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   7836篇
  免费   489篇
  国内免费   262篇
  8587篇
  2025年   17篇
  2024年   117篇
  2023年   117篇
  2022年   179篇
  2021年   258篇
  2020年   264篇
  2019年   279篇
  2018年   222篇
  2017年   210篇
  2016年   244篇
  2015年   230篇
  2014年   264篇
  2013年   392篇
  2012年   272篇
  2011年   281篇
  2010年   233篇
  2009年   359篇
  2008年   401篇
  2007年   418篇
  2006年   363篇
  2005年   319篇
  2004年   252篇
  2003年   252篇
  2002年   240篇
  2001年   165篇
  2000年   157篇
  1999年   147篇
  1998年   168篇
  1997年   160篇
  1996年   165篇
  1995年   145篇
  1994年   141篇
  1993年   120篇
  1992年   127篇
  1991年   133篇
  1990年   89篇
  1989年   107篇
  1988年   71篇
  1987年   66篇
  1986年   72篇
  1985年   74篇
  1984年   56篇
  1983年   53篇
  1982年   54篇
  1981年   32篇
  1980年   34篇
  1979年   20篇
  1978年   15篇
  1977年   11篇
  1976年   9篇
排序方式: 共有8587条查询结果,搜索用时 0 毫秒
1.
2.
Synopsis We investigated the ability of two congeneric species of sunfish to learn to forage on a novel prey item in feeding arenas containing structured habitats. Eight bluegill sunfish and eight pumpkinseed sunfish were given the opportunity to forage on whiteworms daily for 10 days. Each day, several behavioural measures were recorded for each fish. Both species of sunfish learned to feed over the 10-day period but the bluegill sunfish learned to feed more quickly than the pumpkinseed sunfish. Pumpkinseeds, however, attained a higher level of foraging efficiency. The differences in learning and foraging efficiency were related to body morphology.  相似文献   
3.
Mate recognition for the leafhopper Graminella nigrifrons(Forbes) occurs when a male spontaneously emits a multisectional vibrational calling song to which females respond by emitting simple pulses. Significant differences were found among males in the duration, number of chirps, and chirp rate within sections of the song and the total song. Repeatability (proportion of total variation due to differences among males) of call features ranged from very low (0.04 for total chirps in song) to high (0.67 for section 3 chirp rate). However, song modification and playback experiments revealed that the variation in the measured song features was not important in determining whether a female will respond. Rather, female response depended only on the presence of two of the three types of pulses which comprise a chirp. These essential pulses were found within chirps of all call sections that contain chirps. Manipulation of chirp rates from 0.58 to 2.70 times the normal rate did not affect female response, nor did changing the period of silence between the essential pulse types from 0.25 to 1.75 times the normal period. These results suggest that components of the male calling song function in mate recognition but are not used by females to discriminate among conspecific males.  相似文献   
4.
5.
Twenty bluespotted sunfish, Enneacanthus gloriosus, were individually tested for thermoregulatory behavior in an electronic shuttlebox. The first fishes of this genus to be tested for thermoregulatory behavior, these individuals were captured near the northern limit of their zoogeographic range, in northeastern Pennsylvania. The mean final preferendum from pooled data of all 20 fish was 28.5°C; there were no significant differences between day and night.  相似文献   
6.
The stomach contents of 372 specimens of Aspitrigla obscura (collected at quarterly intervals) have been analyzed to determine diet according to fish size and season. The results show that crustaceans (%No = 96%, %Weight = 76%) are the basic food. For all sizes, half of diet is assured by necto-benthic crustaceans. In smaller sizes, the remaining half is formed by planktonic species, whilst in larger sizes they are substituted by nycthemeral migrators. The importance of planktonic prey is greater in the winter sample than in the other samples. In every size, the feeding intensity is greater in the midday sample than in the morning one.  相似文献   
7.
We tested two competing models that could explain how differential flagellar activity leads to phototactic turning in spheroids of Volvox carteri f. weismannia (Powers) Iyengar. In one model, turning results from the flagella of anterior cells in the lighted and shadowed hemispheres beating at different frequencies. In a competing model, turning results from a change in beat direction in these flagella. Both models successfully explain phototactic steering under constant illumination, but they make different predictions when colonies are exposed to abrupt changes in light intensity. If turning is due to control of flagellar beat frequency, both progression and rotation rates will change in the same direction and with similar magnitudes. If spheroid turning is due to a change in flagellar beat direction, a decreased rate of progression will accompany an increased rate of rotation and vice versa. We used video-microscopy to observe the behavior of positively phototactic V. carteri spheroids exposed to 10× step-up and step-down stimuli. After a step-up stimulus, spheroids slow their progression and rotation by equal amounts. No significant changes are reported in these parameters after the reciprocal step-down response. These observations are consistent with the variable flagellar frequency model and inconsistent with the variable flagellar direction model for phototactic turning. Switching the direction of light stimulus by 180° results in reorientation of positively phototactic spheroids. The kinetics of this reorientation did not precisely match the predictions of either model.  相似文献   
8.
Spores newly released from plurilocular sporangia of Ectocarpus siliculosus (Dillw.) Lyngb. sporophytes were assayed for chemotaxis to nutrients and for settlement stimulation by nutrients. To enable these measurements with relatively small volumes and numbers of released spores, we developed a computer-assisted motion-analysis assay for spore chemotaxis and verified the results with a more standard, capillary tube chemotaxis assay. The presence of a nutrient gradient did not influence the swimming behavior of E. siliculosus spores in the motion-analysis assay, and likewise no chemotactic effect was measured in the capillary tube assay. Microplate settlement assays previously utilized with bacteria and invertebrates were adapted for use with algal spores. E. siliculosus spores settled at higher rates on a hydrophobic plastic surface than on surfaces with either positively or negatively charged hydrophilic coatings. Nutrient mixtures had no effect on the rate of spore settlement on hydrophobic surfaces.  相似文献   
9.
Summary We studied the nesting success of Evening Grosbeaks (Coccothraustes vespertinus) inhabiting two areas of the Front Range of the Rocky Mountains of Colorado from 1983–1987. Sixty-four nests were followed during building, incubating, brooding, and fledging; 54.7% were successful (young fledged). The largest number of nests failed during incubation. Nests started later were more successful than nests begun earlier in the season. Failure was most likely due to severe weather, abandonment during building, or predation. Specific habitat characteristics of grosbeak nesting sites and where nests were placed in trees were consistently associated with nesting success. Successful nests, when compared with nests that failed, were: (1) built in more open areas characterized by dispersed vegetation and a higher minimum canopy, (2) oriented in more southerly directions, (3) built closer to the main trunk of the nest tree, and (4) built in larger trees. Current ideas about whether or not birds actually select nest-sites are briefly discussed. We conclude that some grosbeaks optimally select nest sites where the likelihood of producing fledglings is higher than in other areas.  相似文献   
10.
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