more rapidly than when they are normally hydrated. There is considerable variability in the magnitude of such differences; they may be small, as seen in aquatic
species like Xenopus laevis, or large as in the terrestrial toad, Bufo carens. Such
observations on these two species led EWER (1952 b) to suggest that such a response
may be better developed in species that are normally terrestrial in their habits as
compared to more aquatic species. Parallel with these observations it was found
that when neurohypophysial pep tides were injected, Bufo retained large amounts
of water, while Xenopus completely failed to respond. These differences are reminiscent of the observations of STEGGERDA (1937) who found that while terrestrial species of the Amphibia accumulated large quantities of water after being injected with
preparations of mammalian neurohypophysial pep tides, aquatic ones retained
little . An ability to rehydrate rapidly could be important to some amphibians, particularly to those living in areas where water is only available sporadically and
quickly disappears either by evaporation or soaking into the soil. It is also possible
that entry into open pools of water may increase the risk of attack. Thus, a
facilitated rate of absorption would reduce exposure to predators. The rates of rehydration in a large number of North American and Australian amphibians have
been measured, and compared in relation to the availability of water in the habitats
where they normally live. It is evident that there is considerable divergence in the
rates of rehydration, but just how uniformly this may be related to the habitat,
thus representing a physiological adaptation, is not clear.
Rehydration in a large variety of North American frogs and toads from the
families Ranidae, Bufonidae, Hylidae and Pelobatidae has been measured (THORSON, 1955; CLAUSSEN, 1969), but no relationship between the rate of water accumulation and the 'dryness' of the habitat was found . A number of Australian frogs,
especially those from the family Leptodactylidae have also been examined for evidence of such a correlation. We (BENTLEY et al., 1958) confined our studies to species within the genera Heleioporus (5 species) and Neobatrachus (4 species). The
habitats of species within these two genera range from areas in which rain falls regularly, on an average of 120 days a year, to desert, or semi-desert regions where the
annual rainfall may be spread over only a few days. Frogs of the genus Heleioporus
all rehydrated at similar rates (about 50 ,u1/cm
2h)
after being dehydrated to 75%
of their normal weight. However, species of Neobatrachus showed considerable
differences in their abilities to absorb water after such dehydration; N. pelobatoides
which lives in areas where rain usually falls on 60-120 days a year gain water at
the rate of 33 ,ul!cm
2h,
while N . wilsmorei which lives in dry areas towards the interior of the continent regain water at the rate of nearly 100 ,Ill!ern-h. It is notable
that such differences in ability to rehydrate were paralleled by the rates at which
these frogs accumulated water after being injected with a neurohypophysial peptide, oxytocin. The failure of Heleioporus to show any evidence of a correlation
between water uptake and the aridity of the habitat could reflect the ability of this
group to avoid the excesses of the climate by very efficient burrowing, thus making
rehydration ability somewhat redundant. WARBURG (1965 b) has also found that
frogs that live in the arid and semi-arid central regions of Australia rehydrate more
rapidly than those species that live in more temperate areas . Few urodeles have
been examined for their ability to rehydrate following dehydration but SPIGHT
(1967b) has measured this in four species of salamanders. He found only relatively
171
species like Xenopus laevis, or large as in the terrestrial toad, Bufo carens. Such
observations on these two species led EWER (1952 b) to suggest that such a response
may be better developed in species that are normally terrestrial in their habits as
compared to more aquatic species. Parallel with these observations it was found
that when neurohypophysial pep tides were injected, Bufo retained large amounts
of water, while Xenopus completely failed to respond. These differences are reminiscent of the observations of STEGGERDA (1937) who found that while terrestrial species of the Amphibia accumulated large quantities of water after being injected with
preparations of mammalian neurohypophysial pep tides, aquatic ones retained
little . An ability to rehydrate rapidly could be important to some amphibians, particularly to those living in areas where water is only available sporadically and
quickly disappears either by evaporation or soaking into the soil. It is also possible
that entry into open pools of water may increase the risk of attack. Thus, a
facilitated rate of absorption would reduce exposure to predators. The rates of rehydration in a large number of North American and Australian amphibians have
been measured, and compared in relation to the availability of water in the habitats
where they normally live. It is evident that there is considerable divergence in the
rates of rehydration, but just how uniformly this may be related to the habitat,
thus representing a physiological adaptation, is not clear.
Rehydration in a large variety of North American frogs and toads from the
families Ranidae, Bufonidae, Hylidae and Pelobatidae has been measured (THORSON, 1955; CLAUSSEN, 1969), but no relationship between the rate of water accumulation and the 'dryness' of the habitat was found . A number of Australian frogs,
especially those from the family Leptodactylidae have also been examined for evidence of such a correlation. We (BENTLEY et al., 1958) confined our studies to species within the genera Heleioporus (5 species) and Neobatrachus (4 species). The
habitats of species within these two genera range from areas in which rain falls regularly, on an average of 120 days a year, to desert, or semi-desert regions where the
annual rainfall may be spread over only a few days. Frogs of the genus Heleioporus
all rehydrated at similar rates (about 50 ,u1/cm
2h)
after being dehydrated to 75%
of their normal weight. However, species of Neobatrachus showed considerable
differences in their abilities to absorb water after such dehydration; N. pelobatoides
which lives in areas where rain usually falls on 60-120 days a year gain water at
the rate of 33 ,ul!cm
2h,
while N . wilsmorei which lives in dry areas towards the interior of the continent regain water at the rate of nearly 100 ,Ill!ern-h. It is notable
that such differences in ability to rehydrate were paralleled by the rates at which
these frogs accumulated water after being injected with a neurohypophysial peptide, oxytocin. The failure of Heleioporus to show any evidence of a correlation
between water uptake and the aridity of the habitat could reflect the ability of this
group to avoid the excesses of the climate by very efficient burrowing, thus making
rehydration ability somewhat redundant. WARBURG (1965 b) has also found that
frogs that live in the arid and semi-arid central regions of Australia rehydrate more
rapidly than those species that live in more temperate areas . Few urodeles have
been examined for their ability to rehydrate following dehydration but SPIGHT
(1967b) has measured this in four species of salamanders. He found only relatively
171