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   123203篇
  免费   3018篇
  国内免费   907篇
  127128篇
  2023年   161篇
  2022年   368篇
  2021年   776篇
  2020年   415篇
  2019年   550篇
  2018年   12367篇
  2017年   11091篇
  2016年   8314篇
  2015年   2204篇
  2014年   1978篇
  2013年   2271篇
  2012年   6780篇
  2011年   15335篇
  2010年   13453篇
  2009年   9494篇
  2008年   11654篇
  2007年   13161篇
  2006年   2172篇
  2005年   2146篇
  2004年   2625篇
  2003年   2479篇
  2002年   2200篇
  2001年   528篇
  2000年   358篇
  1999年   287篇
  1998年   305篇
  1997年   235篇
  1996年   206篇
  1995年   167篇
  1994年   155篇
  1993年   172篇
  1992年   134篇
  1991年   177篇
  1990年   109篇
  1989年   99篇
  1988年   99篇
  1987年   79篇
  1986年   82篇
  1985年   89篇
  1984年   99篇
  1983年   109篇
  1982年   113篇
  1981年   110篇
  1980年   88篇
  1979年   60篇
  1976年   47篇
  1975年   51篇
  1974年   46篇
  1972年   270篇
  1971年   300篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.

Objective

To examine the effect of PCI-24781 (abexinostat) on the blastocyst formation rate in pig somatic cell nuclear transferred (SCNT) embryos and acetylation levels of the histone H3 lysine 9 and histone H4 lysine 12.

Results

Treatment with 0.5 nM PCI-24781 for 6 h significantly improved the development of cloned embryos, in comparison to the control group (25.3 vs. 10.5 %, P < 0.05). Furthermore, PCI-24781 treatment led to elevated acetylation of H3K9 and H4K12. TUNEL assay and Hoechst 33342 staining revealed that the percentage of apoptotic cells in blastocysts was significantly lower in PCI-24781-treated SCNT embryos than in untreated embryos. Also, PCI-24781-treated embryos were transferred into three surrogate sows, one of whom became pregnant and two fetuses developed.

