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   5607篇
  免费   571篇
  国内免费   382篇
  2024年   21篇
  2023年   170篇
  2022年   138篇
  2021年   237篇
  2020年   266篇
  2019年   329篇
  2018年   252篇
  2017年   255篇
  2016年   329篇
  2015年   375篇
  2014年   378篇
  2013年   482篇
  2012年   275篇
  2011年   315篇
  2010年   245篇
  2009年   285篇
  2008年   295篇
  2007年   282篇
  2006年   184篇
  2005年   168篇
  2004年   145篇
  2003年   134篇
  2002年   133篇
  2001年   96篇
  2000年   73篇
  1999年   77篇
  1998年   65篇
  1997年   67篇
  1996年   69篇
  1995年   44篇
  1994年   52篇
  1993年   44篇
  1992年   33篇
  1991年   33篇
  1990年   26篇
  1989年   23篇
  1988年   24篇
  1987年   21篇
  1986年   18篇
  1985年   23篇
  1984年   9篇
  1983年   16篇
  1982年   18篇
  1981年   7篇
  1980年   9篇
  1979年   4篇
  1978年   7篇
  1977年   3篇
  1976年   2篇
  1974年   2篇
排序方式: 共有6560条查询结果,搜索用时 304 毫秒
51.
Depending on spatial requirements and the distribution of key resources in the environment, social behavior among lizards varies from defense of exclusive territories to the establishment of dominance hierarchies. In captivity or under conditions where dispersal is not possible, dominance hierarchies often emerge in species that are otherwise territorial. This review explores some of the morphological, behavioral, and hormonal determinants of social status in male lizards and how these may lead to differential reproductive function in dominant and subordinate individuals. Emphasis is placed on the importance of population density, local resource dispersion, and the composition and stability of social groups in promoting hierarchical behavior. Results of these studies have ramifications for several aspects of zoo management, including exhibit design, choice of animals to be housed together, provision of resources in space and time, and orientation of enclosures within captive breeding facilities. © 1994 Wiley-Liss, Inc.  相似文献   
52.
Many studies have reported on the dental status of elderly patients. The results demonstrate consistent differences between normative and perceived needs in addition to varying levels of awareness of dental education among carers. This review article attempts to integrate a range of social factors which must be considered if holistic and comprehensive care is to be delivered to elderly patients.  相似文献   
53.
We monitored one group of muriquis, or woolly spider monkeys (Brachyteles arachnoides), over a 9-year period at Fazenda Montes Claros, Minas Gerais, Brazil. The group grew from 22 to 42 individuals due to the births of 21 surviving infants. Eight immigrations involving immature females were offset by emigrations and disappearances. The home range of the group expanded as the group size increased. The group traveled as a cohesive unit during the first 6 years of the study, but recently it has begun to show greater tendencies to fission temporarily into smaller subgroups. Six adult males from the other muriqui group at this site have simultaneously increased their associations with the main study group. These observations indicate that the group is in a state of transition which may lead, ultimately, either to its division into two smaller units or to a more fluid social structure.  相似文献   
54.
Polygynous groups of de Brazzas monkeys (Cercopithecus neglectus) were observed in the Kisere Forest Reserve between December 1987 and March 1989. Total number of contact hours was 528. Three troops numbered 11, 13, and 16 monkeys. Three solitary adult males were seen. All troops had a single adult male, at least three adult females, and juveniles. Five births occurred in the course of the study. Home range varied from 4.1 to 6 ha, and flooded areas of the forest were heavily used. This species was only found near rivers and spent more than 50% of the time below 5 m. Daily path length ranged from 330 to 1001 m. The de Brazza's monkeys were mainly frugivorous, but leave and invertebrates also formed a substantial part of the diet. Other guenons avoided de Brazza's, which did not join polyspecific interactions.  相似文献   
55.
Different cultural norms and standards for appropriate female body size might contribute to the disparity in obesity rates between black and white adult females (46.0% and 24.6% respectively). The purpose of this study was to measure adolescents' perceptions of ideal size and social norms regarding female body size as well as adolescents' perceptions of significant others' evaluation and expectations of the adolescents' body size. Subjects included 437 adolescent girls (247 white and 190 black) aged 13 to 19 (x=44.9, SD=.979) from six randomly selected public schools. The subjects, heights and weights were measured. Responses to a body image questionnaire and a series of nine female body drawings (arranged ordinally, 1 to 9, from thinnest to heaviest) were analyzed using the General Linear Model and Logistic Regression. The female body size considered ideal by black females was significantly larger than the size selected as ideal by white females (x= 3.47 and x= 3.13 respectively, p< 0.001). Black females were two times more likely than white females to describe themselves as thinner than other girls their age (O.R. = 2.01, 95% C.I.1.34, 3.01) and seven times as likely to say that they were not overweight (O.R. = 7.08, 95% C.I. 3.72, 13.45). White females wanted to be a smaller size than they currently were and felt encouraged by significant others to lose weight or reduce their size. Black females did not indicate as great a desire as whites to be smaller and they tended to feel that their size was considered satisfactory by significant others. Only subjects from the low SES group perceived that significant others wanted them to gain weight. The differences between black and white subjects' beliefs and perceptions about body size norms may explain, in part, why heavier body weights persist in some cultural groups.  相似文献   
56.
