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   397篇
  免费   28篇
  2023年   3篇
  2021年   3篇
  2020年   4篇
  2019年   3篇
  2018年   7篇
  2017年   6篇
  2016年   8篇
  2015年   12篇
  2014年   13篇
  2013年   13篇
  2012年   28篇
  2011年   22篇
  2010年   14篇
  2009年   12篇
  2008年   18篇
  2007年   15篇
  2006年   14篇
  2005年   11篇
  2004年   9篇
  2003年   16篇
  2002年   12篇
  2001年   18篇
  2000年   11篇
  1999年   16篇
  1998年   11篇
  1997年   3篇
  1996年   4篇
  1995年   5篇
  1994年   2篇
  1993年   8篇
  1992年   5篇
  1991年   4篇
  1990年   4篇
  1989年   9篇
  1988年   8篇
  1987年   5篇
  1986年   8篇
  1985年   5篇
  1984年   8篇
  1983年   5篇
  1982年   7篇
  1981年   6篇
  1977年   3篇
  1976年   2篇
  1974年   4篇
  1973年   2篇
  1972年   3篇
  1970年   2篇
  1968年   2篇
  1967年   2篇
排序方式: 共有425条查询结果,搜索用时 328 毫秒
351.
352.
This work assessed the changes in both social interactions and estrogen metabolite excreted in feces in eight group-living Japanese macaques (Macaca fuscata). We tested the hypothesis that the social behavior of pregnant females shows significant changes during the late prepartum and early postpartum period. We also tested the hypothesis that the marked fluctuation in estrogen levels during the perinatal period is associated with the changes in social behavior. We found that pregnant females withdrew from the social life of their group in preparation for parturition and only slowly regained their normal social activity after delivery. These changes were correlated with the fluctuation in estrogen conjugate excreted in feces, giving further evidence that hormones can enhance responsiveness to the infant and may predict maternal competence in macaques. We also found that the high frequency of self-grooming by pregnant females during the perinatal period may be a functional way to improve the quality of care toward an infant by a simple shifting from the care for oneself to the care for the infant after parturition.  相似文献   
353.

Background

In the Calvin cycle of eubacteria, the dephosphorylations of both fructose-1, 6-bisphosphate (FBP) and sedoheptulose-1, 7-bisphosphate (SBP) are catalyzed by the same bifunctional enzyme: fructose-1, 6-bisphosphatase/sedoheptulose-1, 7-bisphosphatase (F/SBPase), while in that of eukaryotic chloroplasts by two distinct enzymes: chloroplastic fructose-1, 6-bisphosphatase (FBPase) and sedoheptulose-1, 7-bisphosphatase (SBPase), respectively. It was proposed that these two eukaryotic enzymes arose from the divergence of a common ancestral eubacterial bifunctional F/SBPase of mitochondrial origin. However, no specific affinity between SBPase and eubacterial FBPase or F/SBPase can be observed in the previous phylogenetic analyses, and it is hard to explain why SBPase and/or F/SBPase are/is absent from most extant nonphotosynthetic eukaryotes according to this scenario.

Results

Domain analysis indicated that eubacterial F/SBPase of two different resources contain distinct domains: proteobacterial F/SBPases contain typical FBPase domain, while cyanobacterial F/SBPases possess FBPase_glpX domain. Therefore, like prokaryotic FBPase, eubacterial F/SBPase can also be divided into two evolutionarily distant classes (Class I and II). Phylogenetic analysis based on a much larger taxonomic sampling than previous work revealed that all eukaryotic SBPase cluster together and form a close sister group to the clade of epsilon-proteobacterial Class I FBPase which are gluconeogenesis-specific enzymes, while all eukaryotic chloroplast FBPase group together with eukaryotic cytosolic FBPase and form another distinct clade which then groups with the Class I FBPase of diverse eubacteria. Motif analysis of these enzymes also supports these phylogenetic correlations.