Conclusion

PCI-24781 improves nuclear reprogramming and the developmental potential of pig SCNT embryos.
  相似文献   
972.
973.
It is essential to document habitat occupancy patterns and population structure to facilitate the survival of primates in areas of anthropogenic disturbance. The overlapping of the Nearctic and Neotropical regions in the Olmec region of Mexico make this area particularly important as part of a natural biological corridor that harbors a high number of endemic species and connects the Atlantic and Pacific coastal plains. We surveyed Alouatta palliata mexicana (Mexican mantled howlers) in a 300-km2 area to determine if fragment occupation and subpopulation structure were related to the spatial attributes of the fragments. We measured the fragment size and shape as well as the distances to the nearest road, human settlement, agricultural field, and nearest neighboring fragment. During 1 year (ca. 4500 fieldwork hours) we surveyed 48 fragments, 17 of which were occupied, and counted 198 howlers. Larger fragments that were farther from agricultural activities were more likely to be occupied. Subpopulation size and number of individuals in all age–sex classes increased in larger fragments that were closer to other fragments. We found more females and juveniles, as well as more females per male in fragments that were farther from roads and we found fewer immatures per female, females per male, and individuals per area in more irregular fragments. In addition, more males and immatures per female occurred in fragments that were farther from agricultural fields. The Olmec Region is located at the center of the geographic distribution of mantled howlers in Mexico, and could therefore play a fundamental role in maintaining the contact between different populations. However, our study suggests that mantled howlers are highly threatened by anthropogenic habitat disturbance in this area, mainly through the loss of their habitat and contact with humans.  相似文献   
974.
Accurate and precise surveys of primate abundance provide the basis for understanding species ecology and essential information for conservation assessments. Owing to the elusive nature of wild apes and the vast region of dense forest they inhabit, population estimates of central chimpanzees (Pan troglodytes troglodytes) and western lowland gorillas (Gorilla gorilla gorilla) have largely relied on surveys of their nests. Specific information about the nesting behavior of apes permits the estimation of the number of nests built (nest creation rate). Similarly, information on nest characteristics and environmental factors can be used to estimate the time it takes nests to decay (nest decay rate). Nest creation and decay rates are then used to convert nest density estimates to absolute ape densities. Population estimates that use site-specific estimates of nest creation and decay rates are more accurate and precise. However, it is common practice to generalize these conversion factors across sites because of the additional cost of studies required to gather the information to estimate them. Over a 9-mo study period, we detected and monitored the time to decay of gorilla nests (N = 514) and chimpanzee nests (N = 521) in northern Republic of Congo. We investigated the influence of nest characteristics and environmental factors on nest survivorship and estimated the mean time to nest decay (or equivalently survival) using MARK. Key factors influencing nest decay rate included ape species, forest type, nest height, mean rainfall, nest structure, nest type, and primary aspects of nest construction. Our findings highlight the synergistic effect of behavior and environment on great ape nest degradation, as well as providing practical insights for improving measures to monitor remaining populations of these endangered species.  相似文献   
975.
Copper may be involved in the pathophysiology of depression. Clinical data on this issue are very limited and not conclusive. The purpose of the study was to determine the copper concentration in the serum of patients with major depressive disorder and to discuss its potential clinical usefulness as a biomarker of the disease. A case–control clinical study included 69 patients with current depressive episode, 45 patients in remission and 50 healthy volunteers. Cu concentration was measured by electrothermal atomic absorption spectrometry (ETAAS). The mean serum copper level in depressed patients was slightly lower (by 11 %; not statistically significant) than in the control group. Furthermore, there was no significant difference in Cu2+ concentration between depressive episode and remission, nor between remission and control group. In the remission group were observed significant correlations between copper levels and the average number of relapses over the past years or time of remission. There was no correlation between serum copper and severity of depression, as measured by HDRS and MADRS. The obtained results showed no significant differences between the copper concentration in the blood serum of patients (both with current depressive episode and in remission) and healthy volunteers, as well as the lack of correlations between the copper level in the active stage of the disease and clinical features of the population. Our study is the first conducted on such a large population of patients, so the results may be particularly important and reliable source of knowledge about the potential role of copper in depression.  相似文献   
976.
The placement of radio- and GPS-collars on primates has advanced the fields of primate behavior and ecology and yielded considerable conservation gains. However, to ensure that the placement of tracking collars remains a humane practice, the reporting of both positive and negative outcomes is required. Here, we evaluate the impacts of radio-collars on the reproduction, behavior, and health of female mantled howlers (Alouatta palliata) on Barro Colorado Island, Panama. In June 2005, as part of a behavioral study, we fitted 16 female howlers with ball-chain radio-collars weighing 1.2% of the mean adult female weight. We monitored these 16 collared females and 64 noncollared females across an annual cycle, and recaptured the collared females in July 2006 for collar removal. The mortality rate for collared females across the study year was 6.3% (N=1), while the net loss of noncollared females from study groups was 15.6% (N=10). All collared females reproduced. The behaviors of collared and noncollared females were not significantly different. However, on recapture to remove all remaining collars (N = 13), 38% of recaptured collared females presented with damage to the dermal layer on the back of the neck and 31% presented with more severe damage extending into the subcutaneous tissue and muscle. Given this physical damage, which was not apparent through binoculars, we cannot recommend using ball-chain radio-collars on female mantled howlers without careful welfare monitoring.  相似文献   
