152 research outputs found
Etudes thermochimiques à haute température par spectrométrie de masse : emploi de dispositifs à caloducs pour le maintien isotherme de cellules d'effusion
Différentes solutions ont été testées pour résoudre le problÚme délicat du maintien isotherme de cellules d'effusion multiples couplées à un spectromÚtre de masse. Un dispositif constitué par un caloduc, dans lequel sont logées les cellules, permet d'obtenir d'une maniÚre trÚs sûre ce résultat en éliminant pratiquement les risques d'incertitudes dues aux gradients de températur
Optimum control strategies for maximum thrust production in underwater undulatory swimming
Fish, cetaceans and many other aquatic vertebrates undulate their bodies to
propel themselves through water. Numerous studies on natural, artificial or
analogous swimmers are dedicated to revealing the links between the kinematics
of body oscillation and the production of thrust for swimming. One of the most
open and difficult questions concerns the best kinematics to maximize this
later quantity for given constraints and how a system strategizes and adjusts
its internal parameters to reach this maximum. To address this challenge, we
exploit a biomimetic robotic swimmer to determine the control signal that
produces the highest thrust. Using machine learning techniques and intuitive
models, we find that this optimal control consists of a square wave function,
whose frequency is fixed by the interplay between the internal dynamics of the
swimmer and the fluid-structure interaction with the surrounding fluid. We then
propose a simple implementation for autonomous robotic swimmers that requires
no prior knowledge of systems or equations. This application to aquatic
locomotion is validated by 2D numerical simulations
Etudes thermochimiques à haute température par spectrométrie de masse : dispositif pour mesures au moyen de cellules d'effusion multiples
Le dispositif dĂ©crit a Ă©tĂ© rĂ©alisĂ© dans le but d'effectuer des mesures comparatives directes de pressions de vapeur sur cellules d'effusion multiples, le dĂ©tecteur de flux Ă©tant un spectromĂštre de masse. La reproductibilitĂ© des mesures a Ă©tĂ© Ă©tudiĂ©e notamment en fonction du positionnement mĂ©canique des cellules et des conditions d'ionisation du jet molĂ©culaire dans la source d'ions. Les essais prĂ©sentĂ©s montrent que cette mĂ©thode peut ĂȘtre appliquĂ©e de maniĂšre satisfaisante Ă la dĂ©termination d'activitĂ©s thermodynamiques dans des systĂšmes Ă haute tempĂ©rature
Independent Origins of Cultivated Coconut (Cocos nucifera L.) in the Old World Tropics
As a portable source of food, water, fuel, and construction materials, the coconut (Cocos nucifera L.) played a fundamental role in human migrations and the development of civilization across the humid tropics. Here we investigated the coconut's domestication history and its population genetic structure as it relates to human dispersal patterns. A sample of 1,322 coconut accessions, representing the geographical and phenotypic diversity of the species, was examined using ten microsatellite loci. Bayesian analyses reveal two highly genetically differentiated subpopulations that correspond to the Pacific and Indo-Atlantic oceanic basins. This pattern suggests independent origins of coconut cultivation in these two world regions, with persistent population structure on a global scale despite long-term human cultivation and dispersal. Pacific coconuts show additional genetic substructure corresponding to phenotypic and geographical subgroups; moreover, the traits that are most clearly associated with selection under human cultivation (dwarf habit, self-pollination, and âniu vaiâ fruit morphology) arose only in the Pacific. Coconuts that show evidence of genetic admixture between the Pacific and Indo-Atlantic groups occur primarily in the southwestern Indian Ocean. This pattern is consistent with human introductions of Pacific coconuts along the ancient Austronesian trade route connecting Madagascar to Southeast Asia. Admixture in coastal east Africa may also reflect later historic Arab trading along the Indian Ocean coastline. We propose two geographical origins of coconut cultivation: island Southeast Asia and southern margins of the Indian subcontinent
The culture history of Madagascar
Madagascar's culture is a unique fusion of elements drawn from the western, northern, and eastern shores of the Indian Ocean, and its past has fascinated many scholars, yet systematic archaeological research is relatively recent on the island. The oldest traces of visitors are from the first century AD. Coastal settlements, with clear evidence of ties to the western Indian Ocean trading network, were established in several places over the next millennium. Important environmental changes of both plant and animal communities are documented over this period, including the extinctions of almost all large animal species. Urban life in Madagascar began with the establishment of the entrepĂŽt of Mahilaka on the northwest coast of the island in the twelfth century. At about the same time, communities with ties to the trade network were established around the island's coasts. From the fourteenth to the sixteenth century, social hierarchies developed in several regions of the island. During the succeeding two centuries, Madagascar saw the development of state polities.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45256/1/10963_2004_Article_BF00997802.pd
Comparative phylogeography of parasitic Laelaps mites contribute new insights into the specialist-generalist variation hypothesis (SGVH)
BACKGROUND: The specialist-generalist variation hypothesis (SGVH) in parasites suggests that, due to patchiness in
habitat (host availability), specialist species will show more subdivided population structure when compared to
generalist species. In addition, since specialist species are more prone to local stochastic extinction events with their
hosts, they will show lower levels of intraspecific genetic diversity when compared to more generalist.
RESULTS: To test the wider applicability of the SGVH we compared 337 cytochrome oxidase I mitochondrial DNA and
268 nuclear tropomyosin DNA sequenced fragments derived from two co-distributed Laelaps mite species and
compared the data to 294 COI mtDNA sequences derived from the respective hosts Rhabdomys dilectus, R. bechuanae,
Mastomys coucha and M. natalensis. In support of the SGVH, the generalist L. muricola was characterized by a high
mtDNA haplotypic diversity of 0.97 (±0.00) and a low level of population differentiation (mtDNA Fst= 0.56, p < 0.05;
nuDNA Fst = 0.33, P < 0.05) while the specialist L. giganteus was overall characterized by a lower haplotypic diversity of
0.77 (±0.03) and comparatively higher levels of population differentiation (mtDNA Fst = 0.87, P < 0.05; nuDNA Fst = 0.48,
P < 0.05). When the two specialist L. giganteus lineages, which occur on two different Rhabdomys species, are
respectively compared to the generalist parasite, L. muricola, the SGVH is not fully supported. One of the specialist L.
giganteus species occurring on R. dilectus shows similar low levels of population differentiation (mtDNA Fst= 0.53, P < 0.
05; nuDNA Fst= 0.12, P < 0.05) than that found for the generalist L. muricola. This finding can be correlated to
differences in host dispersal: R. bechuanae populations are characterized by a differentiated mtDNA Fst of 0.79 (P < 0.05)
while R. dilectus populations are less structured with a mtDNA Fst= 0.18 (P < 0.05).
CONCLUSION: These findings suggest that in ectoparasites, host specificity and the vagility of the host are both
important drivers for parasite dispersal. It is proposed that the SGHV hypothesis should also incorporate reference to
host dispersal since in our case only the specialist species who occur on less mobile hosts showed more subdivided
population structure when compared to generalist species
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