Human bipedal instability in tree canopy environments is reduced by "light touch" fingertip support.

Whether tree canopy habitats played a sustained role in the ecology of ancestral bipedal hominins is unresolved. Some argue that arboreal bipedalism was prohibitively risky for hominins whose increasingly modern anatomy prevented them from gripping branches with their feet. Balancing on two legs is indeed challenging for humans under optimal conditions let alone in forest canopy, which is physically and visually highly dynamic. Here we quantify the impact of forest canopy characteristics on postural stability in humans. Viewing a movie of swaying branches while standing on a branch-like bouncy springboard destabilised the participants as much as wearing a blindfold. However "light touch", a sensorimotor strategy based on light fingertip support, significantly enhanced their balance and lowered their thigh muscle activity by up to 30%. This demonstrates how a light touch strategy could have been central to our ancestor's ability to avoid falls and reduce the mechanical and metabolic cost of arboreal feeding and movement. Our results may also indicate that some adaptations in the hand that facilitated continued access to forest canopy may have complemented, rather than opposed, adaptations that facilitated precise manipulation and tool use.

Sonation in the male common snipe (Capella gallinago gallinago L.) is achieved by a flag-like fluttering of their tail feathers and consequent vortex shedding.

Male common snipe (Capella gallinago gallinago) produce a 'drumming' sound with their outer tail feathers during their mating dives, but little is known about how this is achieved. We investigated the movements and sound producing capabilities of the outer tail feathers. Using a wind tunnel, we compared observations of the frequencies of sound produced with the predictions from aerodynamic theory. The feathers were also filmed in an air-flow with a high speed video camera, and subjected to morphological examination and biomechanical testing. We propose a mechanistic hypothesis of how the modified outer feathers of the male common snipe generate sound, and the adaptations that facilitate this. Video and audio analysis of the feather demonstrated that a fluttering of the trailing vane generated the sound. The flutter of the vane is facilitated by the rearward curvature of the feather shaft, reduced branching angles of the barbs in the trailing vane and the lack of hooks on the barbs along a hinge region, all of which increase its flexural compliance. Sound production occurred at the same frequency as the vane movements, at frequencies consistent with it being produced by a fluttering flag mechanism powered by vortex shedding.

Analysing lodging of the panicle bearing cereal teff (Eragrostis tef).

Lodging, the permanent displacement of crop plants from their vertical because of root or shoot failure, is a major yield constraint of the gluten free, panicle bearing cereal teff. The objective of this paper was to analyse the causes of lodging of teff by using, modifying and validating conventional biomechanical models. The model parameters were obtained from a field trial with two contrasting teff cultivars, using novel in situ and laboratory measurements under wet and dry conditions. Cross-species model validation was done with rice (Oryza sativa). Teff is more susceptible to root lodging than to shoot lodging, although the data indicated that shoot strength is also insufficient. Hence, simultaneously breeding for both improved root anchorage and shoot strength is advocated. The study showed that the lodging model, derived for the spike-bearing cereal wheat, needed modifications in order to be able to deal with panicle-bearing plants such as teff and rice. Water adhering to plants owing to rain or dew increased calculated lodging susceptibility. To prevent underestimation of lodging susceptibility, future lodging research should be done under completely wet conditions (water saturated soil and wetted shoots).

Transverse stresses and modes of failure in tree branches and other beams.

The longitudinal stresses in beams subjected to bending also set up transverse stresses within them; they compress the cross section when the beam's curvature is being increased and stretch it when its curvature is being reduced. Analysis shows that transverse stresses rise to a maximum at the neutral axis and increase with both the bending moment applied and the curvature of the beam. These stresses can qualitatively explain the fracture behaviour of tree branches. Curved 'hazard beams' that are being straightened split down the middle because of the low transverse tensile strength of wood. By contrast, straight branches of light wood buckle when they are bent because of its low transverse compressive strength. Branches of denser wood break, but the low transverse tensile strength diverts the crack longitudinally when the fracture has only run half-way across the beam, to produce their characteristic 'greenstick fracture'. The bones of young mammals and uniaxially reinforced composite beams may also be prone to greenstick fracture because of their lower transverse tensile strength.