Conclusions

There are two evolutionarily distant classes of eubacterial bifunctional F/SBPase. Eukaryotic FBPase and SBPase do not diverge from either of them but have two independent origins: SBPase share a common ancestor with the gluconeogenesis-specific Class I FBPase of epsilon-proteobacteria (or probably originated from that of the ancestor of epsilon-proteobacteria), while FBPase arise from Class I FBPase of an unknown kind of eubacteria. During the evolution of SBPase from eubacterial Class I FBPase, the SBP-dephosphorylation activity was acquired through the transition ??from specialist to generalist??. The evolutionary substitution of the endosymbiotic-origin cyanobacterial bifunctional F/SBPase by the two light-regulated substrate-specific enzymes made the regulation of the Calvin cycle more delicate, which contributed to the evolution of eukaryotic photosynthesis and even the entire photosynthetic eukaryotes.  相似文献   
354.
Global Positioning System (GPS) collars have revolutionized the field of spatial ecology, but to date, few primate studies have used them. We fitted a free-ranging, semi-habituated, juvenile male chacma baboon (Papio hamadryas ursinus) with an automatic self-releasing GPS collar and tracked his movements for 359?days. The collar captured 4254 fixes out of 5719 programmed opportunities, a 74.4?% acquisition rate, suggesting that the collar effectively tracked this baboon in a variety of habitat types. Of the data points captured, 73.7?% were three-dimensional fixes, and of these fixes, 66.9?% were highly accurate, having a dilution of precision of less than four. We calculated home range using three protocols with three estimation methods: minimum convex polygon, fixed kernel-density estimation (KDE), and fixed r local convex hull. Using all data points and the 95?% contour, these methods created home range estimations ranging from 10.8 to 23.1?km(2) for this baboon troop. Our results indicate that the KDE output using all data locations most accurately represented our data set, as it created a continuous home range boundary that excluded unused areas and outlying, potentially exploratory data points while including all seven sleeping sites and a movement corridor. However, home range estimations generated from KDE varied from 15.4 to 18.8?km(2) depending on the smoothing parameter used. Our results demonstrated that the ad hoc smoothing parameter selection technique was a better method for our data set than either the least squares cross-validation or biased cross-validation techniques. Our results demonstrate the need for primatologists to develop a standardized reporting method which documents the tool, screening protocol, and smoothing parameter used in the creation of home range estimations in order to make comparisons that are meaningful.  相似文献   
355.
Numerous behavioral and ecological factors are associated with parasite transmission. One factor explored in human research, but absent from nonhuman primate research, is parasite transmission from soil ingestion. Human studies suggest geophagy, the regular and deliberate consumption of soil, increases risk of soil‐transmitted helminth (STH) infection. Geophagy, which is prevalent in nonhuman primates, has several positive associations: gastrointestinal distress alleviation, possible mineral supplementation, and bacterial infection prevention. Our objective was to determine whether STH transmission was possible from deliberate or accidental soil ingestion, in a troop of chacma baboons (Papio hamadryas ursinus) that engaged in geophagy, foraged on the terrestrial substrata, and had a Trichuris sp. sample prevalence of 100%. We collected and analyzed 80 soil samples from geophagy and ground foraging sites on and around Wildcliff Nature Reserve, South Africa. Forty soil samples were collected from sites where soil was consumed, and 40 were collected from sites where soil was not consumed. At geophagy sites, the number of Trichuris sp. eggs recovered varied significantly between areas used and unused by the baboons, suggesting behavior is an important factor. In contrast, at foraging sites, there was only a tendency to recover more eggs at used than unused areas, and we propose egg recovery was influenced by fecal contamination that occurred throughout foraging stands. The difference in egg recovery between used areas at geophagy sites and used areas at foraging sites was not significant. These preliminary findings suggest both geophagy and foraging sites are a potential source of STH infection for this troop. Whether geophagy benefits outweigh the potential cost of parasite reinfection for this baboon troop is unknown, so we encourage future research on the influence that host foraging behavior may have on parasite reinfection. Am. J. Primatol. 74:940‐947, 2012. © 2012 Wiley Periodicals, Inc.  相似文献   
356.
Primates are among the most threatened of animal taxa, and it is therefore increasingly important to examine ways to monitor their health in the wild. We here investigate trends in nutrition and health in Japanese macaques (Macaca fuscata yakui) under natural conditions using a common tool from human and veterinary health practices: urinalysis. Between October 2007 and August 2009, we collected 103 urine samples ad libitum from 29 identified individuals of a group of macaques on Yakushima Island. We used multireagent dipstick analysis to examine variation in 10 urine chemistry parameters across subjects in combination with systematic monitoring of clinical signs of disease (via focal sampling). Positive tests tended to be transient and weak, i.e., analytes detected in trace amounts, and were not associated with clinical signs of disease. In addition, most urine samples we collected were highly concentrated (urine specific gravity ≥1.030), which could have increased the likelihood of detecting clinically insignificant amounts of analytes that can occur in urine. However, we found that ketone bodies, which are not normally found in urine, were more prevalent during winter than summer, which may indicate that Japanese macaques inhabiting the richest habitat available to the species nonetheless experience an energy deficit during periods of low food quality and high thermoregulatory costs. We conclude that dipstick urinalysis can be used to examine general trends in nutrition and health, but that results must be interpreted with care because positive tests may not reflect clinically significant urological conditions in many cases.  相似文献   
