TY - JOUR A1 - Linsenmair, Karl Eduard A1 - Schmuck, R. T1 - Adaptations of the reed frog Hyperbolius viridiflavus to its arid environment. III. Aspects of nitrogen metabolism and osmuregulation in the reed frog, H. viridiflavus taeniatus, with special reference to the role of iridophores N2 - Reed frogs of the superspecies Hyperolius viridiflavus occur throughout the seasonally very dry and hot African savannas. Despite their small size (300-700 mg), estivating reed frogs do not avoid stressful conditions above ground by burrowing into the soil, but endure the inhospitable climate relatively unprotected, clinging to mostly dry grass sterns. They must have emcient mechanisms to enable them to survive e.g. very high temperatures, low relative hurnidities, and high solar radiation loads. Mechanisms must also have developed to prevent poisoning by the nitrogenous wastes that inevitably result from protein and nucleotide turnover. In contrast to fossorial amphibians, estivating reed frogs do not become torpid. Reduction in metabolism is therefore rather Iimited so that nitrogenous wastes accumulate faster in these frogs than in fossorial amphibians. This severely aggravates the osmotic problems caused by dehydration. During dry periods total plasma osmolarity greatly increases, mainly due to urea accumulation. Of the total urea accumulated over 42 days of experimental water deprivation, 30% was produced during the first 7 days. In the next 7 days rise in plasma urea content was negligible. This strong initial increase of urea is seen as a byproduct of elevated amino acid catabolism following the onset of dry conditions. Tbe rise in total plasma osmolarity due to urea accumulation, however, is not totally disadvantageous, but enables fast rehydration when water is available for very short periods only. Voiding of urine and feces eeases once evaporative water loss exceeds 10% of body weight. Tberefore, during continuous water deprivation, nitrogenous end products are not excreted. After 42 days of water deprivation, bladder fluid was substantially depleted, and urea coneentration in the remaining urine (up to 447 mM) was never greater than in plasma fluid. Feces voided at the end of the dry period after water uptake contained only small amounts of nitrogenous end products. DSF (dry season frogs) seemed not to be uricotelic. Instead, up to 35% of the total nitrogenous wastes produced over 42 days of water deprivation were deposited in an osmotically inert and nontoxic form in iridophore crystals. The increase in skin purine content averaged 150 µg/mg dry weight. If urea had been the only nitrogenous waste product during an estivation period of 42 days, lethal limits of total osmolarity (about 700 mOsm) would have been reached 10-14 days earlier. Thus iridophores are not only involved in colour change and in reducing heat load by radiation remission, but are also important in osmoregulation during dry periods. The seIective advantages of deposition of guanine rather than uric acid are discussed. KW - Biologie KW - Zoologie KW - Frosch KW - Hyperolius viridiflavus KW - Estivation KW - Osmoregulation KW - Nitrogen metabolism KW - lridophores Y1 - 1988 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-78108 ER - TY - JOUR A1 - Schmuck, R. A1 - Kobelt, F. A1 - Linsenmair, Karl Eduard T1 - Adaptations of the reed frog Hyperbolius viridiflavus (Anura, Hyperbolidae) to its arid environment: V. Iridophores and nitrogen metabolism N2 - Ofall amphibians living in arid habitats, reed frogs (belonging to the super species Hyperolius viridiflavus) are the most peculiar. Froglets are able to tolerate dry periods of up to 35 days or longer immediately after metamorphosis, in climatically exposed positions. They face similar problems to estivating juveniles, i.e. enduranee of long periods of high temperature and low RH with rather limited energy and water reserves. In addition, they must have had to develop meehanisms to prevent poisoning by nitrogenous wastes that rapidly accumulate during dry periods as a metabolie consequenee of maintaining a non-torpid state. During dry periods, plasma osmolarity of H. v. taeniatus froglets strongly increased, mainly through urea accumulation. Urea accumulation was also observed during metamorphic climax. During postmetamorphic growth, chromatophores develop with the density and morphology typical of the adult pigmentary pattern. The dermal iridophore layer, which is still incomplete at this time, is fully developed within 4-8 days after metamorphosis, irrespective of maintenance conditions. These iridophores mainly contain the purines guanine and hypoxanthine. The ability of these purines to reflect light provides an excellent basis for the role of iridophores in temperature regulation. In individuals experiencing dehydration stress, the initial rate of purine synthesis is doubled in eomparison to specimens continuously maintained under wet season conditions. This increase in synthesis rate leads to a rapid increase in the thiekness of the iridophore layer, thereby effectively reducing radiation absorption. Thus, the danger of overheating is diminished during periods of water shortage when evaporative cooling must be avoided. After