4,476 research outputs found
A new function of hydrocarbons in insect communication : maternal care and offspring signalling in the European earwig
International audienc
Hydrodynamic stress on fractal aggregates of spheres
We calculate the average hydrodynamic stress on fractal aggregates of spheres using Stokesian dynamics. We find that for fractal aggregates of force-free particles, the stress does not grow as the cube of the radius of gyration, but rather as the number of particles in the aggregate. This behavior is only found for random aggregates of force-free particles held together by hydrodynamic lubrication forces. The stress on aggregates of particles rigidly connected by interparticle forces grows as the radius of gyration cubed. We explain this behavior by examining the transmission of the tension along connecting lines in an aggregate and use the concept of a persistance length in order to characterize this stress transmission within an aggregate
The phylogenetic origin and evolution of acellular bone in teleost fishes: insights into osteocyte function in bone metabolism
Vertebrate bone is composed of three main cell types: osteoblasts, osteoclasts and osteocytes, the latter being by far the most numerous. Osteocytes are thought to play a fundamental role in bone physiology and homeostasis, however they are entirely absent in most extant species of teleosts, a group that comprises the vast majority of bony âfishesâ, and approximately half of vertebrates. Understanding how this acellular (anosteocytic) bone appeared and was maintained in such an important vertebrate group has important implications for our understanding of the function and evolution of osteocytes. Nevertheless, although it is clear that cellular bone is ancestral for teleosts, it has not been clear in which specific subgroup the osteocytes were lost. This review aims to clarify the phylogenetic distribution of cellular and acellular bone in teleosts, to identify its precise origin, reversals to cellularity, and their implications. We surveyed the bone type for more than 600 fossil and extant rayâfinned fish species and optimised the results on recent largeâscale molecular phylogenetic trees, estimating ancestral states. We find that acellular bone is a probable synapomorphy of Euteleostei, a group uniting approximately twoâthirds of teleost species. We also confirm homoplasy in these traits: acellular bone occurs in some nonâeuteleosts (although rarely), and cellular bone was reacquired several times independently within euteleosts, in salmons and relatives, tunas and the opah (Lampris sp.). The occurrence of peculiar ecological (e.g. anadromous migration) and physiological (e.g. redâmuscle endothermy) strategies in these lineages might explain the reacquisition of osteocytes. Our review supports that the main contribution of osteocytes in teleost bone is to mineral homeostasis (via osteocytic osteolysis) and not to strain detection or bone remodelling, helping to clarify their role in bone physiology
Mesoscale dynamics on the Sun's surface from HINODE observations
Aims: The interactions of velocity scales on the Sun's surface, from
granulation to supergranulation are still not understood, nor are their
interaction with magnetic fields. We thus aim at giving a better description of
dynamics in the mesoscale range which lies between the two scales mentioned
above. Method: We analyse a 48h high-resolution time sequence of the quiet Sun
photosphere at the disk center obtained with the Solar Optical Telescope
onboard Hinode. The observations, which have a field of view of 100
\arcsec 100 \arcsec, typically contain four supergranules. We monitor
in detail the motion and evolution of granules as well as those of the radial
magnetic field. Results: This analysis allows us to better characterize Trees
of Fragmenting Granules issued from repeated fragmentation of granules,
especially their lifetime statistics. Using floating corks advected by measured
velocity fields, we show their crucial role in the advection of the magnetic
field and in the build up of the network. Finally, thanks to the long duration
of the time series, we estimate that the turbulent diffusion coefficient
induced by horizontal motion is approximately . Conclusions: These results demonstrate that the long living
families contribute to the formation of the magnetic network and suggest that
supergranulation could be an emergent length scale building up as small
magnetic elements are advected and concentrated by TFG flows. Our estimate for
the magnetic diffusion associated with this horizontal motion might provide a
useful input for mean-field dynamo models.Comment: to appear in A&A - 8 pages, 13 figures (degraded quality) - Full
resolution version available @
http://www.ast.obs-mip.fr/users/rincon/hinode_roudier_aa09.pd
Graphene Ripples as a Realization of a Two-Dimensional Ising Model: A Scanning Tunneling Microscope Study
Ripples in pristine freestanding graphene naturally orient themselves in an
array that is alternately curved-up and curved-down; maintaining an average
height of zero. Using scanning tunneling microscopy (STM) to apply a local
force, the graphene sheet will reversibly rise and fall in height until the
height reaches 60-70 percent of its maximum at which point a sudden, permanent