Tensile and shear properties of fingernails as a function of a changing humidity environment.

The mechanical properties of fingernails are important because of their impact in preventing damage and in maintaining their appearance. In particular, knowing the effect of local environmental conditions can tell us how they might best be protected. In order to better understand this, tensile tests were carried out to characterise the properties of fingernails at different relative humidities. Cyclic tests were also conducted to investigate the ability of the structure to recover deformation at different moisture contents. Torsional tests were performed to determine the shear modulus of the keratinous matrix material which binds together the fibrous components of the fingernails. This enabled an analysis of how the material may resist bending, torsion and permanent deformation in a natural environment. In particular, it is shown that at low relative humidity the nails are more brittle, and at high moisture contents they are more flexible. Increasing relative humidity lowers torsional stiffness much more than tensile stiffness, suggesting that moisture plasticises the matrix rather than affecting the fibres. The twist to bend ratio is minimised at 55% RH, close to the natural condition of nails which should minimise susceptibility to torsional damage due to plasticisation and a disruption of the matrix material binding the keratin fibres.

A novel mechanism by which silica defends grasses against herbivory.

BACKGROUND AND AIMS: Previous studies have shown that silica in grass leaves defends them against small herbivores, which avoid high-silica grasses and digest them less efficiently. This study tested the idea that silica can reduce digestibility by preventing the mechanical breakdown of chlorenchyma cells. METHODS: Both the percentage of total chlorophyll liberated from high- and low-silica grass leaves by mechanical grinding and the chlorophyll content of locust faeces were measured. KEY RESULTS: High-silica grasses released less chlorophyll after grinding and retained more after passing through the gut of locusts, showing that silica levels correlated with increased mechanical protection. CONCLUSIONS: These results suggest that silica may defend grasses at least in part by reducing mechanical breakdown of the leaf, and that mechanical protection of resources in chlorenchyma cells is a novel and potentially important mechanism by which silica protects grasses.

Structural development and stability of rice Oryza sativa L. var. Nerica 1.

The structural development of glasshouse-grown rice Oryza sativa L. var. Nerica 1 was studied in relation to its stability against lodging. The morphology and mechanical properties of both the stem and roots were examined from tillering, 4 weeks after transplantation up to maturity, together with plant weight distribution and anchorage strength. The "factors of safety" against root and stem failure were subsequently calculated throughout development. Rice plants showed similar morphology to wheat, although they possessed around twice as many tillers per plant and 10 times as many coronal roots. The mechanics of anchorage were also similar. The strength and rigidity of individual tillers increased throughout development as the plants grew taller and heavier and were around 15 times greater than in wheat. By contrast, individual root bending strength, the number of roots, and the anchorage strength levelled off earlier, and anchorage strength was only around twice that in wheat. Consequently, while the self-weight safety factor against stem failure was much higher than in wheat, increasing until late on in development from around 30 to 150, the self-weight safety factor against root anchorage failure was similar to wheat, decreasing from around 15 to 5. Consequently, plants subjected to anchorage tests always failed in their root system rather than their shoot system. The results suggest that, in the field, rice plants would be more likely to undergo root lodging than stem lodging, and that breeding efforts to reduce the incidence of lodging should act to strengthen the rather weak coronal roots.

Flight behaviour during foraging of the social wasp Vespula vulgaris (Hymenoptera: Vespidae) and four mimetic hoverflies (Diptera: Syrphidae) Sericomyia silentis, Myathropa florea, Helophilus sp. and Syrphus sp.

Many hoverfly species show specific or non specific morphological resemblance to wasps (Vespula sp.) and it has been suggested that they also show similar flight behaviour. In this study we therefore compared the flight behaviour of wasps with that of four mimetic hoverflies, Sericomyia silentis, Myathropa florea, Helophilus sp. and Syrphus sp., by filming insects while they were foraging on an artificial array of flowers. Films were analysed to determine the routes taken, time spent hovering and flight speed. Of the four flies, only the non specific mimic, Syrphus, showed similar flight behaviour to the wasps; it flew more slowly, and with more roundabout routes than the other flies, hesitating before landing. These results suggest that in hoverflies there is little reason to expect strict correlation between morphological and behavioural mimicry; insects may acquire the similarities to their model more-or-less independently.

