Spatial Response of Mammals to Late Quaternary Environmental Fluctuations     

RW Graham  EL Lundelius  MA Graham  EK Schroeder  RS Toomey  E Anderson  AD Barnosky  JA Burns  CS Churcher  DK Grayson  RD Guthrie  CR Harington  GT Jefferson  LD Martin  HG McDonald  RE Morlan  HA Semken  SD Webb  L Werdelin  MC Wilson 《Science (New York, N.Y.)》1996,272(5268):1601-1606

Analyses of fossil mammal faunas from 2945 localities in the United States demonstrate that the geographic ranges of individual species shifted at different times, in different directions, and at different rates in response to late Quaternary environmental fluctuations. The geographic pattern of faunal provinces was similar for the late Pleistocene and late Holocene, but differing environmental gradients resulted in dissimilar species composition for these biogeographic regions. Modern community patterns emerged only in the last few thousand years, and many late Pleistocene communities do not have modern analogs. Faunal heterogeneity was greater in the late Pleistocene.  相似文献   

Neutron-mapping polymer flow: scattering, flow visualization, and molecular theory     

Bent J  Hutchings LR  Richards RW  Gough T  Spares R  Coates PD  Grillo I  Harlen OG  Read DJ  Graham RS  Likhtman AE  Groves DJ  Nicholson TM  McLeish TC 《Science (New York, N.Y.)》2003,301(5640):1691-1695

Flows of complex fluids need to be understood at both macroscopic and molecular scales, because it is the macroscopic response that controls the fluid behavior, but the molecular scale that ultimately gives rise to rheological and solid-state properties. Here the flow field of an entangled polymer melt through an extended contraction, typical of many polymer processes, is imaged optically and by small-angle neutron scattering. The dual-probe technique samples both the macroscopic stress field in the flow and the microscopic configuration of the polymer molecules at selected points. The results are compared with a recent "tube model" molecular theory of entangled melt flow that is able to calculate both the stress and the single-chain structure factor from first principles. The combined action of the three fundamental entangled processes of reptation, contour length fluctuation, and convective constraint release is essential to account quantitatively for the rich rheological behavior. The multiscale approach unearths a new feature: Orientation at the length scale of the entire chain decays considerably more slowly than at the smaller entanglement length.  相似文献   

A 5-micron-bright spot on Titan: evidence for surface diversity     

Barnes JW  Brown RH  Turtle EP  McEwen AS  Lorenz RD  Janssen M  Schaller EL  Brown ME  Buratti BJ  Sotin C  Griffith C  Clark R  Perry J  Fussner S  Barbara J  West R  Elachi C  Bouchez AH  Roe HG  Baines KH  Bellucci G  Bibring JP  Capaccioni F  Cerroni P  Combes M  Coradini A  Cruikshank DP  Drossart P  Formisano V  Jaumann R  Langevin Y  Matson DL  McCord TB  Nicholson PD  Sicardy B 《Science (New York, N.Y.)》2005,310(5745):92-95

Observations from the Cassini Visual and Infrared Mapping Spectrometer show an anomalously bright spot on Titan located at 80 degrees W and 20 degrees S. This area is bright in reflected light at all observed wavelengths, but is most noticeable at 5 microns. The spot is associated with a surface albedo feature identified in images taken by the Cassini Imaging Science Subsystem. We discuss various hypotheses about the source of the spot, reaching the conclusion that the spot is probably due to variation in surface composition, perhaps associated with recent geophysical phenomena.  相似文献   

Calcium transient in presynaptic terminal of squid giant synapse: detection with aequorin   总被引：14，自引：0，他引：14  

R Llinás  J R Blinks  C Nicholson 《Science (New York, N.Y.)》1972,176(39):1127-1129

Microinjection of aequorin, a bioluminescent protein sensitive tocalcium, into the presynaptic terminal of the squid giant synapse demnonstrated an increase in intracellular calcium ion concentration during repetitive synaptic transmission. Although no light flashes synchronous with individual presynaptic : tion potentials were detected, the results are considered consistent with the hypothesis that entry of calcium into the presynaptic terminal triggers release of e synaptic transmitter substance.  相似文献   

Hillslope Evolution by Bedrock Landslides     

AL Densmore  RS Anderson  BG McAdoo  MA Ellis 《Science (New York, N.Y.)》1997,275(5298):369-372

Bedrock landsliding is a dominant geomorphic process in a number of high-relief landscapes, yet is neglected in landscape evolution models. A physical model of sliding in beans is presented, in which incremental lowering of one wall simulates baselevel fall and generates slides. Frequent small slides produce irregular hillslopes, on which steep toes and head scarps persist until being cleared by infrequent large slides. These steep segments are observed on hillslopes in high-relief landscapes and have been interpreted as evidence for increases in tectonic or climatic process rates. In certain cases, they may instead reflect normal hillslope evolution by landsliding.  相似文献   

COMPARISONS OF GRASS VARIETIES FOR SURFACE SEEDING UPLAND PASTURE TYPES     

I. Molinia  Nardus  Pastures R. Hughes  I. A. Nicholson 《Grass and Forage Science》1961,16(3):210-221

A series of experiments was laid down in 1957 on different upland vegetation types in Scotland to provide a basis of critical information on the performance of 18 herbage varieties. The present paper deals with results on areas dominated originally by Molinia and Nardus at Sourhope farm, Roxburghshire. Data obtained between 1957 and 1954 are discussal in relation to sward development, competition with remnants of native herbage and seasonal productivity.  相似文献   

Serological survey of vector-borne zoonotic pathogens in pet cats and cats from animal shelters and feral colonies     

Case JB  Chomel B  Nicholson W  Foley JE 《Journal of Feline Medicine and Surgery》2006,8(2):111-117

Although cats and their arthropod parasites can sometimes be important sources of zoonotic diseases in humans, the extent of exposure among various cat populations to many potential zoonotic agents remains incompletely described. In this study, 170 domestic cats living in private homes, feral cat colonies, and animal shelters from California and Wisconsin were evaluated by serology to determine the levels of exposure to a group of zoonotic vector-borne pathogens. Serological positive test results were observed in 17.2% of cats for Rickettsia rickettsii, 14.9% for R akari, 4.9% for R typhi, 11.1% for R felis, and 14.7% for Bartonella henselae. Although vector-borne disease exposure has been documented previously in cats, the evaluation of multiple pathogens and diverse cat populations simultaneously performed here contributes to our understanding of feline exposure to these zoonotic pathogens.  相似文献