the development of an iridophore layer of sufficient thickness for effective radiation reflectance, synthesis of iridophore pigments does not cease. Rather, this pathway is further used during the remaining dry season for solving osmotic problems eaused by accumulation of nitrogenous wastes. During prolonged water deprivation, in spite of reduced metabolic rates, purine pigments are produced at the same rate as in wet season conditions. This leads to a higher relative proportion of nitrogen end products being stored in skin pigments under dry season conditions. At the end of an experimental dry season lasting 35 days, up to 38% of the accrued nitrogen is stored in the form of osmotically inactive purines in thc skin. Thus the osmotic problems caused by evaporative water loss and urea production are greatly reduced. KW - Biologie KW - Zoologie KW - Frosch Y1 - 1988 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-78094 ER - TY - THES A1 - Drechsler, Patrick Hans T1 - Mechanics of adhesion and friction in stick insects and tree frogs T1 - Mechanik der Adhäsion und Reibung von Stabheuschrecken und Baumfröschen N2 - Many arthropods and vertebrates can cling to surfaces using adhesive pads on their legs. These pads are either smooth and characterised by a specialised, soft cuticle or they are hairy, i.e. densely covered with flexible adhesive setae. Animals climbing with adhesive organs are able to control attachment and detachment dynamically while running. The detailed mechanisms of how tarsal pads generate adhesive and frictional forces and how forces are controlled during locomotion are still largely unclear. The aim of this study was to clarify the attachment mechanism of smooth adhesive pads as present in many insects and tree frogs. To understand the function of these fluid-based adhesive systems, I characterized their performance under standardized conditions. To this end, experiments were conducted by simultaneously measuring adhesion, friction, and contact area in single adhesive pads. The first result of this study showed that friction in stick insect attachment pads is anisotropic: Attachment pads regularly detached when slid away from the body. Further analyses of "immobilized" arolia revealed that this anisotropy is not caused by an increased shear stress in the proximal direction, but by the instability of the tarsus when pushed distally. In the second part of this study, I analysed the role of the pad secretion present in insects and tree frogs. In stick insects, shear stress was largely independent of normal force and increased with velocity, seemingly consistent with the viscosity effect of a continuous fluid film. However, measurements of the remaining force two minutes after a sliding movement showed that adhesive pads could sustain considerable static friction in insects and tree frogs. Repeated sliding movements and multiple consecutive pull-offs of stick insect single legs to deplete adhesive secretion showed that on a smooth surface, friction and adhesion strongly increased with decreasing amount of fluid in insects. In contrast, stick insect pull-off forces significantly decreased on a rough substrate. Thus, the secretion does not generally increase attachment but does so only on rough substrates, where it helps to maximize contact area. When slides with stick insect arolia were repeated at one position so that secretion could accumulate, sliding shear stress decreased but static friction remained clearly present. This suggests that static friction in stick insects, which is biologically important to prevent sliding, is based on non-Newtonian properties of the adhesive emulsion rather than on a direct contact between the cuticle and the substrate. % Analogous measurements in toe pads of tree frogs showed that they are also able to generate static friction, even though their pads are wetted by mucus. In contrast to the mechanism proposed for insects, static friction in tree frogs apparently results from the very close contact of toe pads to the substrate and boundary lubrication. In the last section of this study, I investigated adhesive forces and the mode of detachment by performing pull-off measurements at different velocities and preloads. These experiments showed that preload has only an increasing effect on adhesion for faster pull-offs. This can be explained by the viscoelastic material properties of the stick insect arolium, which introduce a strong rate-dependence of detachment. During fast pull-offs, forces can spread over the complete area of contact, leading to forces scaling with area. In contrast, the pad material has sufficient time to withdraw elastically and peel during slow detachments. Under these conditions the adhesive force will concentrate on the circumference of the contact area, therefore scaling with a length, supporting models such as the peeling theory. The scaling of single-pad forces supported these conclusions, but large variation between pads of different stick insects did not allow statistically significant conclusions. In contrast, when detachment forces were quantified for whole insects using a centrifuge, forces scaled with pad contact area and not with length. N2 - Viele Arthropoden und