全文获取类型
收费全文 | 847篇 |
免费 | 84篇 |
出版年
2024年 | 2篇 |
2023年 | 9篇 |
2022年 | 10篇 |
2021年 | 21篇 |
2020年 | 12篇 |
2019年 | 23篇 |
2018年 | 9篇 |
2017年 | 16篇 |
2016年 | 29篇 |
2015年 | 42篇 |
2014年 | 42篇 |
2013年 | 65篇 |
2012年 | 65篇 |
2011年 | 53篇 |
2010年 | 43篇 |
2009年 | 31篇 |
2008年 | 60篇 |
2007年 | 62篇 |
2006年 | 59篇 |
2005年 | 64篇 |
2004年 | 48篇 |
2003年 | 41篇 |
2002年 | 27篇 |
2001年 | 11篇 |
1999年 | 11篇 |
1998年 | 11篇 |
1997年 | 6篇 |
1996年 | 3篇 |
1995年 | 3篇 |
1994年 | 2篇 |
1992年 | 5篇 |
1991年 | 2篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1982年 | 2篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 5篇 |
1973年 | 2篇 |
1972年 | 2篇 |
1969年 | 2篇 |
1965年 | 1篇 |
1964年 | 1篇 |
1961年 | 1篇 |
1955年 | 1篇 |
1954年 | 1篇 |
1952年 | 1篇 |
1949年 | 1篇 |
1947年 | 1篇 |
排序方式: 共有931条查询结果,搜索用时 718 毫秒
1.
Sexually selected infanticide is an important source of infant mortality in many mammalian species. In species with long-term male-female associations, females may benefit from male protection against infanticidal outsiders. We tested whether mountain gorilla (Gorilla beringei beringei) mothers in single and multi-male groups monitored by the Dian Fossey Gorilla Fund’s Karisoke Research Center actively facilitated interactions between their infants and a potentially protective male. We also evaluated the criteria mothers in multi-male groups used to choose a preferred male social partner. In single male groups, where infanticide risk and paternity certainty are high, females with infants <1 year old spent more time near and affiliated more with males than females without young infants. In multi-male groups, where infanticide rates and paternity certainty are lower, mothers with new infants exhibited few behavioral changes toward males. The sole notable change was that females with young infants proportionally increased their time near males they previously spent little time near when compared to males they had previously preferred, perhaps to encourage paternity uncertainty and deter aggression. Rank was a much better predictor of females’ social partner choice than paternity. Older infants (2–3 years) in multi-male groups mirrored their mothers’ preferences for individual male social partners; 89% spent the most time in close proximity to the male their mother had spent the most time near when they were <1 year old. Observed discrepancies between female behavior in single and multi-male groups likely reflect different levels of postpartum intersexual conflict; in groups where paternity certainty and infanticide risk are both high, male-female interests align and females behave accordingly. This highlights the importance of considering individual and group-level variation when evaluating intersexual conflict across the reproductive cycle. 相似文献
2.
3.
4.
5.
Novel Pathway of Toluene Catabolism in the Trichloroethylene-Degrading Bacterium G4 总被引:41,自引:31,他引:10 下载免费PDF全文
Malcolm S. Shields Stacy O. Montgomery Peter J. Chapman Stephen M. Cuskey P. H. Pritchard 《Applied microbiology》1989,55(6):1624-1629
o-Cresol and 3-methylcatechol were identified as successive transitory intermediates of toluene catabolism by the trichloroethylene-degrading bacterium G4. The absence of a toluene dihydrodiol intermediate or toluene dioxygenase and toluene dihydrodiol dehydrogenase activities suggested that G4 catabolizes toluene by a unique pathway. Formation of a hybrid species of 18O- and 16O-labeled 3-methylcatechol from toluene in an atmosphere of 18O2 and 16O2 established that G4 catabolizes toluene by successive monooxygenations at the ortho and meta positions. Detection of trace amounts of 4-methylcatechol from toluene catabolism suggested that the initial hydroxylation of toluene was not exclusively at the ortho position. Further catabolism of 3-methylcatechol was found to proceed via catechol-2,3-dioxygenase and hydroxymuconic semialdehyde hydrolase activities. 相似文献
6.
The effects of dopamine and octopamine on adenylate cyclase activity were studied on the head homogenate of adult Culex pipiens mosquitoes in vitro. Both dopamine and octopamine were shown to increase the cyclic AMP content in the homogenate. The antagonist haloperidol blocked the production of cyclic AMP induced from dopamine but had no effect on the production of cyclic AMP induced by octopamine at the concentrations tested. The opiate agonist etorphine was ineffective at reducing cyclic AMP levels induced by either dopamine or octopamine at the concentrations tested. 相似文献
7.
Skeleton, muscles and tendons were investigated in 3 lower legs with aplasia of the tibia. The tibiofibular joint appears to be rather a syndesmosis than a diarthrosis. In all 3 cases, talus and calcaneus are connected by a synostosis. Also in the other parts of the skeleton of the feet, synarthroses with different extensions are found. The muscles, normally originating from the tibia, have shifted their origin to the fibula. The proximal part of the inserting tendons under the retinacula is quite normal. Also, their distal part in the region of the metatarsalia and of the digits exhibits no alterations. In the region of the tarsus in which already alterations in bone formation are found, the tendons of the flexor muscles of the toes exhibit some variations. Often the muscles show a new, strong insertion into the bones of the tarsus. The normal connection between the proximal and distal parts of the tendons is sometimes still indicated by a thin bundle of collagenous fibers. In all 3 cases, the tibialis anterior muscle inserts into the distal part of the tendons of the flexor muscles of the toes, indicating that there are also connections between tendons of muscles from different blastemas. 相似文献
8.
Stacy K. Li Donald H. Owings 《Ethology : formerly Zeitschrift fur Tierpsychologie》1978,46(4):359-371
The purpose of this study was to seek evidence for the influence of sexual selection on both male and female three-spined sticklebacks, a species in which the ♂ alone has parental responsibilities. We studied the intrasexual relationships of ♂♂ and ♀♀ prior to courtship, and the intrasexual and intersexual relationships of both sexes during courtship. Results demonstrated that both ♂♂ and ♀♀ compete intrasexually, and suggested that both sexes are discriminative in mate selection. These results are compatible with Trivers' (1972) model of the role of parental investment in sexual selection. 相似文献
9.
10.