48,775 research outputs found
Reciprocal relationships in collective flights of homing pigeons
Collective motion of bird flocks can be explained via the hypothesis of many
wrongs, and/or, a structured leadership mechanism. In pigeons, previous studies
have shown that there is a well-defined hierarchical structure and certain
specific individuals occupy more dominant positions --- suggesting that
leadership by the few individuals drives the behavior of the collective.
Conversely, by analyzing the same data-sets, we uncover a more egalitarian
mechanism. We show that both reciprocal relationships and a stratified
hierarchical leadership are important and necessary in the collective movements
of pigeon flocks. Rather than birds adopting either exclusive averaging or
leadership strategies, our experimental results show that it is an integrated
combination of both compromise and leadership which drives the group's movement
decisions.Comment: 7 pages, 5 figure
Selective vulnerability in neurodegenerative diseases
Neurodegenerative diseases have two general characteristics that are so fundamental we usually take them for granted. The first is that the pathology associated with the disease only affects particular neurons (‘selective neuronal vulnerability’); the second is that the pathology worsens with time and impacts more regions in a stereotypical and predictable fashion. The mechanisms underpinning selective neuronal and regional vulnerability have been difficult to dissect, but the recent application of whole-genome technologies, the development of mouse models that reproduce spatial and temporal features of the pathology, and the identification of intrinsic morphological, electrophysiological, and biochemical properties of vulnerable neurons are beginning to shed some light on these fundamental features of neurodegenerative diseases. Here we detail our emerging understanding of the underlying biology of selective neuronal vulnerability and outline some of the areas in which our understanding is incomplete
Detection of water vapor in the terrestrial planet forming region of a transition disk
We report a detection of water vapor in the protoplanetary disk around DoAr
44 with the Texas Echelon Cross Echelle Spectrograph --- a visitor instrument
on the Gemini north telescope. The DoAr 44 disk consists of an optically thick
inner ring and outer disk, separated by a dust-cleared 36 AU gap, and has
therefore been termed "pre-transitional". To date, this is the only disk with a
large inner gap known to harbor detectable quantities of warm (T=450 K) water
vapor. In this work, we detect and spectrally resolve three mid-infrared pure
rotational emission lines of water vapor from this source, and use the shapes
of the emission lines to constrain the location of the water vapor. We find
that the emission originates near 0.3 AU --- the inner disk region. This
characteristic region coincides with that inferred for both optically thick and
thin thermal infrared dust emission, as well as rovibrational CO emission. The
presence of water in the dust-depleted region implies substantial columns of
hydrogen (>10^{22} cm-2) as the water vapor would otherwise be destroyed by
photodissociation. Combined with the dust modeling, this column implies a
gas/small-dust ratio in the optically thin dusty region of >1000. These results
demonstrate that DoAr 44 has maintained similar physical and chemical
conditions to classical protoplanetary disks in its terrestrial-planet forming
regions, in spite of having formed a large gap.Comment: Paper accepted to the Astrophysical Journal Letter
Is Neolithic land use correlated with demography? An evaluation of pollen-derived land cover and radiocarbon-inferred demographic change from Central Europe
The transformation of natural landscapes in Middle Europe began in the Neolithic as a result of the introduction of food-producing economies. This paper examines the relation between land-cover and demographic change in a regionally restricted case study. The study area is the Western Lake Constance area which has very detailed palynological as well as archaeological records. We compare land-cover change derived from nine pollen records using a pseudo-biomisation approach with 14C date probability density functions from archaeological sites which serve as a demographic proxy. We chose the Lake Constance area as a regional example where the pollen signal integrates a larger spatial pattern. The land-cover reconstructions for this region show first notable impacts at the Middle to Young Neolithic transition. The beginning of the Bronze Age is characterised by increases of arable land and pasture/meadow, whereas the deciduous woodland decreases dramatically. Changes in the land-cover classes show a correlation with the 14C density curve: the correlation is best with secondary woodland in the Young Neolithic which reflects the lake shore settlement dynamics. In the Early Bronze Age, the radiocarbon density correlates with open land-cover classes, such as pasture, meadow and arable land, reflecting a change in the land-use strategy. The close overall correspondence between the two archives implies that population dynamics and land-cover change were intrinsically linked. We therefore see human impact as a key driver for vegetation change in the Neolithic. Climate might have an influence on vegetation development, but the changes caused by human land use are clearly detectable from Neolithic times, at least in these densely settled, mid-altitude landscapes
A general condition of inflationary cosmology on trans-Planckian physics
We consider a more general initial condition satisfying the minimal
uncertainty relationship. We calculate the power spectrum of a simple model in
inflationary cosmology. The results depend on perturbations generated below a
fundamental scale, e.g. the Planck scale.Comment: 7 pages, References adde
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