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   217篇
  免费   2篇
  2014年   2篇
  2013年   5篇
  2012年   6篇
  2011年   15篇
  2010年   26篇
  2009年   13篇
  2008年   13篇
  2007年   14篇
  2006年   8篇
  2005年   6篇
  2004年   5篇
  2003年   5篇
  2001年   2篇
  2000年   2篇
  1999年   2篇
  1997年   3篇
  1996年   2篇
  1995年   2篇
  1994年   4篇
  1993年   2篇
  1992年   2篇
  1989年   2篇
  1988年   2篇
  1986年   3篇
  1984年   3篇
  1983年   2篇
  1982年   3篇
  1981年   5篇
  1980年   3篇
  1979年   5篇
  1978年   3篇
  1976年   3篇
  1975年   2篇
  1974年   2篇
  1973年   2篇
  1972年   4篇
  1971年   3篇
  1965年   1篇
  1959年   1篇
  1958年   2篇
  1957年   1篇
  1956年   6篇
  1955年   1篇
  1954年   2篇
  1953年   1篇
  1952年   2篇
  1951年   2篇
  1950年   1篇
  1949年   1篇
  1925年   1篇
排序方式: 共有219条查询结果,搜索用时 15 毫秒
21.
Abstract Coevolution is thought to have led to many small mammal species avoiding the scent marks of their main mammalian predators, as they provide a reliable cue to predation risk. Most support for this hypothesis comes from northern hemisphere predator/prey systems, however, it is unclear whether this avoidance of predator faecal odour occurs in Australia's mammalian fauna, which has evolved in relative isolation from the rest of the world, and is dominated by marsupials rather than placentals. We tested this theory for an Australian system with marsupial and placental predators and prey, that share a long‐term (>1 million years) or short‐term (<150 years) exposure to each other. The predators were the native marsupial tiger quoll Dasyurus maculatus and the introduced placental red fox Vulpes vulpes. The potential prey were three native rodent species, the bush rat Rattus fuscipes, the swamp rat Rattus lutreolus, the eastern chestnut mouse Pseudomys gracilicaudatus, and the marsupial brown antechinus Antechinus stuartii. Small mammals were captured in Elliott traps with 1/3 of traps treated with fox faeces, 1/3 treated with quoll faeces and the remainder left untreated. The native rodent species all showed avoidance of both tiger quoll and red fox odours whereas the marsupial antechinus showed no responses to either odour. Either predator odour avoidance has not evolved in this marsupial or their reaction to predator odours may be exhibited in ways which are not recognizable through trapping. The avoidance by the rodents of fox odour as well as quoll odour indicates this response may either be due to common components in fox and quoll odour, or it may be a recently evolved response.  相似文献   
22.
Powwow     
Powwow. Clyde Ellis, Luke Eric Lassiter, and Gary H. Dunham, eds. Lincoln: University of Nebraska Press, 2005. 309 pp.  相似文献   
23.
The population genetics and systematics of coccidian parasites of the genus Sarcocystis remain poorly defined, notwithstanding their relevency to veterinary and human health. Despite opportunities for sexual recombination, nonrecombinant parasite clones characterized by distinct transmission and pathogenesis traits persist in related parasites (i.e. Toxoplasma gondii). In order to determine whether this may be generally true for parasitic coccidia, and to address evolutionary and taxonomic problems within the genus Sarcocystis, we isolated 12 polymorphic microsatellite markers (four to 14 alleles) for Sarcocystis neurona, the major causative agent of equine protozoal myeloencephalitis (EPM).  相似文献   
24.
Abstract Lynx (Lynx canadensis) occur in the northern counties of Washington state, USA; however, current distribution and status of lynx in Washington is poorly understood. During winters 2002–2004 we snow-tracked lynx for 155 km within a 211-km2 area in northern Washington, to develop a model of lynx-habitat relationships that we could use to assess their potential distribution and status in the state. We recorded movements and behaviors of lynx with a Global Positioning System and overlaid digitized lynx trails on various habitat layers using a Geographic Information System. Based on univariate analyses, lynx preferred Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) forests, with moderate canopy and understory cover, and elevations ranging from 1,525 m to 1,829 m but avoided Douglas-fir (Pseudotsuga menziesii) and ponderosa pine (Pinus ponderosa) forests, openings, recent burns, open canopy and understory cover, and steep slopes. A map of suitable lynx habitat based on a logistic regression model built using these candidate variables revealed that habitats at elevations >1,400 m where lynx historically occurred in Washington are intersected and fragmented by landscape features and forest conditions that are generally avoided by lynx. Our habitat suitability map predicts 3,800 km2 of lynx habitat in Washington that could support 87 lynx, far fewer than previous estimates. Since 1985, natural fires have burned >1,000 km2 of forested habitat in Okanogan County, the only region in Washington where lynx occurrence has been documented during that period. Loss of suitable habitat from natural and human-caused disturbances, and the lack of verifiable evidence of lynx occurrence in historic lynx range, suggests that fragmented landscape conditions may have impeded recolonization of these areas by lynx. Consequently, translocations may be necessary to ensure lynx persistence in Washington. We suggest that managers assess the potential for translocation by first identifying the scale and distribution of potential foraging habitats for lynx based on our or similar habitat models, survey various habitat conditions to obtain reliable estimates of snowshoe hare densities, and identify a genetically compatible source population of lynx. If habitat and source populations are adequate, reintroducing lynx to areas of their historic range may be an appropriate conservation strategy.  相似文献   
25.
