全文获取类型
收费全文 | 899篇 |
免费 | 17篇 |
出版年
2015年 | 9篇 |
2013年 | 20篇 |
2012年 | 12篇 |
2011年 | 11篇 |
2010年 | 29篇 |
2009年 | 22篇 |
2008年 | 27篇 |
2007年 | 46篇 |
2006年 | 15篇 |
2005年 | 21篇 |
2004年 | 13篇 |
2002年 | 14篇 |
2001年 | 9篇 |
1999年 | 13篇 |
1998年 | 9篇 |
1997年 | 16篇 |
1996年 | 13篇 |
1995年 | 12篇 |
1994年 | 12篇 |
1993年 | 17篇 |
1992年 | 9篇 |
1991年 | 18篇 |
1990年 | 16篇 |
1989年 | 11篇 |
1988年 | 10篇 |
1987年 | 17篇 |
1986年 | 13篇 |
1985年 | 11篇 |
1984年 | 13篇 |
1982年 | 14篇 |
1980年 | 12篇 |
1979年 | 11篇 |
1976年 | 12篇 |
1975年 | 10篇 |
1974年 | 15篇 |
1973年 | 18篇 |
1972年 | 16篇 |
1971年 | 10篇 |
1970年 | 8篇 |
1969年 | 9篇 |
1968年 | 10篇 |
1965年 | 36篇 |
1964年 | 19篇 |
1963年 | 32篇 |
1962年 | 23篇 |
1961年 | 33篇 |
1960年 | 21篇 |
1959年 | 27篇 |
1958年 | 27篇 |
1957年 | 12篇 |
排序方式: 共有916条查询结果,搜索用时 31 毫秒
81.
Abstract. 1. Predators, including insect parasitoids, often eavesdrop on prey signals, and as a result, predation can have important effects on the evolution of prey signalling behaviour.
2. The phonotactic parasitoid fly, Ormia ochracea , uses the calling songs of male field crickets to locate their field crickets hosts. In the western USA, this fly parasitises the variable field cricket, Gryllus lineaticeps . Previous work with one fly population suggested that female flies, like female field crickets, preferentially orient to male songs with higher chirp rates and longer chirp durations, although a limited range of male song types was used in this previous study. The current study, with a different fly population, used field-based, two-speaker choice tests to examine: (1) the effect of male chirp rate and chirp duration on fly attraction, using a natural range of song types; and (2) the relative importance of these song types in host selection by the flies.
3. Three lines of evidence suggested that chirp rate is more important than chirp duration in host selection. (a) The flies consistently preferred higher chirp rates but only sometimes preferred longer chirp durations. (b) The flies consistently preferred higher chirp rate/shorter chirp duration songs to lower chirp rate/longer chirp duration songs. (c) Preferences for longer chirp durations could be eliminated by increasing the amplitude of the less attractive song type, while preferences for higher chirp rates could only sometimes be eliminated by increasing the amplitude of the less attractive song type.
4. Fly predation may favour lower chirp rates and shorter chirp durations in G. lineaticeps , and may have resulted in stronger selection on chirp rate than on chirp duration. 相似文献
2. The phonotactic parasitoid fly, Ormia ochracea , uses the calling songs of male field crickets to locate their field crickets hosts. In the western USA, this fly parasitises the variable field cricket, Gryllus lineaticeps . Previous work with one fly population suggested that female flies, like female field crickets, preferentially orient to male songs with higher chirp rates and longer chirp durations, although a limited range of male song types was used in this previous study. The current study, with a different fly population, used field-based, two-speaker choice tests to examine: (1) the effect of male chirp rate and chirp duration on fly attraction, using a natural range of song types; and (2) the relative importance of these song types in host selection by the flies.
3. Three lines of evidence suggested that chirp rate is more important than chirp duration in host selection. (a) The flies consistently preferred higher chirp rates but only sometimes preferred longer chirp durations. (b) The flies consistently preferred higher chirp rate/shorter chirp duration songs to lower chirp rate/longer chirp duration songs. (c) Preferences for longer chirp durations could be eliminated by increasing the amplitude of the less attractive song type, while preferences for higher chirp rates could only sometimes be eliminated by increasing the amplitude of the less attractive song type.