357.
Reduced uterine perfusion initiated in late gestation in the rat results in intrauterine growth restriction (IUGR) and development of hypertension by 4 wk of age. We hypothesize that the renin angiotensin system (RAS), a regulatory system important in the long-term control of blood pressure, may be programmed by placental insufficiency and may contribute to the etiology of IUGR hypertension. We previously reported that RAS blockade abolished hypertension in adult IUGR offspring; however, the mechanisms responsible for the early phase of hypertension are unresolved. Therefore, the purpose of this study was to examine RAS involvement in early programmed hypertension and to determine whether temporal changes in RAS expression are observed in IUGR offspring. Renal renin and angiotensinogen mRNA expression were significantly decreased at birth (80 and 60%, respectively); plasma and renal RAS did not differ in conjunction with hypertension (mean increase of 14 mmHg) in young IUGR offspring; however, hypertension (mean increase of 22 mmHg) in adult IUGR offspring was associated with marked increases in renal angiotensin-converting enzyme (ACE) activity (122%) and renal renin and angiotensinogen mRNA (7-fold and 7.4-fold, respectively), but no change in renal ANG II or angiotensin type 1 receptor. ACE inhibition (enalapril, 10 mg x kg(-1) x day(-1), administered from 2 to 4 wk of age) abolished hypertension in IUGR at 4 wk of age (decrease of 15 mmHg, respectively) with no significant depressor effect in control offspring. Therefore, temporal alterations in renal RAS are observed in IUGR offspring and may play a key role in the etiology of IUGR hypertension.  相似文献   
358.
Primates - Demographic studies on translocated primate groups provide a unique opportunity to study population dynamics, social strategies, and reproductive parameters of a species adapting to new...  相似文献   
359.
Macrophages regulate lymphatic vasculature development; however, the molecular mechanisms regulating their recruitment to developing, and adult, lymphatic vascular sites are not known. Here, we report that resting mice deficient for the inflammatory chemokine‐scavenging receptor, ACKR2, display increased lymphatic vessel density in a range of tissues under resting and regenerating conditions. This appears not to alter dendritic cell migration to draining lymph nodes but is associated with enhanced fluid drainage from peripheral tissues and thus with a hypotensive phenotype. Examination of embryonic skin revealed that this lymphatic vessel density phenotype is developmentally established. Further studies indicated that macrophages and the inflammatory CC‐chemokine CCL2, which is scavenged by ACKR2, are associated with this phenotype. Accordingly, mice deficient for the CCL2 signalling receptor, CCR2, displayed a reciprocal phenotype of reduced lymphatic vessel density. Further examination revealed that proximity of pro‐lymphangiogenic macrophages to developing lymphatic vessel surfaces is increased in ACKR2‐deficient mice and reduced in CCR2‐deficient mice. Therefore, these receptors regulate vessel density by reciprocally modulating pro‐lymphangiogenic macrophage recruitment, and proximity, to developing, resting and regenerating lymphatic vessels.  相似文献   
360.
We have developed a novel hybridization platform that utilizes nuclear male sterility to produce hybrids in maize and other cross‐pollinating crops. A key component of this platform is a process termed Seed Production Technology (SPT). This process incorporates a transgenic SPT maintainer line capable of propagating nontransgenic nuclear male‐sterile lines for use as female parents in hybrid production. The maize SPT maintainer line is a homozygous recessive male sterile transformed with a SPT construct containing (i) a complementary wild‐type male fertility gene to restore fertility, (ii) an α‐amylase gene to disrupt pollination and (iii) a seed colour marker gene. The sporophytic wild‐type allele complements the recessive mutation, enabling the development of pollen grains, all of which carry the recessive allele but with only half carrying the SPT transgenes. Pollen grains with the SPT transgenes exhibit starch depletion resulting from expression of α‐amylase and are unable to germinate. Pollen grains that do not carry the SPT transgenes are nontransgenic and are able to fertilize homozygous mutant plants, resulting in nontransgenic male‐sterile progeny for use as female parents. Because transgenic SPT maintainer seeds express a red fluorescent protein, they can be detected and efficiently separated from seeds that do not contain the SPT transgenes by mechanical colour sorting. The SPT process has the potential to replace current approaches to pollen control in commercial maize hybrid seed production. It also has important applications for other cross‐pollinating crops where it can unlock the potential for greater hybrid productivity through expanding the parental germplasm pool.  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