首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   754篇
  免费   70篇
  国内免费   33篇
  857篇
  2024年   3篇
  2023年   13篇
  2022年   17篇
  2021年   11篇
  2020年   32篇
  2019年   46篇
  2018年   33篇
  2017年   27篇
  2016年   33篇
  2015年   23篇
  2014年   56篇
  2013年   56篇
  2012年   41篇
  2011年   43篇
  2010年   36篇
  2009年   44篇
  2008年   43篇
  2007年   31篇
  2006年   28篇
  2005年   34篇
  2004年   22篇
  2003年   24篇
  2002年   16篇
  2001年   8篇
  2000年   7篇
  1999年   15篇
  1998年   16篇
  1997年   6篇
  1996年   14篇
  1995年   8篇
  1994年   7篇
  1993年   7篇
  1992年   5篇
  1991年   1篇
  1990年   3篇
  1989年   4篇
  1987年   1篇
  1986年   1篇
  1985年   4篇
  1984年   6篇
  1983年   3篇
  1982年   4篇
  1981年   3篇
  1980年   5篇
  1979年   6篇
  1978年   3篇
  1977年   1篇
  1976年   4篇
  1974年   2篇
  1973年   1篇
排序方式: 共有857条查询结果,搜索用时 0 毫秒
841.
842.
Increased body size in Brachyteles has been regarded as an important evolutionary adaptation that allowed a greater reliance on leaves compared to other more frugivorous Atelidae, but its association with muriqui positional behavior and substrate use is still unknown. Here, we present original data on the feeding and resting postures of the northern muriqui (Brachyteles hypoxanthus) and evaluate predictions about the relationships between body size, postural behavior, and substrate use derived from previously published data for other atelids (e.g. Alouatta, Ateles, and Lagothrix). The study was undertaken from August 2002 to July 2003 on a large group of well‐habituated muriquis inhabiting the Reserva Particular do Patrimônio Natural – Felíciano Miguel Abdala in Minas Gerais, Brazil. Consistent with our predictions, we found that B. hypoxanthus was highly suspensory during postural feeding (60.9%) and commonly used tail–hind limb suspension/horizontal tripod (38.0%) or tail–forelimb/hind limb suspension (21.4%). However, although tail‐suspensory postures permitted the muriquis to use the terminal canopy and small‐sized substrates, these areas were also accessed via tail‐assisted above‐branch postural behaviors involving multiple substrates. Unexpectedly, tail‐suspensory postures were found to be frequently associated with large substrates, tree trunks, and the understory. We suggest that Brachyteles’ ability to access food resources from all areas of a feeding tree and from tree crowns at different canopy levels may account for their ability to efficiently exploit food resources in seasonal disturbed forest fragments of the Brazilian Atlantic Forest today. Am. J. Primatol. 75:74‐87, 2013. © 2012 Wiley Periodicals, Inc.  相似文献   
843.
An increase in renewable energy and the planting of perennial bioenergy crops is expected in order to meet global greenhouse gas (GHG) targets. Nitrous oxide (N2O) is a potent greenhouse gas, and this paper addresses a knowledge gap concerning soil N2O emissions over the possible “hot spot” of land use conversion from established pasture to the biofuel crop Miscanthus. The work aims to quantify the impacts of this land use change on N2O fluxes using three different cultivation methods. Three replicates of four treatments were established: Miscanthus x giganteus (Mxg) planted without tillage; Mxg planted with light tillage; a novel seed‐based Miscanthus hybrid planted with light tillage under bio‐degradable mulch film; and a control of uncultivated established grass pasture with sheep grazing. Soil N2O fluxes were recorded every 2 weeks using static chambers starting from preconversion in April 2016 and continuing until the end of October 2017. Monthly soil samples were also taken and analysed for nitrate and ammonium. There was no significant difference in N2O emissions between the different cultivation methods. However, in comparison with the uncultivated pasture, N2O emissions from the cultivated Miscanthus plots were 550%–819% higher in the first year (April to December 2016) and 469%–485% higher in the second year (January to October 2017). When added to an estimated carbon cost for production over a 10 year crop lifetime (including crop management, harvest, and transportation), the measured N2O conversion cost of 4.13 Mg CO2‐eq./ha represents a 44% increase in emission compared to the base case. This paper clearly shows the need to incorporate N2O fluxes during Miscanthus establishment into assessments of GHG balances and life cycle analysis and provides vital knowledge needed for this process. This work therefore also helps to support policy decisions regarding the costs and benefits of land use change to Miscanthus.  相似文献   
844.
