全文获取类型
收费全文 | 12263篇 |
免费 | 1110篇 |
国内免费 | 427篇 |
出版年
2023年 | 185篇 |
2022年 | 94篇 |
2021年 | 273篇 |
2020年 | 335篇 |
2019年 | 428篇 |
2018年 | 362篇 |
2017年 | 459篇 |
2016年 | 391篇 |
2015年 | 454篇 |
2014年 | 493篇 |
2013年 | 615篇 |
2012年 | 430篇 |
2011年 | 480篇 |
2010年 | 414篇 |
2009年 | 642篇 |
2008年 | 690篇 |
2007年 | 787篇 |
2006年 | 625篇 |
2005年 | 582篇 |
2004年 | 486篇 |
2003年 | 473篇 |
2002年 | 386篇 |
2001年 | 342篇 |
2000年 | 281篇 |
1999年 | 303篇 |
1998年 | 277篇 |
1997年 | 234篇 |
1996年 | 226篇 |
1995年 | 215篇 |
1994年 | 176篇 |
1993年 | 197篇 |
1992年 | 184篇 |
1991年 | 154篇 |
1990年 | 124篇 |
1989年 | 136篇 |
1988年 | 119篇 |
1987年 | 109篇 |
1986年 | 102篇 |
1985年 | 80篇 |
1984年 | 83篇 |
1983年 | 50篇 |
1982年 | 62篇 |
1981年 | 62篇 |
1980年 | 40篇 |
1979年 | 28篇 |
1978年 | 42篇 |
1977年 | 15篇 |
1976年 | 15篇 |
1975年 | 11篇 |
1973年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
991.
992.
Pollination and phenology of flowers in the canopy of two contrasting rain forest types in Amazonia,Colombia 总被引:3,自引:0,他引:3
The main objective of this investigation was to study the pollination characteristics of two types of Amazonian rain forest at plant community level. Seasonally inundated forest was compared with upland (tierra firme) forest. The study focused on plant species in the canopy. The pollination spectra show that in both forests most canopy trees and lianas are pollinated by small bees, large bees, butterflies or by small, relatively unspecialized insects. In the upland forest small bees are the most important pollinators (32% of all species of trees and lianas are pollinated by them), whereas large bees are predominant in the floodplain (22%). Other pollinators, like hummingbirds, bats, moths, and beetles are less common (>10%), but always somewhat more important in the flood plain than in the upland forest. Bees are the most common pollinators of epiphytes. In the flood plain forest, flies are also important as epiphyte pollinators (19%), whereas in the upland forest hummingbirds pollinate more epiphytes. The phenological patterns are quite similar in both the upland and the flood plain. We found a peak in flowering in the transition period between the wet and the dry season. Flowering activity was lowest during the wet season. Differentiation in sexual systems was correlated with life form. Dioecy and monoecy were found mostly among tree species. Most species of all life forms though were hermaphroditic. No difference with respect to the relative importance of sexual systems was found between the two forest types. 相似文献
993.
The infectivity of infective juveniles(IJs) of Heterorhabditis megidis (strain NLH-E87.3) produced on small, medium and large larvae ofGalleria mellonella, and on medium and largelarvae of Otiorhynchus sulcatus was tested underlaboratory conditions against G. mellonella andO. sulcatus larvae. Infective juvenilesoriginating from small G. mellonella exposed toan initial dose of one IJ were more infectious thanthose from small cadavers exposed to a dose of 30 IJs.Independent of the initial inoculum size, IJs fromsmall cadavers of G. mellonella were moreinfectious than those from medium and large cadavers.At a dose of one IJ per larva, IJs originating frommedium size O. sulcatus cadavers were moreinfective against G. mellonella than againstO. sulcatus larvae. Large G. mellonellalarvae were less susceptible to all IJ batches thanmedium and small sized larvae. 相似文献
994.
Tae Kyung Kim Joo Young Chung Yun Hee Sung Gyun Min Lee 《Biotechnology and Bioprocess Engineering》2001,6(5):332-336
When parental Chinese hamster ovary (CHO) cell clones that are capable of producing thrombopoietin (TPO) were subjected to
high methotrexate (MTX) concentrations, clonal variations in cell growth were apparent. In the clones that had no significant
enhancement in specific TPO productivity (q
Tpo) when a higher level of MTX was administered, their growth was not depressed significantly nor their cell size changed significantly.
