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 months, indicating
regulation of the extracellular fluid and water content of the body tissues
by reabsorption of water from the
urinary bladder. The bladder thus acts both as a store for nitrogenous
waste and K+ and as a water reservoir
during droughts. Following rain showers, there is a sharp decline in tritium
activity correlated with copious
drinking from temporary pools of rain water. The old bladder urine is voided
and most of the water drunk
is stored as a highly dilute urine.
In 1676 Perrault observed that in a freshwater turtle, Emys orbicularis,
but not in the giant tortoise, two
other bladders opened into the cloaca. By the mid-twentieth century it
had been established that these
cloacal bladders typically were restricted to species of chelonians that
led a semi-terrestrial or semi-aquatic
life. The function of the bladders has been debated since Townson observed
in 1799 that dehydrated
freshwater turtles took up water by anal drinking, suggesting that anal
drinking served in the water economy
of semi-terrestrial turtles. Since then, the bladders have been ascribed
hydrostatic and respiratory functions,
but the recent literature mostly argues for a respiratory function. The
possible role of the cloacal bladders
as a water reservoir in amphibious turtles is still open.
Terrestrial amphibians and tortoises are unique among vertebrates in
possessing large urinary bladders
that may function as water reservoirs in dry environments. This function
depends upon copious water intake
when water becomes available combined with discontinued voiding of urine
in the absence of water.
Adaptation to terrestrial habitats in ureotelic amphibians is correlated
with tolerance of high urea
concentrations in the body fluids. In arid-zone tortoises and uricotelic
tree frogs, nitrogenous waste products
are precipitated in the bladder, which functions as the main sink. Renewed
contact with water releases
drinking behaviour and voiding of the bladder urine until the accumulated
excretory products are
eliminated from the body and/or bladder, preparing the organism for
re-exposure to arid conditions.