Using an antibody against a lizard telomerase‐1 component the presence of telomerase has been detected in regenerating lizard tails where numerous cells are proliferating. Immunoblots showed telomerase positive bands at 75–80 kDa in normal tissues and at 50, 75, and 90 kDa in those regenerating. Immunofluorescence and ultrastructural immunolocalization showed telomerase‐immunoreactivity in sparCe (few/diluted) mesenchymal cells of the blastema, early regenerating muscles, perichondrium of the cartilaginous tube, ependyma of the spinal cord, and in the regenerating epidermis. Clusters of gold particles were detected in condensing chromosomes of few mesenchymal and epithelial cells in the regenerating tail, but a low to undetectable labeling in interphase cells. Telomerase‐immunoreactivity was intense in the nucleus and sparCe (few/diluted) in the cytoplasm of spermatogonia and spermatocytes and drastically decreased in early spermatids where some nuclear labeling remains. Some intense immunoreactivity was seen in few cells near the basal membrane of intestinal enterocytes or in leukocytes (likely lymphocytes) of the intestine mucosa. In spermatogonia, spermatids and in enterocytes part of the nuclear labeling formed cluster of gold particles in dense areas identified as Cajal Bodies, suggesting that telomerase is a marker for these stem cells. This therefore suggests that also the sparCe (few/diluted) telomerase positive cells detected in the regenerating tail may represent sparCe (few/diluted) stem cells localized in regenerating tissues where transit amplifying cells are instead preponderant to allow for tail growth. This observation supports previous studies indicating that few stem cells are present in the stump after tail amputation and give rise to transit amplifying cells for tail regeneration. J. Morphol. 276:748–758, 2015. © 2015 Wiley Periodicals, Inc.  相似文献   
845.
846.
847.
The ability of an animal to shed its tail is a widespread antipredator strategy among lizards. The degree of expression of this defense is expected to be shaped by prevailing environmental conditions including local predation pressure. We test these hypotheses by comparing several aspects of caudal autotomy in 15 Mediterranean lizard taxa existing across a swath of mainland and island localities that differ in the number and identity of predator species present. Autotomic ease varied substantially among the study populations, in a pattern that is best explained by the presence of vipers. Neither insularity nor the presence of other types of predators explain the observed autotomy rates. Final concentration of accumulated tail muscle lactate and duration of movement of a shed tail, two traits that were previously thought to relate to predation pressure, are in general not shaped by either predator diversity or insularity. Under conditions of relaxed predation selection, an uncoupling of different aspects of caudal autotomy exists, with some elements (ease of autotomy) declining faster than others (duration of movement, lactate concentration). We compared rates of shed tails in the field against rates of laboratory autotomies conducted under standardized conditions and found very high correlation values ( r > 0.96). This suggests that field autotomy rates, rather than being a metric of predatory attacks, merely reflect the innate predisposition of a taxon to shed its tail.  相似文献   
848.
849.
850.
Health in relation to tail-biting behaviour was investigated on a problem farm. Quartets (n = 16) of age- and gender-matched fattening pigs including a tail biter (TB, n = 16), a victim (V, n = 16), a control in the same pen (Ctb, n = 10) and a control in a pen where no tail biting was observed (Cno, n = 14) were chosen by direct behavioural observation. Haematological and clinicochemical analyses, autopsy and histological examination of 16 different tissues were carried out. Tail lesion severity was evaluated both macroscopically, on the basis of inspection, and histologically, in the sagittally cut tail. Category effects were tested using Friedman's ANOVA by Ranks, Cochran's Q or a repeated-measure GLM and, if significant, pair-wise tests were conducted using Wilcoxon Signed Ranks or McNemar's Test. The number of received tail bites correlated better with histological than with macroscopic tail lesion scoring because of deep inflammation beneath healthy skin in some cases. Most individuals had mild inflammatory lesions in internal organs suggestive of generalized activation of the immune system, and 30% of the animals were anaemic, possibly because of systemic spread of infectious agents. V had more severe respiratory organ lesions and higher serum protein concentrations than all other categories of pigs. Liver- and muscle-specific enzymes (alanine aminotransferase, alkaline phosphatase and creatine kinase) differed between categories. In conclusion, most animals had signs of generalized activation of the immune system, possibly because of systemic spread of infectious agents. V pigs suffered from more severe inflammatory lesions than TB, Ctb or Cno. Deep infections may exist under healthy skin in the tail of bitten pigs.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号