jump occurs. We successfully model the ripples as a spin-half Ising magnetic
system, where the height of the graphene is the spin. The permanent jump in
height, controlled by the tunneling current, is found to be equivalent to an
antiferromagnetic-to-ferromagnetic phase transition. The thermal load
underneath the STM tip alters the local tension and is identified as the
responsible mechanism for the phase transition. Four universal critical
exponents are measured from our STM data, and the model provides insight into
the statistical role of graphenes unusual negative thermal expansion
coefficient.Comment: 12 pages, 5 figures, 1 tabl
Dynamic reconfiguration of human brain networks during learning
Human learning is a complex phenomenon requiring flexibility to adapt
existing brain function and precision in selecting new neurophysiological
activities to drive desired behavior. These two attributes -- flexibility and
selection -- must operate over multiple temporal scales as performance of a
skill changes from being slow and challenging to being fast and automatic. Such
selective adaptability is naturally provided by modular structure, which plays
a critical role in evolution, development, and optimal network function. Using
functional connectivity measurements of brain activity acquired from initial
training through mastery of a simple motor skill, we explore the role of
modularity in human learning by identifying dynamic changes of modular
organization spanning multiple temporal scales. Our results indicate that
flexibility, which we measure by the allegiance of nodes to modules, in one
experimental session predicts the relative amount of learning in a future
session. We also develop a general statistical framework for the identification
of modular architectures in evolving systems, which is broadly applicable to
disciplines where network adaptability is crucial to the understanding of
system performance.Comment: Main Text: 19 pages, 4 figures Supplementary Materials: 34 pages, 4
figures, 3 table
Interpretation of Light-Quenching Factor Measurements
We observe that the pattern of the quenching factors for scintillation light
from various ions, recently studied in in connection with dark matter
detectors, can be understood as a saturation phenomenon in which the light
output is simply proportional to track length, independent of the ion and its
energy. This observation is in accord with the high dE/dx limit of Birks' law.
It suggests a simple model for the intrinsic resolution of light detectors for
low energy ions, which we briefly discuss.Comment: Seven pages, seven figures, some with colo
Using the Sun to estimate Earth-like planets detection capabilities. III. Impact of spots and plages on astrometric detection
Stellar activity is a potential important limitation to the detection of low
mass extrasolar planets with indirect methods (RV, photometry, astrometry). In
previous papers, using the Sun as a proxy, we investigated the impact of
stellar activity (spots, plages, convection) on the detectability of an
Earth-mass planet in the habitable zone (HZ) of solar-type stars with RV
techniques. We extend here the detectability study to the case of astrometry.
We used the sunspot and plages properties recorded over one solar cycle to
infer the astrometric variations that a Sun-like star seen edge-on, 10 pc away,
would exhibit, if covered by such spots/bright structures. We compare the
signal to the one expected from the astrometric wobble (0.3 {\mu}as) of such a
star surrounded by a one Earth-mass planet in the HZ. We also briefly
investigate higher levels of activity. The activity-induced astrometric signal
along the equatorial plane has an amplitude of typ. less than 0.2 {\mu}as
(rms=0.07 {\mu}as), smaller than the one expected from an Earth-mass planet at
1 AU. Hence, for this level of activity, the detectability is governed by the
instrumental precision rather than the activity. We show that for instance a
one Earth-mass planet at 1 AU would be detected with a monthly visit during
less than 5 years and an instrumental precision of 0.8 {\mu}as. A level of
activity 5 times higher would still allow such a detection with a precision of
0.35 {\mu}as. We conclude that astrometry is an attractive approach to search
for such planets around solar type stars with most levels of stellar activity.Comment: Letter accepted by A&
Rythmes saisonniers de la croissance chez quelques Téléostéens de Guyane française
Des études squeletto-chronologiques de diverses espÚces de Guyane française, #Arius couma, #A. proops, et #A. parkeri (#Siluriformes, #Arridae), #Leporinus friderici (#Characiformes, #Anostomidae) et #Myleus rhomboidalis (#Characiformes, #Serrasalmidae), permettent de proposer quelques hypothÚses sur les modalités de leur croissance dans le milieu naturel, notamment dans le fleuve Sinnamary. Toutes semblent présenter, chaque année, deux phases de croissance active séparées par des périodes de ralentissement correspondant respectivement aux saisons humides et aux saisons sÚches. C'est trÚs probablement la disponibilité en nourriture, abondante en saison des pluies, qui rÚgle les performances de croissance. A partir de l'hypothÚse de deux cycles de croissance annuels, des courbes de croissance sont proposées pour chaque espÚce. (Résumé d'auteur
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