全文获取类型
收费全文 | 206篇 |
免费 | 2篇 |
出版年
2017年 | 1篇 |
2015年 | 1篇 |
2014年 | 1篇 |
2013年 | 8篇 |
2012年 | 7篇 |
2011年 | 8篇 |
2010年 | 13篇 |
2009年 | 14篇 |
2008年 | 13篇 |
2007年 | 9篇 |
2006年 | 11篇 |
2005年 | 10篇 |
2004年 | 6篇 |
2003年 | 3篇 |
2002年 | 3篇 |
2001年 | 1篇 |
2000年 | 2篇 |
1999年 | 4篇 |
1998年 | 3篇 |
1997年 | 7篇 |
1996年 | 6篇 |
1995年 | 2篇 |
1994年 | 3篇 |
1993年 | 5篇 |
1992年 | 5篇 |
1991年 | 1篇 |
1990年 | 3篇 |
1989年 | 6篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 4篇 |
1972年 | 2篇 |
1971年 | 1篇 |
1970年 | 1篇 |
1969年 | 1篇 |
1968年 | 2篇 |
1959年 | 1篇 |
排序方式: 共有208条查询结果,搜索用时 15 毫秒
31.
The dentition of the American opossum, Didelphis marsupialis , has been examined using prepared skulls, occlusal casts and, during normal function in chewing, by cinefluorography. The information obtained from these studies has been used in a simulation of the power stroke of mastication on a stereotaxic machine. The directions and amplitudes of the movements used in chewing have been demonstrated.
Two types of power stroke are described; a crushing-puncturing stroke associated with tooth-food-tooth contact occurring in the early stages of mastication, and a shearing stroke with tooth-tooth contact occurring later. The first type produces a rapid flattening of the tips of the cusps, the second a series of striated wear facets on the slopes of the cusps. In the shearing stroke the food is triturated by an anteromedial movement of the paracristid of the lower molars across the metacrista of the upper. Active chewing occurs on one side of the mouth only at any one time and for a considerable number of cycles. The symphysis does not cross the midline during any sequence of cycles. The anteromedially directed power stroke is mediated in part by the mobile symphysis which allows asymmetrical movement of the two dentaries and by a Bennett shift of the mandibular condyles.
The similarity between the molars of Didelphis and those of the early therians, notably Alphadon, Pappotherium and Holoclemensia is recognized. It is suggested that the information obtained from this study may assist in the interpretation of early therian molars. 相似文献
Two types of power stroke are described; a crushing-puncturing stroke associated with tooth-food-tooth contact occurring in the early stages of mastication, and a shearing stroke with tooth-tooth contact occurring later. The first type produces a rapid flattening of the tips of the cusps, the second a series of striated wear facets on the slopes of the cusps. In the shearing stroke the food is triturated by an anteromedial movement of the paracristid of the lower molars across the metacrista of the upper. Active chewing occurs on one side of the mouth only at any one time and for a considerable number of cycles. The symphysis does not cross the midline during any sequence of cycles. The anteromedially directed power stroke is mediated in part by the mobile symphysis which allows asymmetrical movement of the two dentaries and by a Bennett shift of the mandibular condyles.
The similarity between the molars of Didelphis and those of the early therians, notably Alphadon, Pappotherium and Holoclemensia is recognized. It is suggested that the information obtained from this study may assist in the interpretation of early therian molars. 相似文献
32.
Ready and Waiting: Delayed Hatching and Extended Incubation of Anamniotic Vertebrate Terrestrial Eggs 总被引:1,自引:1,他引:0
Some anamniotic aquatic vertebrates lay eggs in a terrestrialhabitat that is hostile to the survival of hatchings or larvae.These terrestrial eggs are ready and able to hatch at a particulardevelopmental time, but do not hatch until presented with suitableconditions for aquatic larval survival. Beyond this time, hatchingis possible whenever aquatic conditions occur. The durationof extended terrestrial incubation is dependent on the availabilityof energy for metabolism from the yolk. Extended incubationis useful for anamniotic eggs laid in terrestrial habitats whereconditions suitable for larval survival arrive with unpredictableor variable timing. Examples of anamniotes with delayed hatchingand extended terrestrial incubation can be found among teleostfishes, anurans, and caudate amphibians. This paper characterizesthe embryonic period, compares this mode with other forms ofdevelopmental plasticity in anamniotes, evaluates the constraintsand advantages of this life history mode, and examines how somefishes and amphibians are able to obtain the benefits of terrestrialityfor their eggs when the timing of the return to aquatic conditionsis not entirely predictable. 相似文献
33.
Eight microsatellite loci were isolated from the lance‐tailed manakin (Chiroxiphia lanceolata), a polygynous lek‐breeding bird from Central America. Five of these loci were polymorphic (two to seven alleles per locus), with observed levels of heterozygosity ranging from 0.100 to 0.860 (n = 50 individuals). These variable loci provide a valuable tool for assessing patterns of parentage and relatedness within lance‐tailed manakin social groups. 相似文献
34.
