Some physiological consequences of estivation by freshwater crocodiles, Crocodylus johnstoni

Estivating crocodiles were studied at their dry season refuge in northern Australia over six years. The crocodiles spent three or four months a year underground without access to water. Doubly labeled water was used to measure field metabolic rates and water flux, and plasma and cloacal fluid samples were taken at approximately monthly intervals during some years to monitor the effects of estivation with respect to the accumulation of nitrogenous wastes and electrolyte concentrations. Estivating crocodiles expended a mean of 26.1 kJ kg-1 day-1. Although environmental and, hence, body temperatures increased with increasing time in estivation, water efflux decreased with increasing time in estivation to 9.3 mL kg-1 day-1, which was only 23% of the flux rate of crocodiles in water before estivation (40.2 mL kg-1 day-1). The absolute size of body water pools declined proportionately with body mass and body solids; thus the crocodiles were not dehydrated even after three months without access to water. With increasing time in estivation, the following parameters increased in the cloacal fluid: osmolality, potassium, and magnesium; and the following increased in the blood plasma: osmolality, and protein concentration. Electrolytes in plasma were highest when the crocodiles were in the water and late in the estivation period. Freshwater crocodiles do not appear to have any specific adaptations for estivation, but given an adequate refuge, they can survive many months without access to water.