全文获取类型
收费全文 | 1187篇 |
免费 | 38篇 |
国内免费 | 94篇 |
专业分类
1319篇 |
出版年
2024年 | 4篇 |
2023年 | 20篇 |
2022年 | 37篇 |
2021年 | 29篇 |
2020年 | 17篇 |
2019年 | 32篇 |
2018年 | 28篇 |
2017年 | 31篇 |
2016年 | 51篇 |
2015年 | 28篇 |
2014年 | 43篇 |
2013年 | 78篇 |
2012年 | 24篇 |
2011年 | 24篇 |
2010年 | 24篇 |
2009年 | 43篇 |
2008年 | 39篇 |
2007年 | 53篇 |
2006年 | 41篇 |
2005年 | 32篇 |
2004年 | 33篇 |
2003年 | 32篇 |
2002年 | 19篇 |
2001年 | 22篇 |
2000年 | 16篇 |
1999年 | 30篇 |
1998年 | 18篇 |
1997年 | 4篇 |
1996年 | 18篇 |
1995年 | 21篇 |
1994年 | 74篇 |
1993年 | 9篇 |
1992年 | 13篇 |
1991年 | 60篇 |
1990年 | 72篇 |
1989年 | 65篇 |
1988年 | 33篇 |
1987年 | 5篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 59篇 |
1978年 | 5篇 |
1977年 | 2篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1962年 | 1篇 |
排序方式: 共有1319条查询结果,搜索用时 0 毫秒
101.
102.
The claim that men prefer women with low waist-to-hip ratios (WHR) has been vigorously disputed. We examine self-report data
from 359 primiparous Polish women (with normal singleton births and healthy infants) and show that WHR correlates with at
least one component of a woman’s biological fitness (her first child’s birth weight, a variable that significantly affects
infant survival rates). However, a woman’s Body Mass Index (BMI) is a better predictor of her child’s neonatal weight in small-bodied
women (<54 kg). The failure to find a preference for low WHR in some traditional populations may thus be a consequence of
the fact that, even in western populations, body mass is a better predictor of fitness in those cases characterized by low
maternal body weight.
Boguslaw Pawłowski Ph.D., D.Sc., is a researcher and lecturer in biological anthropology at the University of Wrocław, Poland.
His research interests focus on mechanisms of human evolution (with special attention to the evolution of subcutaneous fat
tissue distribution) and human mate choice.
Robin Dunbar Ph.D., FBA is British Academy Research Professor of Evolutionary Psychology at the University of Liverpool, England,
and co-Director of the British Academy Centenary Research Project. His research interests focus on the evolutionary and environmental
determinants of social and reproductive strategies, with particular references to humans, nonhuman primates, and ungulates. 相似文献
103.
Modifications and use of Karr’s Index of Biotic Integrity (IBI) for assessing the effects of anthropogenic impacts to aquatic ecosystems have typically occurred using data collected at the macro scale. However, some non-point sources of habitat degradation occur at small scales. One possible source of perturbation to fish habitat in boreal rivers is the application of rip rap shoreline armouring in human use areas. In this study we assess the use of IBI in a small scale setting and discuss the potential impact of rip rap shoreline armouring. We captured small and juvenile fishes weekly during 2002–2003 from 12 sample sites within the littoral zone of a human use area using a modified beach seine. Principal component analysis (PCA) was performed on the data to examine the relationship between species composition and IBI scores. We also performed PCA on the IBI metrics to assess our modifications. The IBI method produced higher scores for armoured sites than for unarmoured sites. We found a strong rank order correlation (Spearman’s ρ > 0.93; p < 0.001) between the modified IBI scores and the first principal component, suggesting that Karr’s original empirically-based IBI is strongly linked to species composition. We found a high degree of redundancy between the metrics of the IBI that validate our modifications. These results suggest that IBI can be a suitable method for assessing non-point impacts from within a small study area. Our results also indicate that fish habitat from rip rap armoured sites within the test area had consistently higher IBI scores that unarmoured sites. 相似文献
104.