Skeletal bone morphology is resistant to the high amplitude seasonal leptin cycle in the Siberian hamster.

Recent studies have suggested that the adipocyte-derived hormone, leptin, plays a role in the regulation of metabolism. Here, we tested this hypothesis in the seasonally breeding Siberian hamster, as this species exhibits profound seasonal changes in adiposity and circulating leptin concentrations driven by the annual photoperiodic cycle. Male hamsters were kept in either long (LD) or short (SD) photoperiods. Following exposure to short photoperiods for 8 weeks animals exhibited a significant weight-loss and a 16-fold reduction of serum leptin concentrations. At Week 9, animals in both photoperiods were infused with leptin or PBS via osmotic mini-pump for 14 days. Chronic leptin infusion mimicked LD-like concentrations in SD-housed animals and caused a further decline in body weight and adipose tissue. In LD-housed animals, leptin infusion resulted in a significant elevation of serum concentrations above natural LD-like levels, but had no discernable effect on body weight or overall adiposity. Both bending and compression characteristics and histomorphometric measurements of trabecular bone mass were unaltered by leptin treatment or photoperiod. Our data therefore show that despite a high natural amplitude cycle of leptin, this hormone has no apparent role in the regulation of bone metabolism, and therefore do not support recent propositions that this hormone is an important component in the metabolism of bone tissue.

The fracture properties and mechanical design of human fingernails.

Fingernails are a characteristic feature of primates, and are composed of three layers of the fibrous composite keratin. This study examined the structure and fracture properties of human fingernails to determine how they resist bending forces while preventing fractures running longitudinally into the nail bed. Nail clippings were first torn manually to examine the preferred crack direction. Next, scissor cutting tests were carried out to compare the fracture toughness of central and outer areas in both the transverse and longitudinal direction. The fracture toughness of each of the three isolated layers was also measured in this way to determine their relative contributions to the toughness. Finally, the structure was examined by carrying out scanning electron microscopy of free fracture surfaces and polarized light microscopy of nail sections. When nails were torn, cracks were always diverted transversely, parallel to the free edge of the nail. Cutting tests showed that this occurred because the energy to cut nails transversely, at approximately 3 kJ m(-2), was about half that needed (approx. 6 kJ m(-2)) to cut them longitudinally. This anisotropy was imparted by the thick intermediate layer, which comprises long, narrow cells that are oriented transversely; the energy needed to cut this layer transversely was only a quarter of that needed to cut it longitudinally. In contrast the tile-like cells in the thinner dorsal and ventral layers showed isotropic behaviour. They probably act to increase the nail's bending strength, and as they wrap around the edge of the nail, they also help prevent cracks from forming. These results cast light on the mechanical behaviour and care of fingernails.

Muscle activity during gait initiation in normal elderly people.

The purpose of this study was to describe the patterns of phasic muscle during gait initiation in normal elderly people. Bilateral surface EMG recordings were made of tibialis anterior, medial gastrocnemius and gluteus medius activity throughout gait initiation in 21 subjects. A variable expression of the onset muscle pattern is shown, with a tendency for muscle activity to be more variable in the preparatory phase. These results provide a baseline of normal gait initiation muscle activity against which to compare that of patients with gait initiation and balance difficulties.

Fungi are the predominant micro-organisms responsible for degradation of soil-buried polyester polyurethane over a range of soil water holding capacities.