In ants, workers of different sizes may perform various tasks, even in so-called monomorphic species with relatively low body size variation. However, it is unclear if the body size diversity of monomorphic workers correlates with task efficiency, especially in stressful contingencies. Here we tested if the body size variation of workers corresponds with its efficiency in transferring pupae. Transferring brood is a pre-set behavioral response to stress, e.g. suboptimal temperature. Here we applied a laboratory experiment simulating nest damage. The study was performed on the common garden ant (Lasius niger (Linnaeus, 1758)) – a species with no distinct worker subcastes. The efficiency of workers was measured as the latency of transferring pupae from a lit part of the experimental colony to a darkened part, while the body size diversity was expressed as the within-colony coefficient of variation in head width. We did not find any significant correlation between efficiency and body size variation. Summarizing the existing studies and the present results, we propose the hypothesis that the body size diversity of L. niger may have implications for workers’ division of labor but not for their task efficiency in a stressful contingency.  相似文献   
57.
Now is the time to refocus efforts in urban research and design. A changing climate and extreme weather events are presenting unique challenges to urban systems around the world. These challenges illuminate the social barriers that accompany disruptive events such as resource inequities and injustices. In this perspective, we provide three research priorities for just and sustainable urban systems that help to address these matters. The three research priorities are: (1) social equity and justice, (2) circularity, and (3) digital twins. Conceptual context and future research directions are provided for each. For social equity and justice, the future directions are mandatory equity analysis and inclusionary practices, understanding and reconciling historical injustices, and intentional integration with diverse community stakeholders. For circularity applications, they are better metrics for integration, more robust evaluation frameworks, and dynamic modeling at multiple spatial and temporal scales. Future directions for digital twins include developing principles to reduce complexity, integrating model and system components, and reducing barriers to data access. These research priorities are core to meeting several of the United Nations Sustainable Development Goals (i.e., 1—No Poverty, 8—Decent Work and Economic Growth, 10—Reduced Inequalities, and 11—Sustainable Cities and Communities). Useful social and technical matters are discussed throughout, where we highlight the importance of prioritizing localized research efforts, provide guidance for community-engaged research and co-development practices, and explain how these priorities interact to align with the evolving field of industrial ecology.  相似文献   
58.
The cultural reproduction of lithic technology, long an implicit assumption of archaeological theories, has garnered increasing attention over the past decades. Major debates ranging from the origins of the human culture capacity to the interpretation of spatiotemporal patterning now make explicit reference to social learning mechanisms and cultural evolutionary dynamics. This burgeoning literature has produced important insights and methodological innovations. However, this rapid growth has sometimes led to confusion and controversy due to an under-examination of underlying theoretical and methodological assumptions. The time is thus ripe for a critical assessment of progress in the study of the cultural reproduction of lithic technology. Here we review recent work addressing the evolutionary origins of human culture and the meaning of artifact variation at both intrasite and intersite levels. We propose that further progress will require a more extended and context-specific evolutionary approach to address the complexity of real-world cultural reproduction.  相似文献   
59.
The snare web is used as a medium for communication betweenindividuals within colonies of social spiders and has thereforebeen suggested as necessary for the evolution of sociality inthe Araneae. The social spider Diaea socialis (Thomisidae) isan exception because it does not build a snare web. Experimentsdemonstrate that silk attracts all spiders and that a chemicaldeposited onto the silk attracts adult female spiders, suggestingthat the group living of this species is mediated by a pheromone.The pheromone attracts spiders differentially: females are notattracted to juvenile silk, and it repels gravid females. Thepheromone appears to be stable but volatile, is ether-soluble,and retains its viability after dissolution. Molecular-ionicmasses for 7-8 different compounds were found in the range 220–281atomic units; the pheromone may be one or a combination of severalof these.  相似文献   
60.
Earwigs (Insecta, Dermaptera) are characterized by uniquelyelaborated cerci, commonly called forceps, the function of whichremains unclear. We studied intrasexual and intersexual interactionsin the laboratory to examine the context and pattern of forcepsuse in the toothed earwig. Vostox apictdenlatus (Caudell). Interactionsbetween pairs of earwigs were recorded in four social situations:(1) two males, (2) two males plus a virgin female, (3) two females,and (4) one male and one female. Forceps were used as both weaponsand display structures by males and females in all of thesesocial contexts. During pairwise male-male interactions, onemale clearly dominated the other male. Dominant males were moreactive and more likely to use their forceps in intrasexual interactionsthan were subordinate males. In interactions where there weretwo males and one female present, the male that dominated male-maleinteractions was able to maintain exclusive access to the female.There was no indication of active female choice during or aftercourtship. During intersexual interactions, only males usedtheir forceps during courtship. The behavioral repertoire involvingforceps was greater for males than for females, especially inintrasexual contests. There was no clear outcome of intrasexualinteractions among females. These results suggest that forcepsfunction mainly as weapons in male-male interactions and mayhave evolved, at least in part, as a result of sexual selection.Further research is required to test for female mate choiceand to separate the various mechanisms of sexual selection ifmate choice exists. Comparative studies are needed to determineif sexual selection was the original evolutionary mechanismleading to the development of these unusual structures or ifsexual selection is relegated to a secondary effect, leadingto the elaboration and sexualdimorphism of these structuresin selected groups of earwigs.  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