of distribution, such as Microtus arvalis, Mus musculus, Apodemus sylvaticus, Apodemus flavicollis and Clethrionomys glareolus. Each one of the studied traits manifested some polymorphism. Moreover, all the calculated epigenetic distances (MMD) were statistically insignificant (P < 0.05) and epigenetic cranial uniqueness (MU) of any studied population was not found. These results reveal lack of expressed geographic relationship of population epigenetic variability in East European vole. The revealed populations epigenetic polymorphism of M. levis gives an opportunity for more complete assessment of variability and biological diversity of this species, but further research is necessary to elucidate its population epigenetics, especially as the data obtained in recent investigations of cranial morphology of the sibling species from the group the M. arvalis (sensu lato) added new locations to the distribution map of the East European vole in Eurasia.     

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.     

根田鼠视网膜的胚后发育

视觉对动物的生活习性尤其是取食具有重要意义。本文对根田鼠视网膜的胚后发育进行了研究,结果表明:出生3d内根田鼠视网膜分化程度较低,神经节母细胞层尚未分化,占据了视网膜层的一半以上;5日龄时,外网层开始出现;6日龄时,外网层开始清晰,外核层与内核层更加清晰;18日龄时,视网膜结构与成年根田鼠结构相似,各层结构清晰可见。测量了神经节细胞层和外核层的细胞密度以及核层厚度,结果表明:随着个体发育,外核层细胞层厚度及细胞密度不断增加;而神经节细胞层厚度及细胞密度不断减少。与褐家鼠、黑线姬鼠、大仓鼠、棕色田鼠、甘肃鼢鼠、达乌尔黄鼠、岩松鼠视网膜相比,根田鼠视网膜结构介于夜行性与昼行性鼠类之间[动物学报52(2):376-382,2006]。     

Characterization of several LINE-1 elements in Microtus kirgisorum

Several LINE-1s have been isolated and characterized from genomic DNA of the vole, Microtus kirgisorum. Blot hybridization revealed specific restriction patterns of L1 elements in vole genomes. Rehybridization of the genomic blot with a cloned 5′-end fragment revealed two major bands indicating the presence of two different L1 subfamilies. The copy numbers are estimated for different parts of M. kirgisorum L1 elements. Data also demonstrate that most vole L1 elements are truncated at the 5′-end; however, in contrast to mouse, the ORF1 copy number is higher in vole. A difference between the substitution rates of the ORF1 5′-region (approximately 330 nucleotides) and the rest of the L1 coding regions is revealed. Received: 12 January, 1999 / Accepted: 18 March, 1999