Horses damp the spring in their step.

The muscular work of galloping in horses is halved by storing and returning elastic strain energy in spring-like muscle-tendon units.These make the legs act like a child's pogo stick that is tuned to stretch and recoil at 2.5 strides per second. This mechanism is optimized by unique musculoskeletal adaptations: the digital flexor muscles have extremely short fibres and significant passive properties, whereas the tendons are very long and span several joints. Length change occurs by a stretching of the spring-like digital flexor tendons rather than through energetically expensive length changes in the muscle. Despite being apparently redundant for such a mechanism, the muscle fibres in the digital flexors are well developed. Here we show that the mechanical arrangement of the elastic leg permits it to vibrate at a higher frequency of 30-40 Hz that could cause fatigue damage to tendon and bone. Furthermore, we show that the digital flexor muscles have minimal ability to contribute to or regulate significantly the 2.5-Hz cycle of movement, but are ideally arranged to damp these high-frequency oscillations in the limb.     

Magma storage beneath Axial volcano on the Juan de Fuca mid-ocean ridge.

Axial volcano, which is located near the intersection of the Juan de Fuca ridge and the Cobb-Eickelberg seamount chain beneath the northeast Pacific Ocean, is a locus of volcanic activity thought to be associated with the Cobb hotspot. The volcano rises 700 metres above the ridge, has substantial rift zones extending about 50 kilometres to the north and south, and has erupted as recently as 1998 (ref. 2). Here we present seismological data that constrain the three-dimensional velocity structure beneath the volcano. We image a large low-velocity zone in the crust, consisting of a shallow magma chamber and a more diffuse reservoir in the lower crust, and estimate the total magma volume in the system to be between 5 and 21 km3. This volume is two orders of magnitude larger than the amount of melt emplaced during the most recent eruption (0.1-0.2 km3). We therefore infer that such volcanic events remove only a small portion of the reservoir that they tap, which must accordingly be long-lived compared to the eruption cycle. On the basis of magma flux estimates, we estimate the crustal residence time of melt in the volcanic system to be a few hundred to a few thousand years.     

Chemistry of ion coordination and hydration revealed by a K+ channel-Fab complex at 2.0 A resolution.

Ion transport proteins must remove an ion's hydration shell to coordinate the ion selectively on the basis of its size and charge. To discover how the K+ channel solves this fundamental aspect of ion conduction, we solved the structure of the KcsA K+ channel in complex with a monoclonal Fab antibody fragment at 2.0 A resolution. Here we show how the K+ channel displaces water molecules around an ion at its extracellular entryway, and how it holds a K+ ion in a square antiprism of water molecules in a cavity near its intracellular entryway. Carbonyl oxygen atoms within the selectivity filter form a very similar square antiprism around each K+ binding site, as if to mimic the waters of hydration. The selectivity filter changes its ion coordination structure in low K+ solutions. This structural change is crucial to the operation of the selectivity filter in the cellular context, where the K+ ion concentration near the selectivity filter varies in response to channel gating.     

Host-induced morphological variations in the strigeoid trematode Posthodiplostomum minimum (Trematoda: Diplostomatidae). II. Body measurements and tegument modifications

Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;} Extensive feeding experiments were undertaken to determine if physiological host specificity was a major characteristic of Posthodiplostomum minimum. This involved the feeding of experimentally infected sunfish livers containing metacercariae of P. minimum to amphibian, reptilian, avian and mammalian hosts. Host - induced morphological variations of adult P. minimum were shown to be associated with the genus and class of host employed as well as with the inherent variability of P. minimum exclusive of host factors. Morphological characters such as body size and shape and tegumental surface and spines are indicated as undergoing morphological variation. Of all the experimental definitive hosts used, avian and mammalian are the most suitable for normal development of P. minimum. Amphibian and reptilian hosts demonstrate marked variation in the adult worm development usually manifested by pronounced size decrease.     

Desert rodents reduce seedling recruitment of Salsola paulsenii

Heteromyid rodents in the deserts of North America have been shown to harvest large quantities of seeds of both native and introduced plants from soil seed banks, but rarely has the impact of this seed removal been demonstrated experimentally. I used a series of fenced plots (some of which excluded rodents) to demonstrate that heteromyids at a western Nevada study site can measurably reduce seed banks and subsequent seedling establishment of Salsola paulsenii , an introduced invasive weed that has become a significant problem over much of the desert Southwest. The frequency of S. paulsenii seedlings in both 2004 and 2005 was significantly greater around the interior perimeters of plots that permitted access by rodents than in plots that excluded rodents. Density of S. paulsenii seedlings was significantly greater inside than outside rodent exclusion plots, but there was no such difference in seedling density inside versus outside plots that permitted rodent access. Salsola paulsenii has such a conspicuous presence in many desert environments that the effect of rodents in reducing its abundance may not be visually apparent; however, rodents may still ameliorate competitive effects of this weed on coexisting plants. Heteromyid rodents disperse seeds through caching, and they also consume them. Caching may enhance establishment of native plant seedlings, but is unlikely to benefit exotics such as S. paulsenii .     

