Role of urinary and cloacal bladders in chelonian water economy: historical and comparative perspectives

The Parisian comparative anatomist Claude Perrault, dissecting an Indian giant tortoise in 1676, was the first to observe that the urinary bladder is of an extraordinary size in terrestrial tortoises. In 1799, the English comparative physiologist Robert Townson suggested that the bladder functioned as a water reservoir, as he had shown previously for frogs and toads. However, these observations went unnoticed in subsequent reports on tortoise water economy that were made by travellers and naturalists visiting the Galapagos Archipelago and marvelling over the huge numbers of giant tortoises that inhabited these desert-like islands. The first such report was by an American naval officer, David Porter, who was a privateer in the 1812–15 war with England. In his journal he referred to the constant supply of water which the Galapagos tortoises carried with them. References to the location in the body, as well as the amounts and quality of the water stored, were, however, contradictory. The confusion concerning the anatomical identity of the water reservoir in the Galapagos tortoise, Geochelone elephantopus, persisted throughout the nineteenth century, and continued when studies of tortoise water economy and drinking behaviour in arid environments were taken up independently in the desert tortoise, Gopherus agassizii, which inhabits the desert regions in the south-western United States. In 1881 Cox found large sacs filled with clear water under the carapace, but it was half a century later that these sacs were identified as the large bilobed bladder; references to specific water sacs continued to appear in the literature until the 1960s. Since 1970, information on the water economy of desert tortoises has been obtained from extensive field studies. Rates of disappearance of tritiated water injected into the body have shown that during the drought periods of the summer, water turnover (intake) rates do not differ from the rates of metabolic water production. Under these conditions urine is not voided, but is stored in the large bladder. During a drought period the bladder urine increases from initially low osmolality finally to reach isosmolality with the blood plasma. Soluble K+ is the major cation of the urine, but large amounts of K+ are also present as precipitated urates. During a drought period the body is in negative water balance, but despite substantial losses of total body water, the plasma concentrations of Na+ and Cl− can remain constant for many mont