687 research outputs found
A phylogenetic comparative analysis on the evolution of sequential hermaphroditism in seabreams (Teleostei : Sparidae)
The Sparids are an ideal group of fish in which to study the evolution of sexual systems since they
exhibit a great sexual diversity, from gonochorism (separate sexes) to protandrous (male-first) and
protogynous (female-first) sequential hermaphroditism (sex-change). According to the size-advantage
model (SAM), selection should favour sex change when the second sex achieves greater reproductive
success at a larger body size than the first sex. Using phylogenetic comparative methods and a sample
of 68 sparid species, we show that protogyny and protandry evolve from gonochorism but evolutionary
transitions between these two forms of sequential hermaphroditism are unlikely to happen. Using male
gonadosomatic index (GSI) as a measure of investment in gametes and proxy for sperm competition,
we find that, while gonochoristic and protogynous species support the predictions of SAM, protandrous
species do not, as they exhibit higher GSI values than expected even after considering mating
systems and spawning modes. We suggest that small males of protandrous species have to invest
disproportionally more in sperm production than predicted not only when spawning in aggregations
with high levels of sperm competition, but also when spawning in pairs due to the need to fertilize
highly fecund females, much larger than themselves. We propose that this compensatory mechanism,
together with Bateman’s principles in sequential hermaphrodites, should be formally incorporated in
the SAM
A discrepancy principle for the Landweber iteration based on risk minimization
In this paper we propose a criterion based on risk minimization to stop the Landweber algorithm for estimating the solution of a linear system with noisy data. Under the hypothesis of white Gaussian noise, we provide an unbiased estimator of the risk and we use it for defining a variant of the classical discrepancy principle. Moreover, we prove that the proposed variant satisfies the regularization property in expectation. Finally, we perform some numerical simulations when the signal formation model is given by a convolution or a Radon transform, to show that the proposed method is numerically reliable and furnishes slightly better solutions than classical estimators based on the predictive risk, namely the Unbiased Predictive Risk Estimator and the Generalized Cross Validation
The Progenitor of the Type IIb SN 2008ax Revisited
Hubble Space Telescope observations of the site of the supernova (SN) 2008ax
obtained in 2011 and 2013 reveal that the possible progenitor object detected
in pre-explosion images was in fact multiple. Four point sources are resolved
in the new, higher-resolution images. We identify one of the sources with the
fading SN. The other three objects are consistent with single supergiant stars.
We conclude that their light contaminated the previously identified progenitor
candidate. After subtraction of these stars, the progenitor appears to be
significantly fainter and bluer than previously measured. Post-explosion
photometry at the SN location indicates that the progenitor object has
disappeared. If single, the progenitor is compatible with a supergiant star of
B to mid-A spectral type, while a Wolf-Rayet (WR) star would be too luminous in
the ultraviolet to account for the observations. Moreover, our hydrodynamical
modelling shows the pre-explosion mass was and the radius was
, which is incompatible with a WR progenitor. We present a
possible interacting binary progenitor computed with our evolutionary models
that reproduces all the observational evidence. A companion star as luminous as
an O9-B0 main-sequence star may have remained after the explosion.Comment: ApJ accepted, 14 pages, 7 figure
Quantifying pigment cover to assess variation in animal colouration
The study of animal colouration addresses fundamental and applied aspects relevant to a wide range
of fields, including behavioural ecology, environmental adaptation and visual ecology. Although a
variety of methods are available to measure animal colours, only few focus on chromatophores
(specialized cells containing pigments) and pigment migration. Here, we illustrate a freely available
and user friendly method to quantify pigment cover (PiC) with high precision and low effort using
digital images, where the foreground (i.e., pigments in chromatophores) can be detected and
separated from the background. Images of the brown shrimp, Crangon crangon were used to
compare PiC with the traditional Chromatophore Index (CI). Results indicate that PiC outcompetes CI
for pigment detection and transparency measures in terms of speed, accuracy and precision. The
proposed methodology provides researchers with a useful tool to answer essential physiological,
behavioural and evolutionary questions on animal colouration in a wide range of species
iPTF13bvn: The First Evidence of a Binary Progenitor for a Type Ib Supernova
The recent detection in archival HST images of an object at the the location
of supernova (SN) iPTF13bvn may represent the first direct evidence of the
progenitor of a Type Ib SN. The object's photometry was found to be compatible
with a Wolf-Rayet pre-SN star mass of ~11 Msun. However, based on
hydrodynamical models we show that the progenitor had a pre-SN mass of ~3.5
Msun and that it could not be larger than ~8 Msun. We propose an interacting
binary system as the SN progenitor and perform evolutionary calculations that
are able to self-consistently explain the light-curve shape, the absence of
hydrogen, and the pre-SN photometry. We further discuss the range of allowed
binary systems and predict that the remaining companion is a luminous O-type
star of significantly lower flux in the optical than the pre-SN object. A
future detection of such star may be possible and would provide the first
robust identification of a progenitor system for a Type Ib SN.Comment: Accepted to AJ on July 26. Slight changes from original, however
delayed by slow refereeing proces
Snapping shrimp and their crustaceous cacophony
We humans are a noisy bunch. Our sounds fill the land and air around us, and even the oceans and seas. But we are not the only ones filling the sea with sound. Tiny snapping shrimp, also known as pistol shrimp, are some of the loudest animals in the ocean! They capture their prey by blasting it with a powerful shockwave from an enlarged claw. While the sound from each individual shrimp is small, the noise they make as a group has been known to mask the presence of submarines! How does something so small make such a loud noise?How can scientists use this noise to better understand the health of the seabed
Make Bow-tie Dynamic by Rethinking it as a Hierarchical Bayesian Network. Dynamic Risk Assessment of an LNG Bunkering Operation
In the present era, the spread of cyber-physical systems in the framework of the so-called Industry 4.0, is leading towards a complete automation of industrial processes, which are increasingly decentralized, smart, and require fewer and fewer frontline personnel. The risk assessment process is certainly not excluded from the revolution, and in perspective needs to be automatic, dynamic and linked with the conditions that emerge, moment by moment, in the life of a complex system. Analytical techniques can help in converting data in information and hence system knowledge to spot trends in operational performance, thus improving risk assessment quality. Even though the bow-tie approach is widely used within the context of complex systems, it still evidences several limitations, mainly connected to the actual assessment of likelihood and interdependencies in the fault and event trees. This paper shows how a bow tie analysis can be reframed as a Hierarchical Bayesian Network, where the probability distributions of the network nodes are updated with real time predictions during the operations. The proposed model was then applied to the risk assessment of a shore-to-ship LNG bunkering operation
Diffusive Ionization of Relativistic Hydrogen-Like Atom
Stochastic ionization of highly excited relativistic hydrogenlike atom in the
monochromatic field is investigated. A theoretical analisis of chaotic dynamics
of the relativistic electron based on Chirikov criterion is given for the cases
of one- and three-dimensional atoms. Critical value of the external field is
evaluated analitically. The diffusion coefficient and ionization time are
calculated.Comment: 13 pages, latex, no figures, submitted to PR
- …