全文获取类型
收费全文 | 699篇 |
免费 | 51篇 |
出版年
2023年 | 4篇 |
2022年 | 10篇 |
2021年 | 13篇 |
2020年 | 11篇 |
2019年 | 11篇 |
2018年 | 21篇 |
2017年 | 29篇 |
2016年 | 17篇 |
2015年 | 28篇 |
2014年 | 25篇 |
2013年 | 47篇 |
2012年 | 41篇 |
2011年 | 47篇 |
2010年 | 29篇 |
2009年 | 28篇 |
2008年 | 34篇 |
2007年 | 23篇 |
2006年 | 40篇 |
2005年 | 31篇 |
2004年 | 32篇 |
2003年 | 26篇 |
2002年 | 32篇 |
2001年 | 18篇 |
2000年 | 7篇 |
1999年 | 14篇 |
1998年 | 10篇 |
1997年 | 9篇 |
1996年 | 10篇 |
1995年 | 8篇 |
1994年 | 9篇 |
1993年 | 6篇 |
1992年 | 7篇 |
1991年 | 9篇 |
1990年 | 3篇 |
1989年 | 9篇 |
1986年 | 3篇 |
1985年 | 2篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1979年 | 1篇 |
1977年 | 2篇 |
1975年 | 2篇 |
1969年 | 8篇 |
1968年 | 3篇 |
1967年 | 5篇 |
1966年 | 2篇 |
1965年 | 3篇 |
1964年 | 8篇 |
1960年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有750条查询结果,搜索用时 312 毫秒
51.
52.
P. Taraborelli R. Ovejero M. E. Mosca Torres N. M. Schroeder P. Moreno P. Gregorio E. Marcotti A. Marozzi P. Carmanchahi 《Acta theriologica》2014,59(4):529-539
Animals optimize the trade-off between the cost of not fleeing and the benefits of staying because the factors that influence flight decisions and the disturbance level of a particular stimulus can vary both spatially and temporally. Different factors (human impact and habitat characteristics) likely to modify anti-predator behaviour in different types of guanaco social groups were analysed. We found that group size was conditioned by high poaching, vehicle traffic, predation risk and vegetation density. Solitary adult males showed shorter alert and flight initiation distances than bachelor and mixed groups. Alert distance was greater during the summer season, and assessment times were shorter when young were present in the groups. In high-predation-risk environments, guanacos detected threats at greater distances and flight initiation distance was longer. Alert distances were shorter on steeper sloped hills and assessment times were shorter in areas with irregular topography than on flat sites. In high traffic areas, flight initiation distance was longer and assessment times were shorter. And in areas with low poaching intensity, assessment times were greater than in those with high poaching levels. Therefore, guanacos may be able to evaluate a true threat. Social group and anti-predator responses were conditioned by habitat characteristics and human impact. We consider that plasticity of responses could be key to the survival of guanacos. 相似文献
53.
Francisco Guil Sandra Agudín Nuria El-Khadir Mariana Fernandez-Olalla Juan Figueredo Francisco G. Domínguez Paloma Garzon Gregorio Gonzalez Jaime Muñoz-Igualada Javier Oria Fernando Silvestre 《European Journal of Wildlife Research》2010,56(4):633-640
Camera trapping is the most used method for surveying medium-sized carnivores in Spain. The main target for these surveys has been the Iberian lynx, the most endangered cat in the world. The Iberian lynx conservation program has received the largest EU LIFE projects grant. So, efficiency is a key goal for managing this grant. During 2003 and 2007, we have applied these funds to the survey of the Iberian lynx in Eastern Sierra Morena (Spain). Using two different techniques, we have studied both to see which is the most efficient. The survey developed in active latrines resulted more efficient than that of scent stations and live prey camera trapping throughout the years, although there has been a variation between years. Otherwise, the live prey method has been the one providing the greatest speed and number of pictures per entrance. We suggest that camera-trapping surveys can be improved in terms of efficiency for a wide range of species, or at least for the Iberian lynx. To improve the results, cameras might be placed in relation to breeding territories. With this determinant, camera-trapping surveys would be shorter than 120 days. Finally, we suggest how those surveys for medium carnivores should be designed. 相似文献
54.
Valentina Fuentes M Rojas de Astudillo L Diaz A Martínez G 《Revista de biología tropical》2010,58(Z3):129-140
The Gulf of Cariaco is a marine ecosystem with high primary productivity, which gives it an ecological and socioeconomic importance. Nevertheless, anthropogenic activities around the Gulf produce wastes that are deposited directly or by runoff into the sediments, and consequently, increases concentrations of metals in this ecosystem. The objective of this study was to determine the distribution of cadmium, copper, lead, manganese, nickel and zinc in geochemical fractions of surface sediments, using modified BCR sequential extraction procedure. The concentrations were measured using flame atomic absorption spectroscopy. In addition, the contents of soluble and exchangeable metals associated to carbonate fractions, determined by BCR, were compared with those determined by the method of Campanella. Samples were collected in 12 stations during June 2007. The applied methodologies were evaluated with a certified reference material of marine sediments (HISS-1) and the results indicated that these methods provide adequate accuracy and precision for the extraction of metals. The total metal concentrations (microg g(-1)) were, Cd: < limit of detection (LD)-5.0; Pb: 1.79-60.41; Cu: no detected (ND)-42.18; Zn: 25.13-104.57; Mn: 66.31-80.29 and Ni: 3.29-24.58. Cd, Cu, Ni and Pb at several stations, exceeded the Canadian Sediment Quality Guidelines of the Lowest Effect Levels (LEL). Cadmium was identified as being the most mobile of the elements, having the highest concentrations in soluble and exchangeable cations and carbonates. However, Pb, Cu, Mn and Zn levels were found highly associated to organic matter and sulfide fractions. The methods did not show significant statistical differences for the extraction of soluble and exchangeable cations and the metals associated to carbonate fraction. There are several significant correlations between heavy metals, which suggest their common origin. 相似文献
55.