Abstract Effectively managing habitat for threatened populations of Canada lynx (Lynx canadensis) requires knowledge of habitat conditions that provide for the ecological needs of lynx. We snow-tracked lynx to identify habitat conditions associated with hunting behavior and predation during winters of 2002–2003 and 2003–2004 in the northern Cascade Range in Washington state, USA. We recorded number and success of predation attempts, prey species killed, and trail sinuosity on 149 km of lynx trails. Lynx killed snowshoe hares (Lepus americanus), red squirrels (Tamiasciurus hudsonicus), and cricetids more than expected in Englemann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) forests, where snowshoe hare densities were highest. Lynx killed prey less than expected in Douglas-fir (Pseudotsuga menziesii) and ponderosa pine (Pinus ponderosa) forests and forest openings. We used the sinuosity of lynx trails as an index of quality of habitat hunted. Lynx trails that included predation attempts were more sinuous than trail segments without predation attempts. Lynx trails had greater sinuosity in forest stands with high hare densities dominated by Engelmann spruce and subalpine fir than in stands with low hare densities dominated by Douglas-fir and ponderosa pine or in forest openings. We encourage forest managers to maintain or create sufficient understory cover to support high densities of snowshoe hares as foraging habitat for lynx.  相似文献   
26.
The morphospace of 54 species of Commelinaceae from nine genera was examined with simultaneous attention to constraints, adaptive hypotheses and relatedness. Eleven morphological traits, including leaf length and width, angle between the leaves and internode distances, were measured for each species and analysed by principal components analysis and nested analysis of variance. The results revealed a significant signal of relatedness in vegetative morphology; genus explained 20–50% of the variance in a single trait. The relationships between some traits are consistent with adaptive explanations. The findings are consistent with the prediction that evolution for optimal phyllotaxis should be relaxed as self‐shading decreases, and that light availability governs leaf size and branching patterns. Constraints potentially explain some trait correlations, and support was found for the hypothesis that structural constraints govern leaf size and internode size correlations. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158 , 257–268.  相似文献   
27.
Intraspecific Variation in Cryptocaryon irritans   总被引:10,自引:0,他引:10  
ABSTRACT. Intraspecific variation in the ciliate Cryptocaryon irritans was examined using sequences of the first internal transcribed spacer region (ITS-1) of ribosomal DNA (rDNA) combined with developmental and morphological characters. Amplified rDNA sequences consisting of 151 bases of the flanking 18 S and 5.8 S regions, and the entire ITS-1 region (169 or 170 bases), were determined and compared for 16 isolates of C. irritans from Australia, Israela and the USA. There was one variable base between isolates in the 18 S region nd 11 variables indicate that Australian C. irritans isolates from estuarine (Moreton Bay) and coral reef (Heron Island) environments are distinct. The Heron Island isolate was genetically closer to morphologically dissimilar isolates from Israel (1.8% divergence) and USA (2.3 % divergence) than it was to the Moreton Bay isolates. Three isolates maintained in our laboratory since February 1994 originated from the same source. During this time the sequence of the isolates from wild fish in Moreton Bay remained unchanged. These genetic differences indicate the existence of a founder effects in laboratorty populations of C. irritans . The genetic variation found here, combined with known morphological and developmental differences, is used to characterise four strains of C. irritans .  相似文献   
28.
The oviduct of the pink bollworm is innervated by an intrinsic neural network arising from 4 nerves from the terminal ganglion. The nerve tracts in this network often contained elliptical swellings, each with a central nucleus.

A distinct surface topography was evident in the muscular sheath of the common and lateral oviduct. A very thin muscular envelope consisting of an inner band of circular fibers and an outer layer of longitudinal fibers was also found in the ovarioles. Although conventional A and I bands were recognized, the z-disk was composed of an irregular and loose meshwork, suggesting that the visceral muscles of the reproductive tract possess super-contracting properties. Even when the oviduct and the ovarioles were isolated from the central nervous system, an endogenous rhythmic activity was evident.

Two types of spontaneous excitatory postsynaptic potentials were detected in the oviduct. The type most frequently observed had a complex of multiple spikes with a duration of 18–32 msec. The other type had a saw-tooth shape and a duration of 80–160 msec. Spontaneous action potentials with a plateau-type configuration and a duration of 280–320 msec were also observed. After the removal of the terminal ganglion, endogenous electrical activity distinct from the events just described was found in the midand upper common oviduct. Such discharges seem to originate from the intrinsic neural network and had durations similar to those found for neurosecretory cells.  相似文献   
29.
30.
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