In the 1790s, Robert Townson established the main features of
the water economy of
terrestrial amphibians: rapid evaporative water loss in dry surroundings,
‘drinking’ by
absorption of water through the abdominal skin pressed against moist
substrates, and use of
the urinary bladder as a reservoir from which water is reabsorbed on
land. This knowledge was
of little interest to the establishment in the first half of the
nineteenth century of experimental
physiology as a basic medical discipline, when frogs became models
in the elucidation of general physiological processes. Townson's pioneer
contributions to amphibian physiology
were forgotten for 200 years (Jørgensen 1994b).
Durig (1901) and particularly Overton (1904)
restored knowledge about amphibian water economy to the level reached
by Townson, but the
papers had little impact on the young science of animal physiology
because they primarily
aimed at elucidating the transport of fluids across membranes.
Frog skin remained a model
preparation in such studies throughout the century. With the
establishment of terrestrial
ecology early in the century, the relations of animals, including
amphibians, to water became
a central theme. Concurrently with comparative studies of amphibian
water economy in an
ecological setting, the subject proceeded as an aspect of animal
osmoregulation. Adolph
(1920–1930) and Rey (1937a) established the highly dynamic
nature of water balance in
amphibians in water and on land. Their observations indicated
functional links between
environment, skin and kidneys, the nature of which remained to be
explored. Thorson &
Svihla (1943) reopened the ecological approach in a comparative study
of the relations between
amphibian habitat and tolerance of dehydration. By mid-century, the
central themes of
amphibian adaptations to terrestrial modes of life were re-established,
except for the function
of the bladder as a water-depot. During the following decades, a rich
literature appeared,
particularly focusing on adaptations of amphibians to arid environments.
Thus, in the 1970s,
it was found that ‘waterproofing’ of the highly
permeable skins by means of skin secretions
had evolved independently in several families of tropical arboreal
frogs, and that a number of
amphibians that aestivate whilst burrowed in dry soil could reduce
evaporation by forming
cocoons from shed strata cornea. In 1950–1970 the role of
bladder urine as a water depot in
terrestrial amphibians was recognized: this did not change the
established view of water
balance in terrestrial amphibians as alternating between dehydration
on land and rehydration
in response to the deficit in body water. Amphibians may, however,
maintain normal water
balance whether the ambient medium is water or air by means of
little understood integrated
mechanisms in control of cutaneous drinking behaviour, water
permeability of the skin and bladder wall, and urine production.