Núñez Flores M Rodríguez-Quintal JG Cristina Díaz M 《Revista de biología tropical》2010,58(Z3):175-187
Sponges constitute one of the most diverse and abundant animal groups in the marine tropical benthos especially in coral reefs, though poorly studied to species level. The aim of this study is to characterize the sponge community along a depth gradient at Isla Larga (Parque Nacional San Esteban, Venezuela) fringe reef. Net and total sedimentation, roughness index, sponge species richness, density and proportion of the bottom covered by sponges, were evaluated at seven depths (1, 3, 6, 9, 12, 15, 18 m), 17 species were identified grouped in 10 demosponges families. The highest densities and coverage corresponded to 6 m of depth (6.03ind/m2; 11%), that coincides with the lowest net sedimentation and highest substrate heterogeneity. Most abundant species were Desmapsamma anchorata, Amphimedon erina and Scopalina rueztleri. Principal component analysis divided this community in three zones according to depth. The shallow zone of the reef (1 and 3 m), where wave force and high irradiance exert a constant stress sponges, shows the lowest density and coverage by sponges. In contrast, medium depth (6, 9 y 12 m) and deep zone (15 y 18 m) with lower light and sedimentation levels seem to enhance sponge growth and survival that are reflected on the higher densities and coverage of sponges. 相似文献
56.
Raquel Gutiérrez-González Gregorio R Boto Cristina Fernández-Pérez Náyade del Prado 《BMC neurology》2010,10(1):93
Background
Infection is a major complication of cerebrospinal fluid shunting procedures. The present report assesses the efficacy of such catheters in both shunts and external ventricular drains (EVDs) against infection and particularly against Staphylococcus spp. infection. 相似文献57.
58.
59.
Gregorio Moreno-Rueda 《Journal of Ethology》2007,25(3):215-222
Offspring should demand more food than the optimal amount for the parents to bring (parent–offspring conflict), and models
on the evolution of parent–offspring communication suggest that an equilibrium is reached when the costs associated with begging
make it unprofitable for the offspring to increase its level of begging. Empirical evidence for this cost, however, is mixed,
and the conclusions of most of authors are that begging is inexpensive. In this study, the existing empirical evidence for
this cost is reviewed. One cost proposed is the attraction of predators due to begging calls, but empirical support for this
cost is low. However, studies performed cannot dismiss such a cost. Another possible cost is the metabolic expenditure, but
empirical evidence for this cost is mixed, with some works contending that it is low, while others deem it important. Other
possible metabolic costs have not been studied. A loss of inclusive fitness may be an important cost for the evolution of
begging, and robust empirical evidence does exist for this cost. Costs associated with brood reduction also are reviewed.
In conclusion, there is not enough empirical evidence to test the models on the evolution of begging. Most costs proposed
have not yet been studied or the approach used has been insufficient to reject the null hypothesis (i.e., absence of cost). 相似文献
60.
Jesudason EP Baben B Ashok BS Masilamoni JG Kirubagaran R Jebaraj WC Jayakumar R 《Molecular and cellular biochemistry》2007,298(1-2):69-81
Aβ vaccination as a therapeutic intervention of Alzheimer’s has many challenges, key among them is the regulation of inflammatory
processes concomitant with excessive generation of free radicals seen during such interventions. Here we report the beneficial
effects of melatonin on inflammation associated with Aβ vaccination in the central and peripheral nervous system of mice.
Mice were divided into three groups (n = 8 in each): control, inflammation (IA), and melatonin-treated (IAM). The brain, liver, and spleen samples were collected
after 5 days for quantitative assessment of plasma lipid peroxides (LPO), an oxidative stress marker, and antioxidant enzymes
such as superoxide dismutase (SOD), catalase (CAT), glutathione (GSH), and glutathione peroxidase (Gpx). IA group mice have
shown the elevated concentration of LPO significantly while there was a reduction at antioxidant enzyme levels. In addition,
a significant (P < 0.05) reduction in neurotransmitters like dopamine (DA), 5-hydroxytryptamine (5-HT), and norepinephrine (NE) was also observed
in the IA group mice. Nevertheless, their metabolites, such as homovanillic acid (HVA) and 5-hydroxyindole acetic acid (5-HIAA)
increased significantly (P < 0.05) as compared to control. Samples were further evaluated at microscopic level to examine the neuropathological changes
by immunohistochemical methods. Melatonin treatment effectively reversed these above changes and normalized the LPO and antioxidant
enzyme levels (P < 0.05). Furthermore, melatonin salvaged the brain cells from inflammation. Our Immunohistochemical findings in the samples
of melatonin-treated animals (IAM group) indicated diminished expression of glial fibrillary acidic protein (GFAP) and nuclear
factor kappa B (NfκB) than those observed in the IA group samples. Our results suggest that administration of melatonin protects
inflammation associated with Aβ vaccination, through its direct and indirect actions and it can be an effective adjuvant in
the development of vaccination in immunotherapy for Alzheimer’s disease (AD). 相似文献