V. A. FUNK KAREN S. RICHARDSON SIMON FERRIER 《Biological journal of the Linnean Society. Linnean Society of London》2005,85(4):549-567
Research expeditions into remote areas to collect biological specimens provide vital information for understanding biodiversity. However, major expeditions to little‐known areas are expensive and time consuming, time is short, and well‐trained people are difficult to find. In addition, processing the collections and obtaining accurate identifications takes time and money. In order to get the maximum return for the investment, we need to determine the location of the collecting expeditions carefully. In this study we used environmental variables and information on existing collecting localities to help determine the sites of future expeditions. Results from other studies were used to aid in the selection of the environmental variables, including variables relating to temperature, rainfall, lithology and distance between sites. A survey gap analysis tool based on ‘ED complementarity’ was employed to select the sites that would most likely contribute the most new taxa. The tool does not evaluate how well collected a previously visited site survey site might be; however, collecting effort was estimated based on species accumulation curves. We used the number of collections and/or number of species at each collecting site to eliminate those we deemed poorly collected. Plants, birds, and insects from Guyana were examined using the survey gap analysis tool, and sites for future collecting expeditions were determined. The south‐east section of Guyana had virtually no collecting information available. It has been inaccessible for many years for political reasons and as a result, eight of the first ten sites selected were in that area. In order to evaluate the remainder of the country, and because there are no immediate plans by the Government of Guyana to open that area to exploration, that section of the country was not included in the remainder of the study. The range of the ED complementarity values dropped sharply after the first ten sites were selected. For plants, the group for which we had the most records, areas selected included several localities in the Pakaraima Mountains, the border with the south‐east, and one site in the north‐west. For birds, a moderately collected group, the strongest need was in the north‐west followed by the east. Insects had the smallest data set and the largest range of ED complementarity values; the results gave strong emphasis to the southern parts of the country, but most of the locations appeared to be equidistant from one another, most likely because of insufficient data. Results demonstrate that the use of a survey gap analysis tool designed to solve a locational problem using continuous environmental data can help maximize our resources for gathering new information on biodiversity. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 85 , 549–567. 相似文献
35.
Many hypotheses attempt to explain why younger, less experienced birds have a relatively low reproductive output. We evaluated reproductive patterns of marked American Kestrels Falco sparverius nesting in boxes in southwestern Idaho from 1992 to 2006 to test predictions of these hypotheses. Results were consistent with the selection (differential mortality) hypothesis and did not support the constraint, restraint or recruitment hypotheses. Most known-age Kestrels nested in their first year of life, and there was no apparent short-term or long-term reproductive advantage to delayed breeding. The number of years that Kestrels nested in study area boxes ranged from 1 to 6 years, with most Kestrels nesting in only 1 year. Reproductive rates were higher for birds with at least 1 year of nesting experience than for birds nesting in boxes for the first time. After 2 years of nesting, reproductive rates levelled off; there was no evidence for additional improvement or for senescence. Differences in reproductive output with experience/age were due to variation among and not within individuals. Individuals that nested in more than 1 year had similar reproductive rates in their first and second years. Poor producers either died or dispersed after 1 year of nesting in study area boxes. Successful females that nested early in the season and successful males that had been produced locally had the highest probability of returning to nest in a subsequent year. 相似文献
36.
37.
ALISTAIR S. GLEN MARIA J. CARDOSO CHRIS R. DICKMAN KAREN B. FIRESTONE 《Biological journal of the Linnean Society. Linnean Society of London》2009,96(1):1-7
Female promiscuity is common among mammals but its advantages, particularly for marsupials, remain unclear. Using microsatellite DNA from pouch young of known mothers, we identified the most likely fathers of 25 wild spotted-tailed quolls ( Dasyurus maculatus ) from six litters. We aimed to determine whether young within the same litter had different fathers, and whether breeding success of males was associated with large body mass (consistent with inter-male competition) or scrotal width (consistent with sperm competition). We also explored the possible influence of promiscuity on relatedness within litters. Finally, we used data on paternity and relatedness to make inferences regarding movement and dispersal.
Four litters were sired by more than one male, and three males sired offspring in more than one litter. Known fathers had higher body mass, but not scrotal width, than males of unknown paternity status, suggesting that males may compete for access to females. Sires were less related to dams than expected by chance, and litters with multiple paternity had lower relatedness than litters sired by a single male. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 1–7. 相似文献
Four litters were sired by more than one male, and three males sired offspring in more than one litter. Known fathers had higher body mass, but not scrotal width, than males of unknown paternity status, suggesting that males may compete for access to females. Sires were less related to dams than expected by chance, and litters with multiple paternity had lower relatedness than litters sired by a single male. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 1–7. 相似文献
38.
