Ecophysiology of Australian cocooning and non-cocooning, burrowing, desert frogs

Kellie McMaster

    Research output: ThesisDoctoral Thesis

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    Abstract

    [Truncated abstract] This thesis explores the ecophysiology of cocooning and non-cocooning Australian burrowing desert frogs in order to specify the details of these two alternative survival strategies. Many details of these life-history strategies have not previously been determined for Australian frogs. In this study the detailed method of function and formation of the cocoon itself was investigated for the first time, as were the moulting cycles, which drive cocoon formation. Cocooning species had consistently shorter inter-moult-periods than non-cocooning species, which is presumably adaptive, allowing production of cocoon layers at a faster rate. Transmission electron microscopy revealed cocoon formation in Cyclorana platycephala to be a plastic, rather than static adaptation. The initial stages of cocoon formation appear to be the most critical. Cocoon layer formation is fastest in the first month and the initial layers produced are thicker than subsequent layers. Also, only in this initial stage does layer thickness appear to be influenced by soil moisture. Scanning electron microscopy confirmed the lips, eyelids and cloaca of cocooned frogs to be sealed by continuous layers of cocoon. However, it remains unclear precisely how the continuous junctions between cells from discreet sources, such as the upper and lower lip, are formed. Measurements of evaporative water loss and resistance over various relative humidity gradients suggested that water content within the cocoon structure is very low unless the external RH is > 93%. Therefore, anuran cocoon is confirmed to function as a specialised and highly effective barrier to water diffusion in arid-zone frogs. ... Within a dune swale, N. aquilonius burrows were an average depth of 1.13 m in loamy sand (clay = 6%, silt = 7%). Neobatrachus aquilonius excavated from the claypan had well formed cocoons, while those found in the dune swale had only one or two thin layers, indicating cocoon formation had only just commenced, although the last rainfall had been over six months prior. This suggests that N. aquilonius is able to switch between the cocooning and non-cocooning strategy in different soil types and different conditions. In summary, cocoon formation was found not to be an entirely passive accumulation of layers. Cocoon formation involved specialisation in the rate and thickness of epithelial layer production and the sealing of layers over lips, eyes and cloaca. Moreover, the cocoon was confirmed to operate effectively as a diffusion barrier at all except the highest external relative humidity. Like cocoon-forming species, Notaden nichollsi was found to metabolically depress to survive for longer periods underground. However, they cannot survive for the entire period
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2006

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