Experimental evidence for predator learning and Müllerian mimicry in Peruvian poison frogs (Ranitomeya, Dendrobatidae)

Poison frogs are characterized by bright coloration, striking patterns, and toxicity; they have thus become a classic example of aposematism. Ranitomeya imitator mimics three congeneric model species (R. fantastica, R. summersi, and two morphs of R. variabilis), creating geographically distinct populations of the species, including four allopatric mimetic morphs. These complexes are thought to represent a case of Müllerian mimicry, but no empirical data exist on learned avoidance by predators to support this claim. In this study we used young chickens (Gallus domesticus) as naïve predators to determine if a mimetic morph of R. imitator and R. variabilis contribute to reciprocal learned avoidance by predators--a key component of Müllerian mimicry. Chickens exposed to either stimulus species demonstrated learned avoidance of both species; thus our results indicate that this complex functions as a Müllerian mimicry system. Our study shows no difference between learned avoidance in stimuli frogs and a "novel" morph of R. imitator which differed in both colour and pattern--indicating that predator learned avoidance may be generalized in this system. This study provides empirical evidence demonstrating the first known case of Müllerian mimicry in anurans. Further, it demonstrates generalized learning which provides a plausible mechanism for the maintenance of both polymorphic mimicry and the maintenance of intrapopulation phenotypic heterogeneity.M.S

Elucidating structure-property-performance relationships of plasma modified tin(IV) oxide nanomaterials for enhanced gas sensing applications

2017 Spring.Includes bibliographical references.This dissertation examines structure-property-performance relationships of plasma modified tin(IV) oxide (SnO2) nanomaterials to successfully and efficiently create sensitive targeted gas sensors. Different project aspects include (1) materials characterization before and after plasma modification, (2) plasma diagnostics with and without a SnO2 nanomaterial, (3) sensor performance testing, and ultimately (4) elucidation of gas-surface relationships during this project. The research presented herein focuses on a holistic approach to addressing current limitations in gas sensors to produce desired capabilities for a given sensing application. Strategic application of an array of complementary imaging and diffraction techniques is critical to determine accurate structural information of nanomaterials, especially when also seeking to elucidate structure-property relationships and their effects on performance in specific applications such as gas sensors. In this work, SnO2 nanowires and nanobrushes grown via chemical vapor deposition (CVD) displayed the same tetragonal SnO2 structure as revealed via powder X-ray diffraction (PXRD) bulk crystallinity data. Additional characterization using a range of electron microscopy imaging and diffraction techniques, however, revealed important structure and morphology distinctions between the nanomaterials. Tailoring scanning transmission electron microscopy (STEM) modes and combining these data with transmission electron backscatter diffraction (t-EBSD) techniques afforded a more detailed view of the SnO2 nanostructures. Indeed, upon deeper analysis of individual wires and brushes, we discovered that despite a similar bulk structure, wires and brushes grew with different crystal faces and lattice spacings. Had we not utilized multiple STEM diffraction modes in conjunction with t-EBSD, differences in orientation related to bristle density would have been overlooked. Thus, it is only through methodical combination of several analysis techniques that precise structural information can be reliably obtained. To begin considering what additional features can affect gas sensing capabilities, we needed to understand the driving force behind SnO2 sensors. SnO2 operates widely as a gas sensor for a variety of molecules via a mechanism that relies on interactions with adsorbed oxygen. To enhance these interactions by increasing surface oxygen vacancies, commercial SnO2 nanoparticles and CVD-grown SnO2 nanowires were plasma modified by Ar/O2 and H2O(v) plasmas. Scanning electron microscopy (SEM) revealed changes in nanomaterial morphology between pre- and post-plasma treatment using H2O plasma treatments but not when using Ar/O2 plasmas. PXRD patterns of the bulk SnO2 showed the Sn4+ is reduced by H2O and not Ar/O2 plasma treatments. X-ray photoelectron spectroscopy (XPS) indicated Ar/O2 plasma treatment increases oxygen adsorption with increasing plasma power and treatment time, without changing Sn oxidation. With the lowest plasma powers and treatment times, however, H2O plasma treatment results in nearly complete bulk Sn reduction. Although both plasma systems increased oxygen adsorption over the untreated (UT) materials, there were clear differences in the tin and oxygen species as well as morphological variations upon plasma treatment. Given that H2O plasma modification of SnO2 nanomaterials resulted in reduction of Sn+4 to Sn0, this phenomenon was further explored. To develop a deeper understanding of the mechanism for this behavior, gas-phase species were detected via optical emission spectroscopy (OES) during H2O plasma processing (nominally an oxidizing environment), both with and without SnO2 substrates in the reactor. Gas-phase species were also detected in the reducing environment of H2 plasmas, which provided a comparative system without oxygen. Sn* and OH* appear in the gas phase in both plasma systems when SnO2 nanowire or nanoparticle substrates are present, indicative of SnO2 etching. Furthermore, H2 and H2O plasmas reduced the Sn in both nanomaterial morphologies. Differences in H* and OH* emission intensities as a function of plasma parameters show that plasma species interact differently with the two SnO2 morphologies. The H2O plasma gas-phase studies found that under most plasma parameters the ratio of reducing to oxidizing gas-phase species was ≥1. The final consideration in our holistic approach relied on sensor performance studies of SnO2 nanomaterials. Resistance was recorded as a function temperature for UT, Ar/O2 and H2O plasma treated nanoparticles and nanowires exposed to air, carbon monoxide (CO), or benzene (C6H6). Resistance data were then used to calculate sensor response (Rair/Rgas) and sensitivity (Rair/Rgas > 1 or Rgas/Rair > 1). Specifically, Ar/O2 and H2O plasma modification increase CO and C6H6 sensitivity under certain conditions, but H2O plasma was more successful at increasing sensitivity over a wider range of plasma parameters. In particular, certain H2O plasma conditions resulted in increased sensitivity over the UT nanomaterials at 25 and 50 °C. Overall, H2O plasma appears to be more effective at increasing sensitivity than Ar/O2 plasma. Furthermore, although certain treatments and temperatures for nanoparticles had greater CO or C6H6 sensitivity than nanowires, nanowire sensitivity was less temperature dependent than nanoparticle sensitivity. Prior materials characterization data were combined with resistance data to elucidate specific structure-property-performance relationships for the different UT and plasma treated materials

