全文获取类型
收费全文 | 6423篇 |
免费 | 472篇 |
国内免费 | 437篇 |
出版年
2024年 | 23篇 |
2023年 | 98篇 |
2022年 | 153篇 |
2021年 | 237篇 |
2020年 | 234篇 |
2019年 | 256篇 |
2018年 | 208篇 |
2017年 | 193篇 |
2016年 | 222篇 |
2015年 | 214篇 |
2014年 | 235篇 |
2013年 | 354篇 |
2012年 | 223篇 |
2011年 | 240篇 |
2010年 | 210篇 |
2009年 | 295篇 |
2008年 | 325篇 |
2007年 | 363篇 |
2006年 | 291篇 |
2005年 | 273篇 |
2004年 | 218篇 |
2003年 | 215篇 |
2002年 | 219篇 |
2001年 | 154篇 |
2000年 | 144篇 |
1999年 | 134篇 |
1998年 | 143篇 |
1997年 | 145篇 |
1996年 | 147篇 |
1995年 | 115篇 |
1994年 | 118篇 |
1993年 | 98篇 |
1992年 | 98篇 |
1991年 | 112篇 |
1990年 | 75篇 |
1989年 | 89篇 |
1988年 | 53篇 |
1987年 | 53篇 |
1986年 | 59篇 |
1985年 | 56篇 |
1984年 | 46篇 |
1983年 | 42篇 |
1982年 | 47篇 |
1981年 | 26篇 |
1980年 | 29篇 |
1979年 | 17篇 |
1978年 | 10篇 |
1977年 | 9篇 |
1976年 | 6篇 |
1973年 | 4篇 |
排序方式: 共有7332条查询结果,搜索用时 15 毫秒
101.
In short-term field trials at combinations of ambient temperature (°C) and insolation (W·m−2), larval Colorado potato beetles (Leptinotarsa decemlineata [Say] [Coleoptera: Chrysomelidae]) were observed after their release on the adaxial surface of leaflets on potato plants
(Solanum tuberosum L. Solanaceae). The larvae either began feeding or moved under the leaflet; mean interval from release to expression of these
behaviors (2.9±0.05 min [n =358]) was independent of air temperature and insolation. Proportion of larvae moving under the
leaflet increased logistically with both air temperature and insolation. A 1 W·m−2 change in insolation (P) evoked the same effect on this proportion as a 0.0838 °C change in air temperature (T
a
), so the two quantities were combined as T*=T
a
+P·0.0838 °C/(W·m−2), which has units of °C. The proportion of larvae moving under the leaflet increased logistically with T*.
In 1-day field trials we monitored air temperature, insolation and proportion of larvae under the leaflet, and compared the
latter to predictions from the logistic regression derived from the short-term trials. Consistently more larvae occurred under
leaflets than predicted from the logistic regression; this bias diminished as T* increased until at T*≥40 °C, observed and predicted proportions were equal. This pattern of deviation from the predictions of the logistic regression
is consistent with a thermoregulatory strategy in which larvae move away from hostile conditions, rather than seek optimal
conditions. 相似文献
102.
Deborah A. Gust 《Primates; journal of primatology》1994,35(3):375-383
A group of 38 free ranging crested mangabeys in the Tana River Primate Reserve was studied over a 6-week period for a total
of 209.5 hr and behavioral comparisons made to a captive group of sooty mangabeys. Although quantitative comparisons between
these two mangabey species are not possible, the present data suggest that these geographically separated mangabeys share
several behavioral similarities. Copulatory behavior in both species involved a pattern of female darting and female vocalizing
following the mount by the male. Several similarities in agonistic behavior also existed: 1) The victim often returned or
stayed within 1 m of the aggressor following an agonistic episode; 2) retaliation in which the victim first fled or avoided
the aggressor, then subsequently, chased or lunged at the aggressor, typically while screaming; and 3) frequent redirection
of aggression by the victim following an agonistic episode. A dominance rank reversal occurred between the two adult male
crested mangabeys with no severe wounding. The rank reversal seemed to be related to two subsequent behaviors which included
infant carriage by the deposed alpha in the presence of the new alpha male and female demonstrations of extreme protectiveness
of their infants in the presence of the new alpha male. Both of these behaviors have been reported in the sooty mangabey group. 相似文献
103.
Shyamala Ratnayeke 《International journal of primatology》1994,15(3):445-469
I compared the behavior of three old postreproductive females in a wild population of toque macaques (Macaca sinica)in Polonnaruwa with those of reproductive females via focal-animal sampling techniques. Postreproductives foraged less, slept
more, and were less active overall than reproductive females were. They also had significantly lower rates of agonistic behavior,
were more peripheral, and had lower frequencies of overall affiliative contact. Although postreproductives initiated contact
with others as frequently as reproductives did, group members initiated contact with them significantly less than they did
with reproductive females. Postreproductives associated more with adult females than reproductives did and less with adult
and subadult males than high-ranking reproductives did. Juvenile and infant females associated more frequently with reproductive
females of high or low rank than with postreproductives. Postreproductives resembled low-ranking reproductive females in giving
less grooming to others than they received. This contrasts with high-ranking females, which gave more grooming to others than
they received. The results suggest that old age and cessation of reproduction are evident through the manifestation of distinct
behavioral characteristics in toque macaque females. 相似文献
104.
Filippo Aureli Marjolijn Das Désirée Verleur Jan A. R. A. M. van Hooff 《International journal of primatology》1994,15(3):471-485
We investigated the occurrence of postconflict social interactions in a group of Barbary macaques (Macaca sylvanus)that live in a large enclosure. We collected data during the mating and the nonmating seasons and analyzed them via two methods.