Vertebraten können sich mit Hilfe tarsaler Haftorgane an Oberflächen festhalten. Diese Organe sind entweder glatt, mit einer spezialisierten, weichen Cuticula oder haarig, d.h. dicht besetzt mit mikroskopisch kleinen, biegsamen Hafthaaren. Mit Haftorganen kletternde Tiere können während des Laufens Haftkräfte dynamisch kontrollieren. Die genaueren Mechanismen, mit denen Adhäsions- und Reibungskräfte erzeugt werden und mit denen die Kräfte während des Laufens schnell kontrolliert werden können, sind allerdings noch immer weitgehend unklar. Das Ziel dieser Arbeit war es, den Haftmechanismus von glatten Haftorganen bei Insekten und Baumfröschen näher aufzuklären. Um die Funktion dieser flüssigkeitsbasierten Haftsysteme zu verstehen, charakterisierte ich ihr Adhäsions- und Reibungsverhalten unter standardisierten Bedingungen. Dazu führte ich Experimente an einzelnen Haftorganen durch, bei denen ich gleichzeitig Adhäsion, Reibung, und Kontaktfläche erfasste. Das erste Ergebnis dieser Arbeit war, dass die Reibung von Insektenhaftorganen von der Bewegungsrichtung abhängt. Ein Haftorgan, das vom Körper weg bewegt wird (distale Richtung), löst sich meist von der Oberfläche ab. Weitere Untersuchungen an Haftorganen bei fixiertem Tarsus zeigten, dass die Richtungsabhängigkeit nicht durch eine erhöhte Scherspannung in der proximalen Richtung hervorgerufen wird, sondern durch die Instabilität des Tarsus, wenn der Fuß vom Körper weg bewegt wird. Im zweiten Teil der Arbeit untersuchte ich die Rolle des Haftsekrets bei Stabheuschrecken und Baumfröschen. Bei Stabheuschrecken war die Scherspannung unabhängig von der Normalkraft und nahm mit der Bewegungsgeschwindigkeit zu, scheinbar in Einklang mit der viskosen Reibung eines durchgehenden Flüssigkeitsfilms. Jedoch ergaben Scherspannungsmessungen bei Stabheuschrecken und Fröschen selbst zwei Minuten nach einer Gleitbewegung ein beträchtliches Maß an statischer "Rest"-Reibung. Um den Einfluss geringer werdender Haftflüssigkeit zu untersuchen, wurden wiederholte Gleitversuche sowie aufeinanderfolgende Ablöseversuche auf glatten Oberflächen durchgeführt. Diese Experimente zeigten, dass sowohl die Reibungs- als auch die Adhäsionskraft mit abnehmender Flüssigkeitsmenge anstieg. Im Gegensatz hierzu nahm die Adhäsionskraft auf rauen Oberflächen mit abnehmender Haftflüssigkeitsmenge ab. Demzufolge führte die Haftflüssigkeit nur auf rauen Oberflächen zu einer Vergrößerung der Kontaktfläche und zu einer Erhöhung der Adhäsionskraft. Reibungskräfte auf glatten Oberflächen wurden bei Stabheuschrecken umso geringer, je häufiger Reibungsversuche an ein und der selben Stelle durchgeführt wurden (um die Menge an Haftflüssigkeit zu erhöhen). Dennoch blieb immer eine statische Reibung vorhanden. Das Vorhandensein von statischer Reibung ist biologisch wichtig um das unfreiwillige Ausrutschen zu verhindern. Meine Ergebnisse weisen darauf hin, dass die Haftreibung bei Insekten nicht auf direkte Kontakte zwischen Cuticula und Untergrund zurückzuführen ist, sondern auf die (scherverdünnende) nicht-Newtonschen Eigenschaften des zweiphasigen Haftsekrets. Analoge Messungen an Haftzehen von Baumfröschen zeigten, dass auch diese statische Reibungskräfte erzeugen können, obwohl sie von einem flüssigen Schleim benetzt sind. Im Gegensatz zu dem bei Insekten gefundenen Mechanismus, entsteht bei Fröschen die statische Reibung wahrscheinlich durch Trockenreibung und den sehr nahen Kontakt zur Oberfläche. Im letzten Teil dieser Arbeit untersuchte ich Adhäsionskräfte und den Ablösevorgang durch Haftkraftmessungen bei verschiedenen Geschwindigkeiten und Normalkräften. Diese Experimente zeigten, dass die Normalkraft nur bei schnellem Ablösen zu höheren Adhäsionskräften führt. Dies ist durch die viskoelastischen Materialeigenschaften der Stabheuschrecken-Arolien erklärbar, die zu einer starken Geschwindigkeitsabhängigkeit des Ablösevorgangs führen. Bei schnellem Ablösen breiten sich die Kräfte über die gesamte Kontaktzone aus, was zu einer Flächenskalierung der Adhäsion führt. Im Gegensatz dazu hat das Haftorgan bei einem langsamen Ablöseprozess genügend Zeit, sich elastisch zurückzuziehen und abzuschälen. Unter diesen Bedingungen konzentriert sich die Kraft am Rand der Kontaktzone, wodurch die Adhäsionskräfte mit einer Länge skalieren, wie z.B. von der "peeling" Theorie vorhergesagt. Die Skalierung von Einzelbein-Haftkräften bestätigte diese Schlußfolgerungen, aber die starke Variation zwischen verschiedenen Stabheuschrecken erlaubte es nicht, diese statistisch abzusichern. Im Gegensatz dazu zeigten die Haftkräfte ganzer Insekten, welche mit Hilfe einer Zentrifuge gemessen wurden, eine deutliche Flächenskalierung. KW - Biomechanik KW - Adhäsion KW - Flüssigkeitsreibung KW - Reibung KW - Frosch KW - Insekten KW - Carausius morosus KW - Haftung KW - Schubspannung KW - Emulsion KW - Schälen KW - Haftmechanismen KW - Haftorgane KW - Haftflüssigkeit KW - Litoria caerulea KW - Scherspannung KW - Wet adhesion model KW - biomechanics KW - adhesion KW - friction KW - attachment structure KW - adhesive fluid KW - wet adhesion KW - shear stress KW - emulsion KW - attachment devices KW - peeling Y1 - 2008 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-26836 ER -