The Effect of Riparian Buffer Zones of Macroinvertebrate Biodiversity and Stream Health

Riparian buffer zones are the forested areas between a stream and the surrounding land. They help preserve stream quality, and in doing so, help preserve biodiversity. Riparian buffer zones have an affect on a vast array of stream attributes, macroinvertebrate biodiversity, and the health of stream ecosystems; it is for this reason that they are important. Macroinvertebrates are often considered an indicator of ecosystem health and therefore the way that buffer zones affect macroinvertebrate species richness and abundance are often indicative of how they affect the rest of the ecosystem. The purpose of this project was to collect macroinvertebrates to test for differences between riparian and non-riparian zones. The data were collected once during each of the calendar seasons (twice during the fall) in the Yellow Breeches Creek in Cumberland County, Pennsylvania. Richness and abundance are not greater in riparian zones. However, the Shannon index and Becks scales are statistically greater in riparian zones; indicating greater biodiversity in riparian zones

Behaviour of oxide layer of zirconium-based fuel rod cladding under steam starvation conditions

Das Verhalten der Oxidschicht von Zirkonium-basierten Brennstabhüllrohren unter Dampfmangelbedingungen Es werden die Ergebnisse von Dauerglühversuchen mit voroxidierten Zircaloy-Hüllrohren in Inertgasatmosphäre bei Glühtemperaturen zwischen 1250 und 1500 °C und ein entsprechendes Modell dargestellt. Ziel der Tests war die Untersuchung der Kinetik der Oxidschichtreduzierung während der Dampfmangelphase bei der Trockenlegung des Reaktorkerns während eines schweren Störfalles. Neben der Abnahme der Oxidschichtstärke wurden die homogene Entwicklung von α-Zr(O) Ausscheidungen innerhalb der Oxidschicht und die Entwicklung einer α-Zr(O) Schicht an der äußeren Oberfläche des Hüllrohres nachgewiesen. Das Phänomen sollte einen starken Einfluss auf die intensive Wasserstofffreisetzung während der folgenden Abschreckphase haben