Un exemple de l’influence du milieu sur les émissions vocales des oiseaux : l’évolution des chants en forêt équatoriale

Une adaptation des chants d’oiseaux au milieu particulier du sous-bois de la forêt équatoriale est suggérée par la simple compa raison globale de la majorité des espèces entendues dans ce type d’habitat avec celles vivant dans des milieux plus ouverts tels que la savane. Pour mieux préciser les caractères de cette adaptation, nous avons choisi une centaine d’espèces, appartenant sensiblement par moitié à chacun de ces deux milieux, au niveau de chaque famille. L’analyse spectrale des enregistrements effectués a fourni des données quantitatives sur les deux milieux, et a permis de montrer que la tonalité des chants était plus grave en milieu dense qu’en milieu ouvert. Le phénomène est surtout marqué pour les petites espèces qui ont les chants les plus aigus (abaissement de 1 000 Hz de la fréquence moyenne en milieu dense par rapport au milieu ouvert) et les espèces de taille moyenne qui utilisent le registre moyen (même différence de 1000 Hz). Par contre les familles à chant grave ont moins évolué, (300 Hz de différence entre les deux milieux). Il semble donc que ce sont les fréquences supérieures à 1 500 Hz qui ont été le plus modifiées, leur transmission étant sans doute altérée en milieu dense. Effectivement, une vérification expérimentale sur la pro pagation des sons, effectuée dans les strates inférieures de la forêt primaire du Gabon, a bien montré que c’est au-dessus de la fréquence de 1 500 Hz que les sons étaient rapidement absorbés par les écrans naturels. Ce fait nous semble suffisamment remarquable pour être signalé. La longueur d’onde de cette « fréquence charnière » de 1 500 Hz correspond sensiblement aux dimensions des plus grandes feuilles de ce milieu dense, et l’on peut donc penser que ce sont les réflexions multiples sur ces feuilles par ailleurs épaisses qui sont la principale cause d’une telle absorption. Parallèlement nous constatons que d’une façon générale les spectres acoustiques sont larges en milieu ouvert, étroits en milieu fermé. Il semble donc que l’oiseau ait, dans les limites de son registre disponible, concentré son énergie sur la note fondamen tale, donc la plus grave, et augmenté ainsi la portée de son chant.An adaptation of bird songs to the undergrowth of the tro pical rain-forest is suggested by the overall comparison of songs in this « closed » habitat with those of the more « open » envi ronment of the savanna. To analyse this adaptation, 78 species (39 for each habitat) were selected and matched for family. Moreover, 21 species from the semi-open habitats of the upper layer of the forest and the tree savanna were also studied (Table I). A spectral analysis of recordings showed that, as a rule, the denser the habitat, the lower the pitch. This phenomenon is particularly marked with the small, shrill-noted species (the « ave rage frequency » is 1000 cycles lower in the forest undergrowth than in the savanna) and in the medium sized species which use middle frequencies (again the average frequency is 1000 cycles lower in dense habitat). On the other hand, differences between families with lowpitched songs are less marked (300 cycles between the two habitats). Thus it would appear that a strong selection pressure has been exerted against frequencies higher than 1500 cycles in dense habitat, their transmission being greatly hindered there. Field experiments on the propagation of sound in the lower stratum of rain forest in Gabon have shown that this is indeed the case : frequencies above 1500 cycles are rapidly absorbed by natural screens. The wavelength of this « critical frequency » of 1 500 cycles corresponds pretty well with the size of the largest leaves in the forest undergrowth (which also have a thick cuticule). On can summise that the multiple reflections from this leaf bar rier are the main cause of such high absorption. Concomitantly with the change in pitch, the harmonic spectra are broad in the open habitat and narrow in the closed one. There, many of the notes are simple ones, without harmonics or with a few weak ones. Therefore it seems that, within the limits of the available sound registers of the birds, the energy has been concentrated in the fundamental frequency, the lowest, thus increasing the carrying power of the songs

The Distance to the Soft Gamma Repeater SGR 1627-41

We report millimeter observations of the line of sight to the recently discovered Soft Gamma Repeater, SGR 1627-41, which has been tentatively associated with the supernova remnant SNR G337.0-0.1 Among the eight molecular clouds along the line of sight to SGR 1627-41, we show that SNR G337.0-0.1 is probably interacting with one of the most massive giant molecular clouds (GMC) in the Galaxy, at a distance of 11 kpc from the sun. Based on the high extinction to the persistent X-ray counterpart of SGR 1627-41, we present evidence for an association of this new SGR with the SNR G337.0-0.1; they both appear to be located on the near side of the GMC. This is the second SGR located near an extraordinarily massive GMC. We suggest that SGR 1627-41 is a neutron star with a high transverse velocity (~ 1,000 \kms) escaping the young (~ 5,000 years) supernova remnant G337.0-0.1Comment: 17 pages, including 2 figures. Accepted for publication in the Astrophysical Journal Letter

Produção e avaliação da qualidade do leite na microrregião de Castanhal.

bitstream/item/63529/1/Oriental-Doc95.PD

Cimetidine, C10H16N6S, formC: crystal structure and modelling of polytypes using suoperspace approach

An efficient method for modelling a polytypic family is presented with the example of cimetidine in the form C polymorph. The method exploits the (3 + 1)-dimensional superspace model, which is a powerful tool for the description, prediction and understanding of polytype modifications in small-molecule crystallography, as illustrated with this pharmaceutical example

Lifetimes of Confined Acoustic Phonons in Ultra-Thin Silicon Membranes

We study the relaxation of coherent acoustic phonon modes with frequencies up to 500 GHz in ultra-thin free-standing silicon membranes. Using an ultrafast pump-probe technique of asynchronous optical sampling, we observe that the decay time of the first-order dilatational mode decreases significantly from \sim 4.7 ns to 5 ps with decreasing membrane thickness from \sim 194 to 8 nm. The experimental results are compared with theories considering both intrinsic phonon-phonon interactions and extrinsic surface roughness scattering including a wavelength-dependent specularity. Our results provide insight to understand some of the limits of nanomechanical resonators and thermal transport in nanostructures

Idiopathic club foot treated with the Ponseti method. Clinical and sonographic evaluation of Achilles tendon tenotomy. A review of 221 club feet

SummaryThe Ponseti method applied to treating idiopathic club foot consists in placing successive corrective casts, possibly an Achilles tendon tenotomy, then derotation braces, a method that has proven its efficacy. This study compared 221 cases of club foot treated with this method between 2002 and 2007 divided into two groups, based on whether or not Achilles tendon tenotomy was performed. Assessment was both clinical and sonographic. We observed clear improvement of the results in the group that underwent Achilles tendon tenotomy and a significant difference in the rate of secondary surgery. The sonographic evaluation also showed improvement of the morphological results. We now systematically propose Achilles tendon tenotomy however severe club foot may be