AIMS: To investigate the relationship between soil water holding capacity (WHC) and biodegradation of polyester polyurethane (PU) and to quantify and identify the predominant degrading micro-organisms in the biofilms on plastic buried in soil. METHODS AND RESULTS: High numbers of both fungi and bacteria were recovered from biofilms on soil-buried dumb-bell-shaped pieces of polyester PU after 44 days at 15-100% WHC. The tensile strength of the polyester PU was reduced by up to 60% over 20-80% soil WHC, but no reduction occurred at 15, 90 or 100% soil WHC. A PU agar clearance assay indicated that fungi, but not bacteria were, the major degrading organisms in the biofilms on polyester PU and 10-30% of all the isolated fungi were able to degrade polyester PU in this assay. A 5.8S rDNA sequencing identified 13 strains of fungi representing the three major colony morphology types responsible for PU degradation. Sequence homology matches identified these strains as Nectria gliocladioides (five strains), Penicillium ochrochloron (one strain) and Geomyces pannorum (seven strains). Geomyces pannorum was the predominant organism in the biofilms comprising 22-100% of the viable polyester PU degrading fungi. CONCLUSIONS: Polyester PU degradation was optimum under a wide range of soil WHC and the predominant degrading organisms were fungi. SIGNIFICANCE AND IMPACT OF THE STUDY: By identifying the predominant degrading fungi in soil and studying the optimum WHC conditions for degradation of PU it allows us to better understand how plastics are broken down in the environment such as in landfill sites.

The effects of air flow and stem flexure on the mechanical and hydraulic properties of the stems of sunflowers Helianthus annuus L.

Many studies have shown that wind affects plant development, causing them to develop shorter and usually stronger stems. Many of these effects have been shown to be due to a response to mechanical flexing of the stem which is known as thigmomorphogenesis. However, it is not known how wind affects the hydraulic properties of stems, nor have the effects of air flow past leaves been examined in isolation from mechanical flexing. This study, therefore, used a factorial experiment to distinguish between the effects of stem flexing and air flow, and examined the morphology, hydraulics and mechanics of developing sunflowers Helianthus annuus. It was found that flexure and air flow had opposite effects on several aspects of development; air flow increased plant height and length-specific stem hydraulic conductivity, k(h), and reduced stem rigidity and strength, while flexing did the reverse. There was also a clear trade-off between hydraulic and mechanical capability: as one increased the other decreased. A plant's response to wind must, therefore, be a complex response to at least two different stimuli and this might help explain why it varies with species and environment.

Modelling the hydrodynamic resistance of bordered pits.

Previous studies of the hydrodynamics of plant stems have shown that resistance to flow through bordered pits on the side walls of tracheids makes up a significant proportion of their total resistance, and that this proportion increases with tracheid diameter. This suggests a possible reason why tracheids with a diameter above around 100 microm have failed to evolve. This possibility has been investigated by obtaining an estimate for the resistance of a single pit, and incorporating it into analytical models of tracheid resistance and wood resistivity. The hydrodynamic resistance of the bordered pits of Tsuga canadensis was investigated using large-scale physical models. The importance of individual components of the pit were investigated by comparing the resistance of models with different pore sizes in their pit membrane, and with or without the torus and border. The estimate for the resistance of a real bordered pit was 1.70x10(15) Pa s m(-3). Resistance of pits varied with morphology as might be predicted; the resistance was inversely proportional to the pore size to the power of 0.715; removing the torus reduced resistance by 28%, while removal of the torus and border together reduced it by 72%. It was estimated that in a 'typical tracheid' pit resistance should account for 29% of the total. Incorporating the results into the model for the resistivity of wood showed that resistivity should fall as tracheid diameter increases. However, to minimize resistance wider tracheids would also need to be proportionally much longer. It is suggested that the diameter of tracheids in conifers is limited by upper limits to cell length or cell volume. This limitation is avoided by angiosperms because they can digest away the ends of their cells to produce long, wide vessels composed of many short cells.

Cloning and characterization of irregular xylem4 (irx4): a severely lignin-deficient mutant of Arabidopsis.

A severe lignin mutant, irx4, has been identified in Arabidopsis thaliana as a result of its collapsed xylem phenotype. In contrast to previously described irx mutants, irx4 plants have 50% less lignin than wild-type plants, whilst the cellulose and hemicellulose content remained unchanged. These alterations in the composition of irx4 secondary cell walls had a dramatic effect on the morphology and architecture of the walls, which expand to fill most of the cell, and also on the physical properties of irx4 stems. Further analysis indicated that the irx4 mutation occurred in a cinnamoyl-CoA reductase (CCR) gene within a highly conserved intron splice site sequence of intron 2. As a result, CCR mRNA transcripts were incorrectly spliced. Transgenic plants expressing an IRX3 promoter-CCR cDNA construct were used to generate a series of plants with varying degrees of lignin content in order to assess the role of lignin content in determining the physical properties of Arabidopsis stems.

