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   1091716篇
  免费   125901篇
  国内免费   634篇
  2018年   13059篇
  2017年   12653篇
  2016年   14827篇
  2015年   17641篇
  2014年   20154篇
  2013年   28537篇
  2012年   33992篇
  2011年   37812篇
  2010年   26667篇
  2009年   25083篇
  2008年   33739篇
  2007年   35697篇
  2006年   28323篇
  2005年   28203篇
  2004年   27312篇
  2003年   26671篇
  2002年   25814篇
  2001年   44819篇
  2000年   45400篇
  1999年   36290篇
  1998年   14079篇
  1997年   14879篇
  1996年   14211篇
  1995年   13445篇
  1994年   13179篇
  1993年   13197篇
  1992年   31009篇
  1991年   30235篇
  1990年   29542篇
  1989年   28878篇
  1988年   26645篇
  1987年   26042篇
  1986年   24235篇
  1985年   24330篇
  1984年   20251篇
  1983年   17714篇
  1982年   14015篇
  1981年   12641篇
  1980年   12088篇
  1979年   19676篇
  1978年   15788篇
  1977年   14387篇
  1976年   13625篇
  1975年   14865篇
  1974年   15821篇
  1973年   15537篇
  1972年   14378篇
  1971年   13249篇
  1970年   11094篇
  1969年   10603篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Emerging infectious diseases threaten a wide diversity of animals, and important questions remain concerning disease emergence in socially structured populations. We developed a spatially explicit simulation model to investigate whether—and under what conditions—disease-related mortality can impact rates of pathogen spread in populations of polygynous groups. Specifically, we investigated whether pathogen-mediated dispersal (PMD) can occur when females disperse after the resident male dies from disease, thus carrying infections to new groups. We also examined the effects of incubation period and virulence, host mortality and rates of background dispersal, and we used the model to investigate the spread of the virus responsible for Ebola hemorrhagic fever, which currently is devastating African ape populations. Output was analyzed using regression trees, which enable exploration of hierarchical and non-linear relationships. Analyses revealed that the incidence of disease in single-male (polygynous) groups was significantly greater for those groups containing an average of more than six females, while the total number of infected hosts in the population was most sensitive to the number of females per group. Thus, as expected, PMD occurs in polygynous groups and its effects increase as harem size (the number of females) increases. Simulation output further indicated that population-level effects of Ebola are likely to differ among multi-male–multi-female chimpanzees and polygynous gorillas, with larger overall numbers of chimpanzees infected, but more gorilla groups becoming infected due to increased dispersal when the resident male dies. Collectively, our results highlight the importance of social system on the spread of disease in wild mammals.  相似文献   
992.
993.
994.
995.
996.
The experimental literature on learning by Primate and non-Primate mammals is reviewed, with the aim of identifying peculiarly Primate features of learning. The evidence indicates that quantitative comparisons of learning by Primates and other mammals are intrinsically equivocal and uninformative because of the impossibility of equating experimental conditions for members of different species. The comparative results of early learning set studies were seriously misleading because the test conditions discriminated against representatives of species in which vision is not a dominant modality.Analyses of transfer between different learning tasks strongly suggest that rhesus macaques differ qualitatively from non-Primates like cats in that they develop generalized, trans-situationally valid response strategies during training on a particular problem which can facilitate learning in other situations. Non-Primate mammals appear not to develop such strategies under the same circumstances. There is also evidence that monkeys are more able than cats to discard previously learned strategies when they are no longer maximally profitable.  相似文献   
997.
N S Swack  G D Hsiung 《In vitro》1974,10(5-6):260-267
  相似文献   
998.
999.
1000.
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