Redox modulated hydrogelation of a self-assembling short peptide amphiphile

Hydrogels resulting from the self-assembly of small peptides are smart nanobiomaterials as their nanostructuring can be readily tuned by environmental stimuli such as pH,ionic strength and temperature,thereby favoring their practical applications.This work reports experimental observations of formation of peptide hydrogels in response to the redox environment.Ac-I 3 K-NH 2 is a short peptide amphiphile that readily self-assembles into long nanofibers and its gel formation occurs at concentrations of about 10 mmol/L.Introduction of a Cys residue into the hydrophilic region leads to a new molecule,Ac-I 3 CGK-NH 2,that enables the formation of disulfide bonds between self-assembled nanofibers,thus favoring cross-linking and promoting hydrogel formation.Under oxidative environment,Ac-I 3 CGK-NH 2 formed hydrogels at much lower concentrations(even at 0.5 mmol/L).Furthermore,the strength of the hydrogels could be easily tuned by switching between oxidative and reductive conditions and time.However,AFM,TEM,and CD measurements revealed little morphological and structural changes at molecular and nano dimensions,showing no apparent influence arising from the disulfide bond formation.     

Vegetation characteristics of mountainous northeastern Nevada sagebrush community types

Vegetation characteristics of 15 sagebrush community types identified on the Humboldt National Forest, northeastern Nevada, are described. A total of 218 plant species were found over the 372 relatively undisturbed rangeland communities sampled. The dominant plant families encountered were the Asteraceae (45 taxa), Poaceae (32 taxa), Scrophulariaceae (14 taxa), and the Fabaceae (12 taxa). Average annual dry weight production of the community types ranged from about 400 kg/ha/yr on types with Artemisia nova as the dominant sagebrush species to 1,200 kg/ha/yr on some A. tridentata ssp. vaseyana community types. A general increase in species richness and vegetation plus litter ground cover was observed within community types as the dominant sagebrush species changed from A. nova to A. arbuscula to A. longiloba to A. tridentata spp. tridentata to A. tridentata ssp. vaseyana . Major differences in plant species production and constancy exist between the sagebrush community types studied.     

Effects of elevation on litter-size variation among lizard populations in the Sceloporus grammicus complex (Phrynosomatidae) in Mexico

We examined the effect of elevation on litter-size variation in viviparous lizards of the Sceloporus grammicus complex in 10 states of Mexico. Female snout–vent length (SVL) decreased with increasing elevation, and absolute litter size based on embryos also tended to de crease with increasing elevation. However, after controlling for variation in female body size, we found that litter sizes tended to be relatively larger at higher elevation. Elevation therefore appears to influence litter size in these lizards; however, relatively little of the variation is explained by elevation; thus, other factors are likely making substantial contributions to the observed litter-size variation. The S. grammicus complex appears to be a good model system for examining the underlying causes of geographic and elevational variation in lizard life histories. Examinamos el efecto del altitud en la variación del tamaño de camada de las lagartijas vivíparas del complejo Sceloporus grammicus en 10 estados de México. La LHC de las hembras disminuyó con la altitud, y el tamaño absoluto de camada, calculado con base en el número de embriones, también tendió a disminuir. No obstante, después de controlar la variación en el tamaño corporal de las hembras, encontramos que los tamaños de camada tendieron a ser relativamente más grandes en altitudes mayores. La altitud, por tanto, parece influir en el tamaño de camada de estas lagartijas; sin embargo, la altitud explica relativamente poco de la variación, por lo que, es probable que otros factores contribuyan substancialmente a la variación observada en el tamaño de camada. El complejo S. grammicus parece ser un buen sistema para estudiar las causas fundamentales de la variación geográfica y altitudinal en la historia de vida de las lagartijas.     

Early development of the razorback sucker, Xyrauchen texanus (Abbott)

  Fertilized ova of razorback sucker, Xyrauchen texanus, were adhesive for 3 to 4 hours after fertilization. Cleavage was completed at 24 hours, gastrulation occurred at 34 hours, and blood circulation was established at 117 hours. Hatching occurred from 5.2 to 5.5 days after fertilization. Larvae were from 6.8 to 7.3 mm TL at hatching. Yolk was assimilated at 13 days (10 mm TL). All fins were formed and had ossified rays at 64 days (27 mm TL). The unique nuchal keel appeared about 200 days after fertilization.