全文获取类型
收费全文 | 1293篇 |
免费 | 26篇 |
国内免费 | 11篇 |
出版年
2022年 | 4篇 |
2021年 | 9篇 |
2020年 | 4篇 |
2019年 | 5篇 |
2018年 | 5篇 |
2017年 | 5篇 |
2016年 | 27篇 |
2015年 | 7篇 |
2014年 | 23篇 |
2013年 | 13篇 |
2012年 | 29篇 |
2011年 | 171篇 |
2010年 | 23篇 |
2009年 | 78篇 |
2008年 | 96篇 |
2007年 | 96篇 |
2006年 | 73篇 |
2005年 | 76篇 |
2004年 | 33篇 |
2003年 | 25篇 |
2002年 | 18篇 |
2001年 | 11篇 |
2000年 | 11篇 |
1999年 | 9篇 |
1998年 | 25篇 |
1997年 | 20篇 |
1996年 | 53篇 |
1995年 | 38篇 |
1994年 | 34篇 |
1993年 | 33篇 |
1992年 | 30篇 |
1991年 | 26篇 |
1990年 | 16篇 |
1989年 | 31篇 |
1988年 | 26篇 |
1987年 | 14篇 |
1986年 | 14篇 |
1985年 | 13篇 |
1984年 | 17篇 |
1983年 | 5篇 |
1982年 | 12篇 |
1981年 | 8篇 |
1980年 | 12篇 |
1979年 | 7篇 |
1978年 | 4篇 |
1977年 | 10篇 |
1976年 | 15篇 |
1974年 | 1篇 |
1972年 | 10篇 |
1971年 | 4篇 |
排序方式: 共有1330条查询结果,搜索用时 31 毫秒
891.
892.
893.
Recently, several authors have proposed that the availability of intermediate hosts (IHs) for definitive hosts (DHs) may contribute to determining the dynamics and evolutionary ecology of parasites with facultative complex life cycles. The protozoa Toxoplasma gondii may be transmitted to DHs either via predation of infected IHs through a complex life cycle (CLC) or directly from a contaminated environment through a simple life cycle (SLC). This parasite is also present in contrasting host density environments. We tested the hypothesis that the relative contributions of the CLC and SLC along an urban-rural gradient depend on the IH supply. We built and analysed a deterministic model of the T. gondii transmission cycle. The SLC relative contribution is important only in urban-type environments, i.e., with low predation rate on IHs. In contrast, the parasite is predominantly transmitted through a CLC in suburban and rural environments. The association of the two cycles enables the parasite to spread in situations of low IH availability and low DH population size for which each cycle alone is insufficient. 相似文献
894.
In nestlings, glucocorticoid (GC) secretion has short-term and long-term f itness consequences. For example, short-time elevations trigger begging activity, whereas chronically elevated GC levels impair body condition, growth and cognitive abilities. Despite a growing body of literature on personality traits, the effects of selection for fast and slow exploration on GC secretion have received little attention. We compared baseline and stress-induced hypothalamic–pituitary–adrenal (HPA) axis activity of hand-reared great tit nestlings of lines selected for fast and slow exploration. Nestling droppings were collected under three conditions: control, test (following handling stress, day 14 after hatching) and the following day. The concentrations of excreted immunoreactive corticosterone metabolites (CM) were determined via an enzyme immunoassay. We also observed nestlings' begging behaviour. CM differed significantly between the lines. Nestlings of the fast line excreted lower CM than slow-line birds. In response to handling stress, nestlings excreted significantly higher concentrations of CM than during the control and on the day after handling. Sex and begging activity were not related to CM levels. Under the control condition, but not after handling, males begged significantly more often than females. In both lines, adults excreted significantly less CM compared to nestlings. Both nestlings and adults of the slow line produced higher baseline CM values than fast-line birds. Fast-line nestlings excreted lower baseline CM than nestlings of a wild population not selected for fast or slow exploration. Slow-line nestlings did not. Our results show that selection on the basis of exploratory behaviour affected HPA axis reactivity. 相似文献
895.
896.
Juliana Vitória Messias Bittencourt Alexandre Magno Sebbenn 《Conservation Genetics》2008,9(4):855-868
As a matter of fact, Araucaria angustifolia populations occur predominately in small and isolated stands; only a minor number of continuous natural forests of this dioecious
wind-pollinated coniferous tree species remain. To implement reasonable conservation, breeding and restorations program it
is necessary to have the knowledge of pollen dispersal distance and fine-scale genetic structure. In this paper, levels and
dispersion distance of pollen and spatial genetic structure of A. angustifolia were investigated in a 14 ha transect in a continuous forest in Paraná State, Brazil. Analyses have been performed by the
use of eight microsatellite loci, paternity and TwoGener approaches, and spatial autocorrelation analysis. In transect, 52 male and 56 female adult trees were mapped and genotyped,
together with 190 seeds. In the present transect, A. angustifolia show spatial genetic structure at distances up to 75 m. Paternity analysis indicated that 54% of seeds were fertilized by
pollen from trees outside the transect. The calculated average pollination distance within transect was 102 and 98 m based
on the paternity analysis and TwoGener analysis, respectively. We found a significant pollen gene pool structure across seed-trees (
, P < 0.01) that corresponds to an effective number of pollen donors of 6.4 male trees or an effective pollination neighbourhood
area (A
ep
) of 2.1 ha. The findings suggest long-distance pollen dispersion (>100 m) inside the continuous forest. However, the high
proportion occurs in short-distance producing biparental and correlated mating as well as reducing the variance effective
size. 相似文献
897.