Separate analyses for the two seasons yielded very similar results. Reconciliation and redirection occurred. But we found
no evidence for consolation and postconflict kin-oriented affiliation. The consistency of these findings with those on other
species of Macacaindicates that reconciliation and redirection are typical postconflict social interactions of macaque victims. 相似文献
105.
人工饲养的蒙新河猪其动情周期可划分为4个阶段:动情前期、动情期(发情交配期)、动情后期和停滞期(动情间隔期)。动情期间雌雄河狸均表现出活跃好动,浮躁不安,食量减小;发情交配期雌雄河狸均发出低沉嘶哑的叫声,雄性叫声频率比性略高,爬跨交配是动情期发情的高峰,动情周期各阶段的行为特征,在雌性与阴道涂片细胞学及外生殖器形态、色泽、阴道分泌物周期性变化特征相吻合,在雄性则与睾丸及阴茎形态特征吻合,在人工饲养 相似文献
106.
Six cases of mother-cub relationships in wild giant pandas (Ailuropoda melanoleuca) were observed in the Qinling Mountains, China. It was found that panda cubs are normally left alone in the den for 4–8 h while mothers forage. The mother's absence during bouts of foraging should be considered when rescuing abandoned cubs in order to avoid adding to the decline of the wild population. © 1994 Wiley-Liss, Inc. 相似文献
107.
Leuprolide acetate acts as a superactive gonadotropin-releasing hormone (GnRH) agonist in mammals. Its administration to humans in a depot formulation (Lupron Depot 3.75 mg; TAP Pharmaceuticals, Deerfield, IL) consisting of microspheres suspended in a diluent of carboxymethylcellulose and other elements leads to an initial increase in serum gonadotropin levels followed by a prolonged suppression of ~ 1 month's duration. To test whether it might act in cockatiels to prevent egg laying, we administered Lupron Depot to groups of pairs (at least 6 pairs/group) stimulated to reproduce by provision of nest boxes and exposure to sexually stimulatory daylengths (15:9 L:D). A single intramuscular injection of Lupron Depot, calculated to achieve a daily release rate of 0 (control; diluent only), 17, 52, or 156 μg/kg/day of leuprolide acetate, was administered on day 0, when birds received nest boxes and after daylength had been stepwise increased. Egg production began on day 12 in the 0 and 17 μg dose groups, and on day 31 in the groups receiving the higher doses. Number of eggs per clutch, candled fertility, and percent hatchability were not significantly different among the groups. In a separate experiment in which leuprolide was administered prior to photostimulation and nest box presentation, nest-inspection behavior was not prevented. We conclude that a single injection of Lupron Depot is effective in reversibly preventing egg laying in cockatiels. © 1994 Wiley-Liss, Inc. 相似文献
108.
Hypoderma (=Oedemagena) tarandi L. (Diptera: Oestridae) is characterized by a mating strategy in which both sexes meet and mate at two types of distinct topographical landmarks. In the expansive, treeless vidda (= tundra-like) biome, mating places are unique, rocky areas located along rivers and streams or in rocky areas of drying river and stream beds. In wooded valleys below the vidda, flies mated at certain topographical areas along dirt road tracks/paths. Thermoregulatory activities of males occupying perches at mating places included selection of substratum at perch site, orientation of body to sun's rays, crouching, stilting, and flights into upper cooler air. On warm sunny days males perched for just 1–2 min before flying up into cooler air to promote cooling. Laboratory and field studies revealed that flies could not metabolically cool down when held at 25–38°C. Time spent at mating places depended on temperature, duration of sunshine, and wind velocity. Males were very aggressive in pursuing allHypoderma-sized objects that passed by them or that landed near them, but they did not defend specific perch sites. Males either pursued and caught females in flight, or they hopped onto females that landed near them. During 5 years, 74 males and 14 females were seen at mating places. Dissection of six females caught at mating places revealed them to be recently eclosed flies full of fat body and with all eggs intact; two not paired with males were non-inseminated. Three experimentally paired females remainedin copulo for 10, 13, and 19.5 min. 相似文献
109.
Variations from the normal female-male sequence of eggs in nests of the leafcutter bee,Megachile rotundata, were examined. Three alternatives were considered: Out-of-sequence males (i) were diploids, (ii) were the result of supersedure of nests or intraspecific brood parasitism, or (iii) were the result of females occasionally laying a male-female sequence. Electrophoretic data provided definitive evidence of diploid males and of multiple females laying in 7 of 18 nests. In the others, the remaining explanation is that females occasionally lay male eggs before female eggs in a nest. 相似文献
110.
In a two-choice test, moreS. furcifera females settled more often on exposed plants than on parafilm-masked ones, regardless of the susceptibility of rice varieties. This indicates that rice volatiles play an important role in the insect's short-range orientation to its host. The fact that more insects settled on exposed resistant Rathu Heenati (RHT) than to masked susceptible Taichung Native 1 (TN1) suggests that there must be certain common volatiles released by both varieties. Few females landed on masked plants of either RHT or TN1. This implies that the insect could not recognize at a distance that a plant was resistant or susceptible without olfactory stimuli.S. furcifera excreted less honeydew on masked plants than on exposed ones for both varieties and more on masked TN1 than on exposed RHT. The electronic monitoring of feeding behavior demonstrates that the insect made more frequent probes and had shorter phloem ingestion durations on exposed RHT than on exposed TN1 and on masked RHT than on masked TN1. Moreover, the insect had longer phloem ingestion durations on masked TN1 than on exposed RHT. These results suggest that volatile chemicals given off by resistant RHT plants have a negative effect on feeding. 相似文献