On the other hand, those clones that showed a significant enhancement inq
Tpo at higher a MTX dosage, cell growth was depressed initially but recovered during successive sub-cultures. Furthermore, their
cell size increased, which suggested that changes in cell size may be indicative of an enhancedq
Tpo. When the enhancement of theq
Tpo of 9 clones after a high MTX dosage was plotted against the extent of the increase of their size, there was a linear correlation
(r
2=0.80,P<0.001, ANOVA), which suggested that an enhancement ofq
Tpo after high MTX administration can be measured by the increase in their cell size. Taken together, our data demonstrate that
the selection of amplified CHO cell clones with enhancedq
Tpo can be done based upon their increased size and growth pattern. This facilitates the development of highly productive recombinant
CHO cell lines. 相似文献
995.
It is generally accepted that high quality males are those that succeed in male–male competition: in either aggression or rivalry to attract and be selected by females. Previous studies of amphibians have suggested that the main characters influencing male mating success include variation in call characteristics (e.g. call rate, call intensity), calling behaviour, body condition, age and chorus tenure. In the present paper, several of the characters influencing female mate choice (male body size, body condition, call rate, call frequency and chorus tenure) are investigated in two closely related, explosive breeding frog species Litoria chloris and Litoria xanthomera. Smaller males of both species are shown to be more successful than larger males and this success is attributed to the increased chorus tenure of smaller males in L. xanthomera. This increased chorus tenure was attributed to the lower total energy used per call by a small male calling at a higher frequency. Whether increased chorus tenure explains female mate choice in L. chloris is uncertain but is highly probable given the strong similarity between the two species in both ecology and call characteristics. 相似文献
996.
Food, vigilance, and sperm: the role of male direct benefits in the evolution of female preference in a polygamous bird 总被引:1,自引:1,他引:0
The adaptive significance of female selection of copulationpartners remains unresolved, particularly in polygamous specieswhere males do not provide paternal care. In these species thepossibility that direct benefits other than paternal care mayplay an important role in the evolution of female choice hasreceived little attention. I tested whether direct benefitsare associated with female choice in the polygamous feral fowl,Gallus g. domesticus, where females prefer socially dominantcopulation partners and males do not care for the young butdo provide females with three commodities: food, vigilance,and sperm. I used a combination of empirical and experimentaldata to show that male propensity to offer food and vigilance,but not sperm, was positively associated with male social status,suggesting that the provision of these resources may be costlyand condition dependent in males. Copulation success was correlatedwith male status but not with the number of feedings a femalereceived from a male, indicating that a female preferred dominantpartners that in general provided any female with more food,rather than partners that provided only her with more food,consistent with the idea that females may use male resourceprovisioning as a proximate mechanism to assess male condition.Together, these results indicate that male resources provisioningis (1) tightly linked to male social status, (2) a potentialindicator of male condition and possibly genetic quality, and(3) a potential criterion for females to select dominant partners,thus playing an important role in the evolution of partner choiceeven in polygamous species lacking paternal care. 相似文献
997.
In fish, brood cycling parental males sometimes eat some orall of their eggs, a behavior termed filial cannibalism. Wetested predictions of filial cannibalism models related to thecost of parental care in the male sand goby, Pomatoschistusminutus, by increasing the parental effort (fanning expenditure)through reduced levels of dissolved oxygen to 39% in an experimentalgroup, whereas a control group had fully saturated water. Malesshowed both full-clutch cannibalism and partial-clutch cannibalismin both treatments. Giving the males one to three females tospawn with, we found that small clutches were completely eatenmore often than were larger ones, whereas partial-clutch cannibalismwas not affected by clutch size. Although treatment did notaffect filial cannibalism, it did affect a male's energy statesuch that males in the low oxygen treatment lost more body fat,indicating a greater fanning effort. This shows that males inthe low oxygen treatment allocated more energy to the presentbrood, potentially at the expense of future reproductive success.Our study strongly suggests that filial cannibalism in malesand gobies represents a strategic life-history decision asan investment in future reproductive success, and is not triggeredby a proximate need for food necessary for the male's own survival.Furthermore, males in the low oxygen treatment built nests withlarger entrances, and were less likely to rebuild their nestsafter destruction. Presumably, this makes fanning easier butthe nest more vulnerable to predators, suggesting a trade-offbetween fanning and nest defense. 相似文献
998.