Direct electron transfer (DET) from bare spectrographic graphite (SPGE) or 3-mercaptopropionic acid-modified gold (MPA-gold) electrodes to Trachyderma tsunodae bilirubin oxidase (BOD) was studied under anaerobic and aerobic conditions by cyclic voltammetry and chronoamperometry. On cyclic voltammograms nonturnover Faradaic signals with midpoint potentials of about 700 mV and 400 mV were clearly observed corresponding to redox transformations of the T1 site and the T2/T3 cluster of the enzyme, respectively. The immobilized BOD was differently oriented on the two electrodes and its catalysis of O2-electroreduction was also massively different. On SPGE, where most of the enzyme was oriented with the T1 copper site proximal to the carbon with a quite slow ET process, well-pronounced DET-bioelectroreduction of O2 was observed, starting already at > 700 mV vs. NHE. In contrast, on MPA-gold most of the enzyme was oriented with its T2/T3 copper cluster proximal to the metal. Indeed, there was little DET-based catalysis of O2-electroreduction, even though the ET between the MPA-gold and the T2/T3 copper cluster of BOD was similar to that observed for the T1 site at SPGE. When BOD actively catalyzes the O2-electroreduction, the redox potential of its T1 site is 690 mV vs. NHE and that of one of its T2/T3 copper centers is 390 mV vs. NHE. The redox potential of the T2/T3 copper cluster of a resting form of BOD is suggested to be about 360 mV vs. NHE. These values, combined with the observed biocatalytic behavior, strongly suggest an uphill intra-molecular electron transfer from the T1 site to the T2/T3 cluster during the catalytic turnover of the enzyme. 相似文献
898.
Xylitol is a sugar alcohol being explored for clinical uses. The aim was to evaluate the effects of xylitol on Leishmania amazonensis-infected J774A.1 macrophages. Macrophages were infected with L. amazonensis for 3h, washed and incubated with 2.5 or 5.0% xylitol for 24, 48, and 72 h at 37 degrees C. Infection indexes for macrophages incubated only in medium were compared to those treated with xylitol. Cell viability and nitric oxide production were determined each time. Xylitol did not affect L. amazonensis or J774A.1 cell viabilities. Xylitol at 5.0% stimulated nitric oxide production by macrophages at 72 h (p<0.01). At 2.5 and 5.0%, xylitol inhibited nitric oxide production by L. amazonensis at 48 h (p<0.05) when compared to control. Infection indexes were significantly lower at 72 h (p<0.05), (16.9% and 9.6%) in cells cultivated with 2.5 and 5.0% xylitol, respectively, compared to control (38.4%). Results suggest a potential leishmanicidal action of the xylitol on infected macrophages. 相似文献
899.
Ruiz-Gomez Mde L Kittilsen S Höglund E Huntingford FA Sørensen C Pottinger TG Bakken M Winberg S Korzan WJ Overli O 《Hormones and behavior》2008,54(4):534-538
Consistent and heritable individual differences in reaction to challenges, often referred to as stress coping styles, have been extensively documented in vertebrates. In fish, selection for divergent post-stress plasma cortisol levels in rainbow trout (Oncorhynchus mykiss) has yielded a low (LR) and a high responsive (HR) strain. A suite of behavioural traits is associated with this physiological difference, with LR (proactive) fish feeding more rapidly after transfer to a new environment and being socially dominant over HR (reactive) fish. Following transport from the UK to Norway, a switch in behavioural profile occurred in trout from the 3rd generation; HR fish regained feeding sooner than LR fish in a novel environment and became dominant in size-matched HR–LR pairs. One year after transport, HR fish still fed sooner, but no difference in social dominance was found. Among offspring of transported fish, no differences in feeding were observed, but as in pre-transported 3rd generation fish, HR fish lost fights for social dominance against size-matched LR opponents. Transported fish and their offspring retained their distinctive physiological profile throughout the study; HR fish showed consistently higher post-stress cortisol levels at all sampling points. Altered risk-taking and social dominance immediately after transport may be explained by the fact that HR fish lost more body mass during transport than did LR fish. These data demonstrate that some behavioural components of stress coping styles can be modified by experience, whereas behavioural plasticity is limited by genetic effects determining social position early in life story. 相似文献
900.