MAXWELL B. JOSEPH JONAH PIOVIA‐SCOTT SHARON P. LAWLER KAREN L. POPE 《Freshwater Biology》2011,56(5):828-838
1. The introduction of trout to montane lakes has negatively affected amphibian populations across the western United States. In northern California’s Klamath–Siskiyou Mountains, introduced trout have diminished the distribution and abundance of a native ranid frog, Rana (=Lithobates) cascadae. This is primarily thought to be the result of predation on frog larvae. However, if trout feed on larval aquatic insects that are available to R. cascadae only after emergence, then resource competition may also affect this declining native amphibian. 2. Stomach contents of R. cascadae were compared between lakes that contained trout and those from which introduced trout were removed. Total prey mass in stomach contents relative to frog body mass was not significantly different between lakes with fish and fish‐removal lakes, but in the former R. cascadae consumed a smaller proportion of adult aquatic insects. The stomach contents of fish included larvae of aquatic insects that are, as adults, eaten by R. cascadae. 3. Rana cascadae consumed fewer caddisflies (Trichoptera) and more grasshoppers (Orthoptera) at lakes with higher densities of fish. At lakes with greater aquatic habitat complexity, R. cascadae consumed more water striders (Hemiptera: Gerridae) and terrestrial spiders (Araneae). 4. We suggest that reductions in the availability of emerging aquatic insects cause R. cascadae to consume more terrestrial prey where trout are present. Thus, introduced trout may influence native amphibians directly through predation and, indirectly, through pre‐emptive resource competition. 相似文献
39.
MATS E. ERIKSSON JOHAN LINDGREN KAREN CHIN URBAN MÅNSBY 《Lethaia: An International Journal of Palaeontology and Stratigraphy》2011,44(4):455-468
Eriksson, M.E., Lindgren, J., Chin, K. & Månsby, U. 2011: Coprolite morphotypes from the Upper Cretaceous of Sweden: novel views on an ancient ecosystem and implications for coprolite taphonomy. Lethaia, Vol. 44, pp. 455–468. Coprolites (fossilized faeces) are common, yet previously unreported, elements in the Campanian (Upper Cretaceous) shallow‐marine strata of Åsen, southern Sweden. They are associated with a diverse vertebrate fauna and comprise at least seven different morphotypes that suggest a variety of source animals. Their faecal origin is corroborated by several lines of evidence, including chemical composition (primarily calcium phosphate), external morphology and nature of the inclusions. Preservation in a fossil coquina, interpreted as a taphocoenosis, suggests early lithification promoted by rapid entombment. This would have prevented disintegration of the faecal matter and facilitated transportation and introduction to the host sediment. The coprofabrics can generally be correlated to specific gross morphologies, supporting a morphology‐determined coprolite classification. Moreover, having been deposited under presumably comparable taphonomic conditions, variations in coprofabrics infer differences in diet and/or digestive efficiency of the host animal. Size and morphology of the coprolites imply that most, if not all, were produced by vertebrates and the largest specimens infer a host animal of considerable size. Two spiralled coprolite morphotypes yield bone fragments and scales of bony fish, suggesting that the producers were piscivorous sharks. Other coprolites contain inclusions interpreted as the remains of shelled invertebrates, thus indicating that they may have derived from durophagous predators and/or scavengers. The occurrence of small scrapes, tracks and traces on several specimens suggest manipulation of the faeces by other (presumably coprophagous) organisms after deposition. The collective data from the Åsen coprolites provide new insights into a shallow‐water Late Cretaceous marine ecosystem hitherto known solely from body fossils. □ Coprolites, vertebrates, coprofabrics, taphonomy, trophic levels, Upper Cretaceous, Sweden. 相似文献
40.
MARY M. PEACOCK KAREN H. BEARD† ERIC M. O'NEILL‡ VERONICA S. KIRCHOFF MAUREEN B. PETERS§ 《Molecular ecology》2009,18(17):3603-3615
The success of non-native species may depend on the genetic resources maintained through the invasion process. The Coqui ( Eleutherodactylus coqui ), a frog endemic to Puerto Rico, was introduced to Hawaii in the late 1980s via the horticulture trade, and has become an aggressive invader. To explore whether genetic diversity and population structure changed with the introduction, we assessed individuals from 15 populations across the Hawaiian Islands and 13 populations across Puerto Rico using six to nine polymorphic microsatellite loci and five dorsolateral colour patterns. Allelic richness ( R T ) and gene diversity were significantly higher in Puerto Rico than in Hawaii populations. Hawaii also had fewer colour patterns (two versus three to five per population) than Puerto Rico. We found no isolation by distance in the introduced range, even though it exists in the native range. Results suggest extensive mixing among frog populations across Hawaii, and that their spread has been facilitated by humans. Like previous research, our results suggest that Hawaiian Coquis were founded by individuals from sites around San Juan, but unlike previous research the colour pattern and molecular genetic data (nuclear and mtDNA) support two separate introductions, one on the island of Hawaii and one on Maui. Coquis are successful invaders in Hawaii despite the loss of genetic variation. Future introductions may increase genetic variation and potentially its range. 相似文献