Water and energy relations of the 'Australian water holding frog' Cyclorana platycephalus.

Abstract

1. Water and energy relations of the Australian Water Holding Frog Cyclorana platycephalus were examined to determine how this species survives in desert conditions.

2. The uncovered skin of E. platycephalus loses water at a lower rate than that of some other anurans.

3. Evaporative water loss is reduced to as little as l/44th by production of a cocoon of shed epidermis and an underlying layer of mucous.

4. Burrowing, formation of a spherical burrow chamber of compacted soil and lowering of metabolic rate, further reduce water loss.

5. C. platycephalus is able to absorb water through the skin from soil of low water content compared to other species.

6. The skin of the ventrum, representing only one third of the total skin surface, is responsible for about 85% of total water uptake.

7. Burrows are always formed near the cover of trees or bushes and the burrowing strategy is well suited to soil of high clay content.

8. Nitrogenous waste is largely excreted as urea but excretion is negligible during dry periods. This allows the frog to absorb most of the urinary water reserve, which can be as much as 57% of wet weight, to prevent drying of body tissues.

9. Small frogs have proportionally larger water reserves than large frogs and as these reserves are proportional to skin surface area the depletion time of two to three years applies to all frogs in the population.

10. Metabolic rate of burrowed frogs is 30% that of frogs resting on the surface. Lipid reserves in the fat pads may amount to one fifth of body weight and chiefly support metabolism in large frogs. Metabolism of body lipid, carbohydrnto and protein may be valuable particularly to small frogs.

11. Ten percent of frogs have sufficient energy reserves to survive for over five years of dormancy given enough rain to replenish their water reserves. Survival of some frogs for this length of time appears to be necessary for survival of the species in inland Australia.

12. The reproductive strategy of C. platycephalus is well adapted to unpredictable and infrequent availability of water and energy (food). Male C. platycephalus mature early and produce mature sperm perennially. Some females carry mature eggs at any time of year.