Elastic strain energy storage in the feet of running monkeys

Monkeys are 'flat–footed' in comparison to humans, but they are still able to utilize elastic strain energy stores in their feet to reduce the metabolic energy cost of running. During contact with the ground, bending moments act on the foot to produce a 'reversed arch', storing strain energy which is returned in the subsequent elastic recoil. This energy–saving mechanism has been investigated in dynamic bending tests and is discussed in relation to the arched foot of humans.     

A new longirostrine dyrosaurid (Crocodylomorpha,Mesoeucrocodylia) from the Paleocene of north‐eastern Colombia: biogeographic and behavioural implications for New‐World Dyrosauridae

Abstract: Fossils of dyrosaurid crocodyliforms are limited in South America, with only three previously diagnosed taxa including the short‐snouted Cerrejonisuchus improcerus from the Paleocene Cerrejón Formation of north‐eastern Colombia. Here we describe a second dyrosaurid from the Cerrejón Formation, Acherontisuchus guajiraensis gen. et sp. nov., based on three partial mandibles, maxillary fragments, teeth, and referred postcrania. The mandible has a reduced seventh alveolus and laterally depressed retroarticular process, both diagnostic characteristics of Dyrosauridae. Acherontisuchus guajiraensis is distinct among known dyrosaurids in having a unique combination of craniomandibular characteristics, and postcranial morphology that suggests it may have occupied a more placid, fluvial habitat than most known Old‐World dyrosaurids. Results from a cladistic analysis of Dyrosauridae, using 82 primarily cranial and mandibular characters, support an unresolved relationship between A. guajiraensis and a combination of New‐ and Old‐World dyrosaurids including Hyposaurus rogersii, Congosaurus bequaerti, Atlantosuchus coupatezi, Guarinisuchus munizi, Rhabdognathus keiniensis and Rhabdognathus aslerensis. Our results are consistent with an African origin for Dyrosauridae with multiple dispersals into the New World during the Late Cretaceous and a transition from marine habitats in ancestral taxa to more fluvial habitats in more derived taxa.     

Colonization and ecological development of new streams in Glacier Bay National Park, Alaska

SUMMARY 1. Colonization and ecological development of postglacial freshwater communities was investigated in Glacier Bay National Park, south-eastern Alaska, following the rapid recession of a Neo-glacial ice sheet within the last 250 years.
2. Environmental variables shown to be most significant in stream development were temperature, flow regime and sedimentation.
3. The Chironomidae (Diptera) were the pioneer invertebrate colonizers of newly emergent streams arising as meltwater from receding ice sheets and displayed a distinct pattern of succession with stream maturity.
4. Ephemeroptera and Plecoptera colonized warmer clearwater streams, but Trichoptera had a minimal role in invertebrate community development.
5. Establishment and production of salmonid fish populations in the new streams related principally to stream flow and sediment characteristics.
6. Future pathways along which the streams may develop is probably dependent on the degree of large organic debris input.
7. Stream development, structure and function are summarized including reference to theories of ecosystem development, ecological succession and community stability.     

Relationships between Acetylene Reduction Activity, Hydrogen Evolution and Nitrogen Fixation in Nodules of Acacia spp.: Experimental Background to Assaying Fixation by Acetylene Reduction under Field Conditions

Hansen, A. P., Pate, J. S. and Atkins, C. A. 1987. Relationshipsbetween acetylene reduction activity, hydrogen evolution andnitrogen fixation in nodules of Acacia spp.: Experimental backgroundto assaying fixation by acetylene reduction under field conditions.—J.exp. Bot. 38: 1–12 Glasshouse grown, symbiotically-dependent seedlings of Acaciaalata R.Br., .A. extensa Lindl., and A. pulchella R.Br. wereexamined for acetylene reduction in closed assay systems usingundisturbed potted plants, excavated whole plants, nodulatedroots or detached nodules. Nitrogenase activity declined sharplyover the first hour after exposure of detached nodules to acetylene(10% v/v in air), less steeply or not at all over a 3 h periodin assays involving attached nodules. Using detached nodules,rates of acetylene reduction, nitrogen (¹⁵N₂) fixation, andhydrogen evolution in air (¹⁵N₂) and acetylene-containing atmosphereswere measured in comparable 30 min assays. Total electron flowthrough nitrogenase in air was determined from rates of nitrogen(¹⁵N₂) fixation ( ? 3) plus hydrogen evolution, that in thepresence of acetylene from rates of acetylene reduction andhydrogen evolution in air: acetylene. Values for the ratio ofelectron flow in air: acetylene to that in air ranged from 0?43to 0?83 in A. pulcheila, from 0?44 to 0?66 in A. alala and from0?37 to 0?70 in A. extensa, indicating substantial inhibitionof electron flow through nitrogenase of detached nodules byacetylene. Relative efficiencies of nitrogenase functioningbased on hydrogen evolution and acetylene reduction were from0?15 to 0?79, those based on nitrogen (¹⁵N₂) fixation and hydrogenevolution from 0?53 to 0?87. Molar ratios of acetylene reducedto nitrogen (¹⁵N₂) fixed were 2?82 ? 0?24, 201 ? 0?15, and 1?91? 0?11 (?s.e.; n = 7) for A. pulcheila,A. extensa and A. alata respectively A standard 5–10 min acetylene reduction assay, conductedon freshly detached unwashed nodules in daytime (12.00–14.00h), was calibrated for field use by comparing total N accumulationof seedlings with estimated cumulative acetylene reduction overa 7-week period of glasshouse culture. Molar ratios for acetylenereduced: nitrogen fixed using this arbitrary method were 3?58for A. alata, 4?82 for A. extensa and 1?60 for A. pulchella.The significance of the data is discussed. Key words: Acacia spp, nitrogenase functioning     