Asymmetric larval mobility and the evolutionary transition from siblicide to nonsiblicidal behavior in parasitoid wasps 总被引:2,自引:0,他引:2
Pexton John J.; Rankin Daniel J.; Dytham Calvin; Mayhew Peter J. 《Behavioral ecology》2003,14(2):182-193
The widespread evolution of gregarious development in parasitoidwasps presents a theoretical challenge because the conditionsunder which larval tolerance can spread in an intolerant populationare very stringent (the individual fitness of larvae developingtogether must increase with clutch size). Recent empirical workhas suggested that gregarious development can arise throughthe loss of larval mobility rather than through the gain oftolerant behavior. Using analytical genetic models, we exploredwhether decreased mobility presents a less stringent route togregariousness than the gain of tolerance. Reduced mobilitycan spread under a wide range of conditions. The critical conditionfor the spread of immobility is much less stringent than thatfor larval tolerance. In contrast with previous models of tolerance,the criterion for the spread of a rare immobility allele isindependent of any bias in the sex ratio and the likelihoodof single sex broods. Superparasitism increases the stringencyof the criterion for the spread of immobility, whereas doublekilling relaxes the criterion. Tolerance can subsequently replaceimmobility if there is any cost to the retention of fightingability. Our results suggest that asymmetric larval mobilitymay explain many instances of the evolution of gregarious development. 相似文献
999.
Alternative models of territoriality are based on contrastingassumptions about the behavioral processes determining territorysize. In a series of controlled field experiments on the fireant Solenopsis invicta, I tested whether territory size is affectedby the availability of food, as predicted by most economic models,and whether territory size is affected by fighting ability,as predicted by models of competition among neighbors. Abundantfood was offered for 3035 days to selected colonies eitherimmediately next to the nest (experiment 1) or at peripheralsites near the territory boundary (experiment 2). These foodsupplements had no detectable effect on territory size. Furthermore,food placed near the periphery of the territory did not significantlyalter local boundary positions. During both experiments, largecolonies lost more territory than did small colonies, reflectingtemporary declines in worker number due to the seasonal productionof reproductives. Such losses by large colonies during the summermonths create opportunities for newly founded colonies to expandterritories. In a third experiment, colonies from which workerswere removed lost significantly more territory than did unmanipulatedcontrols. These results show that territory areas in S. invictaare strongly affected by the relative fighting ability of neighboringcolonies but provide no evidence that colonies adjust territoryarea in response to short-term changes in the availability offood. 相似文献
1000.
Male-biased sexual dimorphism in both size and body mass isa common characteristic of many polygynous mammals and is oftenattributed to sexual selection favoring large males. The degreeof dimorphism is thought to be related to the potential forsome males to monopolize access to estrous females, which isin turn related to the distribution of receptive females inspace and time. In the present study, we investigated the relationshipbetween the temporal distribution of breeding females and thedegree of mass dimorphism among 11 populations of the commonbrushtail possum from northern Australia. Breeding patternsvaried from complete aseasonality in some populations to a seasonalconcentration of births within 2 or 3 months in others. We predictedthat in populations in which mating opportunities were distributedthroughout the year, dominant males would be able to monopolizeaccess to larger numbers of estrous females than in populationsin which matings were more seasonal, and in such populations,large body size in males would be favored. We found that dimorphismwas related to seasonality of breeding, being greatest in populationswith a more aseasonal pattern. Mean body mass of male possumsalso decreased with increasing population density. Populationdensity may influence the degree of breeding synchrony withinpopulations, particularly in locations with a more seasonalclimate. The present study is the first to demonstrate plasticityin mass dimorphism in response to local variation in the synchronyof breeding in a mammal species. 相似文献