Changes in body fluids of the cocooning fossorial frog Cyclorana australis in a seasonally dry environment

Abstract

We investigated changes in the lymph (equivalent to plasma) and urine of the cocooning frog Cyclorana australis during the dry season in monsoonal northern Australia. Frogs in moist soil for two days were fully hydrated (lymph 220 mOsm kg − 1 , urine 49 mOsm kg − 1 ). From five weeks onwards the soil was dry (matric potential <−8000 kPa). Aestivating frogs at three and five months formed cocoons in shallow (< 20 cm) burrows and retained bladder fluid (25–80% of standard mass). After three months, urine but not lymph osmolality was elevated. After five months, lymph (314 mOsm kg − 1 ) and urine (294 mOsm kg − 1 ) osmolality and urea concentrations were elevated. Urea was a major contributing osmolyte in urine and accumulated in lymph after five months. Lymph sodium concentration did not change with time, whereas potassium increased in urine after five months. Active animals had moderate lymph osmolality (252 mOsm kg − 1 ), but urea concentrations remained low. Urine was highly variable in active frogs, suggesting that they tolerate variation in hydration state. Despite prolonged periods in dry soil, osmolality increase in C. australis was not severe. Aestivation in a cocoon facilitates survival in shallow burrows, but such a strategy may only be effective in environments with seasonally reliable rainfall.