4. Fly predation may favour lower chirp rates and shorter chirp durations in G. lineaticeps , and may have resulted in stronger selection on chirp rate than on chirp duration. 相似文献
82.
83.
84.
- 1 Many bat species hibernate to conserve energy during winter and like all hibernators they commonly arouse. During these arousals, some bats may undertake activities away from the hibernation site. Systematic ecological studies of the frequency and purpose of winter activity in temperate zone bats of North America are rare and much of the literature involves observations of single individuals or unmarked populations.
- 2 We review the available literature on winter activity among North American vespertilionid bats to highlight the paucity of data on this subject and to stimulate future research. Due to the lack of repeated, systematic studies on most North American species, the conclusions drawn are general or pertain only to parts of the geographical range of any species.
- 3 We suggest that winter activity is ubiquitous among temperate zone bats, but the degree and purpose of activity varies greatly between and within species. In general, cave‐dwelling bats tend to be relatively inactive compared with tree and foliage roosting bats during winter.
- 4 Cave‐dwelling and tree cavity‐dwelling species do not appear to feed, but they do drink and occasionally copulate during the hibernation period. Species that hibernate in foliage or leaf litter are the most active species during winter and may feed and drink throughout winter, although they probably do not copulate because of their solitary nature.
- 5 We encourage researchers to conduct studies on all aspects of winter activity for North American vespertilionids across wide latitudinal and altitudinal gradients. We suggest studies in the near future be focused on estimating the energetic costs and benefits of winter activity through determination of the frequency and intensity of winter feeding activity and more thorough examinations of movements within and among caves. Studies on common, wide‐ranging species have the potential to illuminate large‐scale patterns and differences and should be favoured over studies on rarer species.
85.
Reversal of histone lysine trimethylation by the JMJD2 family of histone demethylases 总被引:34,自引:0,他引:34
Whetstine JR Nottke A Lan F Huarte M Smolikov S Chen Z Spooner E Li E Zhang G Colaiacovo M Shi Y 《Cell》2006,125(3):467-481
86.
Background
Questions regarding the distribution of stress in the proximal human femur have never been adequately resolved. Traditionally, by considering the femur in isolation, it has been believed that the effect of body weight on the projecting neck and head places the superior aspect of the neck in tension. A minority view has proposed that this region is in compression because of muscular forces pulling the femur into the pelvis. Little has been done to study stress distributions in the proximal femur. We hypothesise that under physiological loading the majority of the proximal femur is in compression and that the internal trabecular structure functions as an arch, transferring compressive stresses to the femoral shaft. 相似文献87.
88.
89.
1. In situ measurements of nitrogen fixation rates for Aphanizomenon in fertile Colorado lakes with low inorganic nitrogen concentrations demonstrated high efficiency of nitrogen fixation at low irradiance. 2. For study populations, rates of N2 fixation in darkness and with alternating exposure to light and darkness were a higher percentage of light‐saturated rates for Aphanizomenon than for Anabaena, suggesting storage of reduced metabolites at high irradiance that are used subsequently by Aphanizomenon when cells are forced by mixing into zones of low irradiance. Also, saturation of N2 fixation occurred over a lower range of irradiance for Aphanizomenon than for Anabaena. 3. High efficiency of N2 fixation in Aphanizomenon at low or fluctuating irradiance is complementary to its previously demonstrated high efficiency of photosynthesis at low irradiance. Nitrogen fixation rate was also strongly related to DIN concentration; fixation was highest at low DIN (maximum < 5 μg L?1) but was also most vulnerable to photoinhibition under such conditions. 4. The fixation capabilities of Aphanizomenon under weak or varying irradiance could explain its commonly observed domination over Anabaena when transparency is low and available nitrogen is scarce. 相似文献
90.
Reinout Raijmakers Joyce JBC van Beers Mahmoud El-Azzouny Natasja FC Visser Borut Bo?i? Ger JM Pruijn Albert JR Heck 《Arthritis research & therapy》2012,14(3):R114-10