Similarity in flight behaviour between the honeybee Apis mellifera (Hymenoptera: apidae) and its presumed mimic, the dronefly Eristalis tenax (Diptera: syrphidae).

It is generally accepted that the dronefly Eristalis tenax is a Batesian mimic of the honeybee Apis mellifera. Previous work has established that the foraging behaviour of droneflies is more similar to that of its model than to that of other more closely related flies, suggesting that behaviour may be important in the mimicry. Locomotor mimicry has been demonstrated in mimetic Heliconius butterflies but not in hoverflies. This study therefore investigated aspects of the flight behaviour of Eristalis tenax, Apis mellifera and two other flies, Syrphus ribesii and a Musca sp. Insects were filmed foraging on Helichrysum bracteum flowers, and flight sequences were analysed to determine flight velocities, flight trajectories and the percentage of time spent hovering. It was found that the flight behaviour of droneflies was more similar to that of honeybees than to that of the other flies. This suggests that the flight behaviour of Eristalis tenax may be mimetic.

Validity and reliability of a kinematic protocol for determining foot contact events.

Timing of foot contact events provides important information for gait studies. The aim of the study is to validate the use of kinematic data, collected at 50 Hz to define foot contact events during gait initiation. Simultaneous kinetic and kinematic data recordings of four discrete foot contact events were made for normal adults. Raters were asked to estimate the timing of the events from kinematic data curves and these timings were compared with those derived from the kinetic data. For the four events, between 88 and 98% of all ratings were accurate to within 0.03 s. Inter-rater reliability was extremely high, reflecting the precision of the definitions used.

The functional morphology of the petioles of the banana, Musa textilis.

Bananas are among the largest herbs in the world and their lightweight petioles hold up huge leaves. This study examined how the petioles manage to achieve adequate rigidity to do this, while allowing extensive and reversible reconfiguration in high winds. Morphological and anatomical examination of the petioles and leaves of Musa textilis suggested how these two apparently incompatible abilities are achieved. The hollow U-shaped section of the petiole and the longitudinal strengthening elements in its outer skin give it adequate rigidity, while its ventral curvature help support the leaf without the need for thick lateral veins. These features, however, also allow the petiole to reconfigure by twisting away from the wind, while the leaf can fold away. In addition, two sets of internal structures, longitudinal partitions and transverse stellate parenchyma plates, help prevent dorsoventral flattening, allowing the petiole to flex further away from the wind without buckling. These ideas were tested and verified by a range of mechanical tests. Simple four-point-bending and torsion tests showed that the petioles are indeed far more compliant in torsion than in bending. Axial bending tests and crushing tests showed that petioles could be flexed twice as far and were four times as resistant to dorsoventral flattening when intact than when the internal tissue is removed. The banana petiole, therefore, seems to be an excellent example of natural integrated mechanical design.

The aerodynamics and hydrodynamics of plants.

Because they grow away from their substratum to compete for light, plants have to withstand hydrodynamic or aerodynamic drag. Both water and land plants reconfigure in response to this drag, and this is presumed to reduce the risk of mechanical failure. However, there is little unequivocal evidence of drag reduction in large trees as a result of reconfiguration. Land plants must also transport water internally to their tissues, and many have developed xylem tracheids and vessels that help speed up this process. Recent evidence that tree height is limited by water supply suggests that water transport efficiency must be a crucial element in tree design. However, the resistance of many parts of the xylem is still unknown. More focused work is urgently required to shed light on the evolution and ecology of plants in relation to the flow of fluids.

Wind as an ecological factor.

Wind has long been regarded as an important ecological factor in forests owing to the dramatic damage hurricanes can wreak. However, the long-term wind regime of a site also exerts a strong influence on the growth of trees. A relatively large amount is known about the acclimation of trees to wind but less about intra- or interspecific adaption to high winds. In fact, changes resulting from the effect of wind may have a greater effect on the ecology of forests than the more acute effects of destructive stroms. Improved understanding of the mechanical effects of wind is helping foresters manage their plantations and may help us to account better for local and geographical variations in forest ecology.