Simonishvili S Tsanava S Sanadze K Chlikadze R Miskalishvili A Lomkatsi N Imnadze P Petri WA Trapaidze N 《Experimental parasitology》2005,110(3):313-317
It is generally accepted that a majority of individuals infected by Entamoeba histolytica do not develop symptomatic disease. However, the parasite and the host factors contributing to the development of the disease, remain undetermined. It is also unclear why certain individuals develop extra-intestinal amebiasis without exhibiting apparent intestinal symptoms. An outbreak of amebic liver abscess in Tbilisi, Georgia in 1998-1999 suggested that the causative E. histolytica strain had an unusual propensity for extra-intestinal spread. To correlate the genetic differences with pathogenic potential of the parasite, we have examined the SREHP gene polymorphisms among Georgian E. histolytica isolates. Comparison of polymorphic patterns revealed the presence of several different genotypes of E. histolytica, thus preventing an association of a single genotype with hepatic disease, but supporting the previous finding of extensive genetic diversity among E. histolytica isolates from the same geographic origin. 相似文献
105.
We sampled moss and vascular forest vegetation in five ancient beech forests from northwest France, embracing in each a wide array of environmental conditions. Indirect (PCA) and direct (RDA) gradient analysis were used to discern local and regional ecological factors which explain the observed variation in species composition. Our results point to a global factor encompassing a large array of soil and light conditions, unravelled when local particularities of studied forests are singled out. The humus form, numerically expressed by the Humus Index, explains a large part of the observed variation in ground vegetation. Our study confirmed opposite trends in vascular and moss species richness according to humus condition. Ecological factors to which vascular and moss forest species respond at the regional level can be estimated directly in the field by visually inspecting humus forms and vegetation strata despite of the confounding influence of local factors. 相似文献
106.
Chen FH 《Journal of mathematical biology》2006,53(2):253-272
An susceptible-infected epidemic model with endogenous behavioral changes is presented to analyze the impact of a prophylactic vaccine on disease prevalence. It is shown that, with voluntary vaccination, whether an endemic equilibrium exists or not does not depend on vaccine efficacy or the distribution of agent-types. Although an endemic equilibrium is unique in the absence of a vaccine, the availability of a vaccine can lead to multiple endemic equilibria that differ in disease prevalence and vaccine coverage. Depending on the distribution of agent-types, the introduction of a vaccine or, if one is available, a subsidy for vaccination can increase disease prevalence by inducing more risky behavior.I would like to thank one of the editors of the journal, Alan Hastings, for his comments and suggestions. 相似文献
107.
AimMigration is a constantly changing adaptation due to the climate condition evolution. The struggle for surviving during harsh winter season is different across Europe, being more complex toward the inner parts of the continent. The current approach explores the Common Buzzard number variation during the cold season and the climatic predictors of birds of prey wintering movements in relation to the possible influences of the Carpathian Mountains, which may act as a geographical barrier providing shelter from cold air outbreak from north and northeast of the continent.LocationRomania (45°N25°E).TaxonBirds of Prey.MethodsWe applied a GLMM to investigate the relation between continental and local climatic factors with the number of Common Buzzard observations in two regions. The first region is located inside the Carpathian Arch and the other one outside, east of this large mountains chain.ResultsThe Common Buzzard numbers wintering Eastern from the Carpathian Mountains are highly influenced by AO (Z = 2.87, p < .05%), while those wintering western are influenced by NAO (Z = 2.17, p < .05%). This is the first proof of separating influences for biodiversity of AO and NAO at continental scale, outlining the influence limit placed over the Eastern Carpathian Mountains.Main conclusionsThe Carpathian Mountains act like a geographic barrier, separating the wintering Common Buzzard populations from both sides of the mountain range. While the high number of individuals in Moldova is related to their eastern and northeastern Europe origins, in Transylvania the large number of individuals observed is related to the more sheltered characteristics of the region attracting individuals from central Europe. Also, since Transylvania region is well sheltered during cold air outbreak, it represents a more favorable region for wintering. From this point of view, we can consider that the Carpathian Mountains are a geographic barrier for wintering birds of prey. 相似文献
108.
