285 research outputs found
Time-varying auditory gain control in response to double-pulse stimuli in harbour porpoises is not mediated by a stapedial reflex
Echolocating animals reduce their output level and hearing sensitivity with decreasing echo delays, presumably to stabilize the perceived echo intensity during target approaches. In bats, this variation in hearing sensitivity is formed by a call-induced stapedial reflex that tapers off over time after the call. Here, we test the hypothesis that a similar mechanism exists in toothed whales by subjecting a trained harbour porpoise to a series of double sound pulses varying in delay and frequency, while measuring the magnitudes of the evoked auditory brainstem responses (ABRs). We find that the recovery of the ABR to the second pulse is frequency dependent, and that a stapedial reflex therefore cannot account for the reduced hearing sensitivity at short pulse delays. We propose that toothed whale auditory time-varying gain control during echolocation is not enabled by the middle ear as in bats, but rather by frequency-dependent mechanisms such as forward masking and perhaps higher-order control of efferent feedback to the outer hair cells
Strong pressure-energy correlations in van der Waals liquids
Strong correlations between equilibrium fluctuations of the configurational
parts of pressure and energy are found in the Lennard-Jones liquid and other
simple liquids, but not in hydrogen-bonding liquids like methanol and water.
The correlations, that are present also in the crystal and glass phases,
reflect an effective inverse power-law repulsive potential dominating
fluctuations, even at zero and slightly negative pressure. In experimental data
for supercritical Argon, the correlations are found to be approximately 96%.
Consequences for viscous liquid dynamics are discussed.Comment: Phys. Rev. Lett., in pres
Isomorphs in model molecular liquids
Isomorphs are curves in the phase diagram along which a number of static and
dynamic quantities are invariant in reduced units. A liquid has good isomorphs
if and only if it is strongly correlating, i.e., the equilibrium
virial/potential energy fluctuations are more than 90% correlated in the NVT
ensemble. This paper generalizes isomorphs to liquids composed of rigid
molecules and study the isomorphs of two systems of small rigid molecules, the
asymmetric dumbbell model and the Lewis-Wahnstrom OTP model. In particular, for
both systems we find that the isochoric heat capacity, the excess entropy, the
reduced molecular center-of-mass self part of the intermediate scattering
function, the reduced molecular center-of-mass radial distribution function to
a good approximation are invariant along an isomorph. In agreement with theory,
we also find that an instantaneous change of temperature and density from an
equilibrated state point to another isomorphic state point leads to no
relaxation. The isomorphs of the Lewis-Wahnstrom OTP model were found to be
more approximative than those of the asymmetric dumbbell model, which is
consistent with the OTP model being less strongly correlating. For both models
we find "master isomorphs", i.e., isomorphs have identical shape in the
virial/potential energy phase diagram.Comment: 20 page
Dynamical Heterogeneities Below the Glass Transition
We present molecular dynamics simulations of a binary Lennard-Jones mixture
at temperatures below the kinetic glass transition. The ``mobility'' of a
particle is characterized by the amplitude of its fluctuation around its
average position. The 5% particles with the largest/smallest mean amplitude are
thus defined as the relatively most mobile/immobile particles. We investigate
for these 5% particles their spatial distribution and find them to be
distributed very heterogeneously in that mobile as well as immobile particles
form clusters. The reason for this dynamic heterogeneity is traced back to the
fact that mobile/immobile particles are surrounded by fewer/more neighbors
which form an effectively wider/narrower cage. The dependence of our results on
the length of the simulation run indicates that individual particles have a
characteristic mobility time scale, which can be approximated via the
non-Gaussian parameter.Comment: revtex, 10 pages, 20 postscript figure
Highly overlapping winter diet in two sympatric lemming species revealed by DNA metabarcoding
Sympatric species are expected to minimize competition by partitioning resources, especially
when these are limited. Herbivores inhabiting the High Arctic in winter are a prime example
of a situation where food availability is anticipated to be low, and thus reduced diet
overlap is expected. We present here the first assessment of diet overlap of high arctic lemmings
during winter based on DNA metabarcoding of feces. In contrast to previous analyses
based on microhistology, we found that the diets of both collared (Dicrostonyx
groenlandicus) and brown lemmings (Lemmus trimucronatus) on Bylot Island were dominated
by Salix while mosses, which were significantly consumed only by the brown lemming,
were a relatively minor food item. The most abundant plant taxon, Cassiope
tetragona, which alone composes more than 50% of the available plant biomass, was not
detected in feces and can thus be considered to be non-food. Most plant taxa that were
identified as food items were consumed in proportion to their availability and none were
clearly selected for. The resulting high diet overlap, together with a lack of habitat segregation,
indicates a high potential for resource competition between the two lemming species.
However, Salix is abundant in the winter habitats of lemmings on Bylot Island and the nonSalix
portion of the diets differed between the two species. Also, lemming grazing impact on
vegetation during winter in the study area is negligible. Hence, it seems likely that the high
potential for resource competition predicted between these two species did not translate
into actual competition. This illustrates that even in environments with low primary productivity
food resources do not necessarily generate strong competition among herbivores
Energy landscape of a Lennard-Jones liquid: Statistics of stationary points
Molecular dynamics simulations are used to generate an ensemble of saddles of
the potential energy of a Lennard-Jones liquid. Classifying all extrema by
their potential energy u and number of unstable directions k, a well defined
relation k(u) is revealed. The degree of instability of typical stationary
points vanishes at a threshold potential energy, which lies above the energy of
the lowest glassy minima of the system. The energies of the inherent states, as
obtained by the Stillinger-Weber method, approach the threshold energy at a
temperature close to the mode-coupling transition temperature Tc.Comment: 4 RevTeX pages, 6 eps figures. Revised versio
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