977.
Stress contributes to the development of chronic degenerative diseases in primates. Allostatic load is an estimate of stress-induced physiological dysregulation based on an index of multiple biomarkers. It has been applied to humans to measure effects of stress and predict health outcomes. Assessing allostatic load in nonhuman primates may aid in understanding factors promoting compromised health and longevity in captive populations, as well as risk assessment among wild populations following human activities. We applied an allostatic load index to gorillas housed at the Columbus Zoo and Aquarium (N = 27, 1956–2014) using data from medical records and biomarkers from banked serum. We estimated allostatic load using seven biomarkers (albumin, cortisol, corticotropin-releasing hormone, dehydroepiandrosterone sulfate, glucose, interleukin-6, and tumor necrosis factor alpha) and then examined this index for associations with age, sex, number of stressful events, parturition, physiological health measures, and age at death. Stressful events were defined as agonistic interactions with wounding, translocations, and anesthetizations. Allostatic load positively associated with age and total number of lifetime stressful events. Allostatic load was significantly higher in females than in males. Allostatic load was not associated with number of pregnancies and was not different between nulliparous and parous females. Allostatic load associated positively with serum creatinine and triglyceride levels, showed a nonsignificant negative association with cholesterol, and did not associate significantly with age at death. These results demonstrate the potential utility of allostatic load for exploring long-term stress and health risks, as well as for evaluating environmental stressors for gorillas and other nonhuman primates in captivity and in the wild.  相似文献   
978.
“Air (aire, also aigre) in the body” is a frequent explanation of illness according to the traditional medical beliefs in Mexico. Anthropologists have generally scrutinized aire in the context of other common folk illnesses treated by traditional healers (curanderas). However, drawing on my research in the communities of Northern Oaxaca I suggest that aire occupies a more distinct position in the folk medical cosmology than it has usually been credited with. This distinction rests on the notion’s exceptional ambivalence and openness to multiple interpretations. “Air” is recurred to as the cause of illness mainly in situations where every other explanation, either “traditional” or “biomedical,” seems to be inadequate. The physical properties of air—its transparency, invisibility, apparent immateriality, near omnipresence, and virtual “nothingness”—render it a suitable explanation of the last resort. Local understandings of what aire “is” are often vague and elusive, and in many respects the term functions in folk medical discourse as an “empty signifier.”  相似文献   
979.
Arboreal primates actively navigate a complex thermal environment that exhibits spatial, daily, and seasonal temperature changes. Thus, temperature measurements from stationary recording devices in or near a forest likely do not reflect the thermal microenvironments that primates actually experience. To better understand the thermal variation primates encounter, we attached automated temperature loggers to anklets worn by free-ranging mantled howling monkeys (Alouatta palliata) to record near-animal ambient temperatures. We compared these measures to conventional, stationary temperature measurements taken from within the forest, in nearby open fields, and at a remote weather station 38.6 km from the field site. We also measured temperatures across vertical forest heights and assessed the effects of wind speed, solar radiation, rain, and vapor pressure on primate subcutaneous temperatures (collected via implanted loggers). Ambient temperatures at measurement sites commonly used by researchers differed from those experienced by animals. Moreover, these differences changed between seasons, indicating dynamic shifts in thermal environment occur through space and time. Temperatures increased with height in the forest, with statistically significant, albeit low magnitude, differences between vertical distances of one meter. Near-animal temperatures showed that monkeys selected relatively warmer microhabitats during nighttime temperature lows and relatively cooler microhabitats during the day. Lastly, the thermal variables wind speed, solar radiation, vapor pressure, and rain were statistically associated with primate subcutaneous temperatures. Our data indicate that the temperatures arboreal primates experience are not well reflected by stationary devices. Attaching automated temperature loggers to animals provides a useful tool for more directly assessing primate microhabitat use.  相似文献   
980.
“Thinking too much”, and variations such as “thinking a lot”, are common idioms of distress across the world. The contextual meaning of this idiom of distress in particular localities remains largely unknown. This paper reports on a systematic study of the content and cause, consequences, and social response and coping related to the local terms |x’an n|a te and |eu-ca n|a te, both translated as “thinking a lot”, and was part of a larger ethnographic study among the Khwe of South Africa. Semi-structured exploratory interviews with community members revealed that “thinking a lot” refers to a common experience of reflecting on personal and interpersonal problems. Consequences were described in emotional, psychological, social, behavioral, and physical effects. Coping strategies included social support, distraction, and religious practices. Our contextualized approach revealed meanings and experiences of “thinking a lot” that go beyond a psychological state or psychopathology. The common experience of “thinking a lot” is situated in socio-political, economic, and social context that reflect the marginalized and displaced position of the Khwe. We argue that “thinking a lot” and associated local meanings may vary across settings, may not necessarily indicate psychopathology, and should be understood in individual, interpersonal, community, and socio-political dimensions.  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