Why are mammalian tendons so thick?

The maximum stresses to which a wide range of mammalian limb tendons could be subjected in life were estimated by considering the relative cross-sectional areas of each tendon and of the fibres of its muscle. These cross-sectional areas were derived from mass and length measurements on tendons and muscles assuming published values for the respective densities. The majority of the stresses are low. The distribution has a broad peak with maximum frequency at a stress of about 13 MPa, whereas the fracture stress for tendon in tension is about 100 MPa. Thus, the majority of tendons are far thicker than is necessary for adequate strength. Much higher stresses are found among those tendons which act as springs to store energy during locomotion. The acceptability of low safety factors in these tendons has been explained previously (Alexander, 1981). A new theory explains the thickness of the majority of tendons. The muscle with its tendon is considered as a combined system which delivers mechanical energy: the thickness of the tendon is optimized by minimizing the combined mass. A thinner tendon would stretch more. To take up this stretch, the muscle would require longer muscle fibres, which would increase the combined mass. The predicted maximum stress in a tendon of optimum thickness is about 10 MPa, which is within the main peak of the observed stress distribution. Individual variations from this value are to be expected and can be understood in terms of the functions of the various muscles.     

Transport and Compartmentation of Abscisic Acid in Roots of Hordeum distichon under Osmotic Stress

Using modified compartmental analysis the unidirectional fluxesof abscisic acid (ABA) and their cytoplasmic and vacuolar contentsin ³H-ABA preloaded barley root segments (Hordeum distichoncv. Aura) have been studied. When root segments were stressedosmotically with sorbitol (osmotic potential of the media ⁰= 0.2 MPa) cytoplasmic and vacuolar contents of ABA were enhanced.Under increased stress cytoplasmic and vacuolar contents weremuch lower than in the unstressed controls. ABA fluxes werevery sensitive to osmotic stress and ABA transport from thecytoplasm of the xylem parenchyma to the xylem vessels (_cx)was rapidly inhibited. The cultivar Aura has higher cytoplasmicand vacuolar ABA contents than the barley cultivar Kocherperle.This correlates well with the higher stress tolerance of theAura cultivar. Key words: Abscisic acid, Compartmentation, Osmotic stress     

Hormonal Control of the Teleost Ovary

The endocrine basis of gonadal development, reproductive processes,and many peculiarities related to reproduction in females ofthe highly diveisified teleosts can not yet be described satisfactorily.Although estrogens have been extracted from ovaries and peripheralblood of several species and various steroids were found tobe produced by ovarian tissue in vitro, there is an almost completelack of knowledge about their physiological role. It is beyond any doubt that hormones of the pituitary have amarked influence on ovarian maturation but many problems awaitfurther clarification. It is not yet clear whether gonadotropichormones might have a direct influence on certain traits ofbehavior in females. Biochemical work and a large number ofhistophysiological studies are at variance with regard to thequestion whether one or two gonadotropic hormones exist in teleosts.Technical difficulties (eg., shortage of hormonal material fromvarious sources, species specificity of gonadotropic hormone(s), widely scattered data from a large array of species withconsiderable differences in reproductive biology) hamper comparativeanalysis, Investigations on the hypothalamic control of endocrineevents are just in their beginning. The unique occurrence ofambisexual species in teleosts raises many questions which remainto be solved.