AbstractThe osmolality and solute composition of body fluids, water exchange with the substrate, role of the bladder, and response to seasonal water availability were examined in frogs from monsoonal northern Australia. Using a technique involving temporary ligation, lymph was more easily obtained from Rhinella marina than from Litoria caerulea and Cyclorana australis. Lymph accumulation was more rapid in hydrated frogs or frogs with full bladders. Differences between the osmolality of lymph and blood-plasma were insubstantial. On semi-solid agar-solute substrates across a range of water potentials, the hygroscopic cocoon of Cyclorana australis absorbed moisture. With the cocoon removed, water exchange depended on the direction of the frog-substrate water potential difference. The cocoon acts as a physical barrier that breaks the continuity between frog and substrate.
The mass of Cyclorana australis on moist soil was variable and the frogs absorbed water from the soil. In the laboratory, frogs with access to water regularly replenished the bladder store, and urine osmolality remained low. With no water available, the osmolality and urea concentration of the bladder store increased, but water reabsorption modulated plasma osmolality. In frogs with empty bladders, plasma and urine osmolality and urea concentration increased with water loss. Models indicate that body size, bladder fluid, initial osmolality and rates of water loss influence osmolality. Cyclorana australis burrowed in soilfilled pots during the dry season. After two days, excavated frogs in moist soil were fully hydrated. As the soil dried, the frogs developed cocoons but retained bladder fluid. Plasma and urine osmolality increased over time, urine osmolality approached plasma osmolality, and urea was a major contributing osmolyte. The cocoon and bladder store ensure survival of this burrowing species during prolonged dry periods. Rhinella marina collected in the late dry season adopted water absorbing postures on moist soil. Water potential of the soil (-31 kPa) permitted cutaneous drinking. Plasma and urine osmolality indicated dehydration, and differed from laboratory hydrated and active toads.
|Date of Award
|Keith Christian (Supervisor), Lindsay B. Hutley (Supervisor) & Christopher Tracy (Supervisor)