Dimming the fire of life: Cocooned aestivating frogs as a model of intrinsic metabolic depression

Metabolic depression occurs across a diverse array of invertebrate and vertebrate animals during aestivation (summer dormancy). Some aestivating vertebrates form a protective cocoon of shed epidermal layers (amphibians) or dried mucus (lungfishes) which precludes activity, and their metabolic rates (0.14–0.44 of standard) are typically lower and more stable than for non-cocooning aestivators. Their transition to aestivation appears to be an intrinsic response to inactivity and adoption of the water-conserving posture rather than extrinsic environmental changes such as temperature or hypoxia. Physiological consequences include a transition from pulmo-cutaneous to primarily pulmonary gas exchange, with consequential changes in ventilation and circulation. Breathing rate increases, heart rate decreases, and there is likely a slight decrease in blood pO 2 and a mild blood acidosis. Cocoon-forming frogs are an ideal model for mechanistic studies of intrinsic metabolic depression, but little progress has been made in resolving underlying molecular mechanisms. To date, ‘omics’ approaches have been largely restricted to genome-wide expression of a single tissue (gastrocnemius muscle) in just one Australian species ( Cyclorana alboguttata ). Future research should expand both taxonomic and physiological coverage by characterising additional metabolically important tissues such as liver, kidney, bladder and skin. It should also investigate multiple levels of biological organisation beyond mRNA, including genome size and structure, CpG methylation, histone acetylation, protein abundance and post-translational modifications, intracellular signalling cascades and metabolite profiles. Comparative analyses of the different dynamics of entry into and arousal from aestivation, as well as steady-state metabolic depression, will be informative regarding mechanisms for metabolic depression. Competing Interests: None of the authors have any competing interests.