Cousson PY Bessadet M Nicolas E Veyrune JL Lesourd B Lassauzay C 《Gerodontology》2012,29(2):e685-e692
doi: 10.1111/j.1741‐2358.2011.00545.x Nutritional status, dietary intake and oral quality of life in elderly complete denture wearers Background and objective: The prevalence of malnutrition increases with age because of many factors. Edentulousness leads to the avoidance of many types of foods. The aim of this study was to determine whether elderly complete denture wearers have a higher risk of malnutrition than dentate controls. Material and methods: A Mini‐Nutritional Assessment (MNA) and a 3‐day dietary record were compiled for a group of fully dentates (21 women and 29 men; mean age 70.1 ± 6.1) and for a group of complete denture wearers (31 women and 16 men; mean age 70.1 ± 8.1). Socio‐demographic data and scores on the General Oral Health Assessment Index (GOHAI) questionnaire were collected. Results: Inter‐group comparison of MNA scores showed that more subjects in the edentulous group (21.3%) risked malnutrition than in the dentate group (0%). The variability of the MNA could be explained for 22% by dental status, 7% by loneliness and 4% by the GOHAI score (regression analysis). Both groups had insufficient energy intakes and deficits in vitamins and micronutrients; moreover, edentulous subjects had lower intakes than dentate subjects. Conclusion: The use of conventional dentures increases the risk of malnutrition in the elderly. 相似文献
109.
Spatial patterns in brown bear Ursus arctos diet: the role of geographical and environmental factors
- 1 We reviewed worldwide spatial patterns in the food habits of the brown bear Ursus arctos in relation to geographical (latitude, longitude, altitude) and environmental (temperature, snow cover depth and duration, precipitation, primary productivity) variables.
- 2 We collected data from 28 studies on brown bear diet based on faecal analysis, covering the entire geographical range of this widely distributed large carnivore. We analysed separately four data sets based on different methods of diet assessment.
- 3 Temperature and snow conditions were the most important factors determining the composition of brown bear diet. Populations in locations with deeper snow cover, lower temperatures and lower productivity consumed significantly more vertebrates, fewer invertebrates and less mast. Trophic diversity was positively correlated with temperature, precipitation and productivity but negatively correlated with the duration of snow cover and snow depth. Brown bear populations from temperate forest biomes had the most diverse diet. In general, environmental factors were more explicative of diet than geographical variables.
- 4 Dietary spatial patterns were best revealed by the relative biomass and energy content methods of diet analysis, whereas the frequency of occurrence and relative biomass methods were most appropriate for investigating variation in trophic diversity.
- 5 Spatial variation in brown bear diet is the result of environmental conditions, especially climatic factors, which affect the nutritional and energetic requirements of brown bears as well as the local availability of food. The trade‐off between food availability on the one hand, and nutritional and energetic requirements on the other hand, determines brown bear foraging decisions. In hibernating species such as the brown bear, winter severity seems to play a role in determining foraging strategies. Large‐scale reviews of food habits should be based on several measures of diet composition, with special attention to those methods reflecting the energetic value of food.
110.
Characterization of rumen bacterial diversity and fermentation parameters in concentrate fed cattle with and without forage 总被引:1,自引:0,他引:1
Petri RM Forster RJ Yang W McKinnon JJ McAllister TA 《Journal of applied microbiology》2012,112(6):1152-1162
Aims: To determine the effects of the removal of forage in high‐concentrate diets on rumen fermentation conditions and rumen bacterial populations using culture‐independent methods. Methods and Results: Detectable bacteria and fermentation parameters were measured in the solid and liquid fractions of digesta from cattle fed two dietary treatments, high concentrate (HC) and high concentrate without forage (HCNF). Comparison of rumen fermentation conditions showed that duration of time spent below pH 5·2 and rumen osmolality were higher in the HCNF treatment. Simpson’s index of 16S PCR‐DGGE images showed a greater diversity of dominant species in the HCNF treatment. Real‐time qPCR showed populations of Fibrobacter succinogenes (P = 0·01) were lower in HCNF than HC diets. Ruminococcus spp., F. succinogenes and Selenomonas ruminantium were at higher (P ≤ 0·05) concentrations in the solid vs the liquid fraction of digesta regardless of diet. Conclusions: The detectable bacterial community structure in the rumen is highly diverse. Reducing diet complexity by removing forage increased bacterial diversity despite the associated reduction in ruminal pH being less conducive for fibrolytic bacterial populations. Quantitative PCR showed that removal of forage from the diet resulted in a decline in the density of some, but not all fibrolytic bacterial species examined. Significance and Impact of the Study: Molecular techniques such as DGGE and qPCR provide an increased understanding of the impacts of dietary changes on the nature of rumen bacterial populations, and conclusions derived using these techniques may not match those previously derived using traditional laboratory culturing techniques. 相似文献