Water loss and colour change in the desert tree frog 
Table 2 Index for chromatophores in the transparent 
toe webbing of the desert tree frog Litoria 
rubella, kept in moist or dry air after transfer 
from black or white backgrounds. Values are 
mean ± standard error; n = 8 for all groups. 
DARK 2 
LIGHT 2 
MOIST' 
3.6 ± 0.4 
3.8 ± 0.4 
DRY' 
2.6 ± 0.2 
2.3 ± 0.1 
1 significant difference (P<0.0005) by two-factor ANOVA 
2 no significant difference (P>0.8) by two-factor ANOVA 
their melanophores were more aggregated and the 
frogs were lighter in colour. 
The reflectance of the dorsal skin from a dark- 
coloured tree frog (toe web chromatophore index = 
5) varied considerably over the visible spectrum 
(Figure 2), but was lower than for the dorsal skin 
of a light-coloured tree frog (toe web 
chromatophore index = 2). The mean reflectance 
for the skin samples, from 400 to 700 nm (visible 
spectrum), was 18.8% for the dorsal skin of the 
dark tree frog, and 31.8% for the dorsal skin of the 
light tree frog. 
200 300 400 500 600 700 800 900 
Wavelength (nm) 
Figure 2 Reflectance of dorsal skin for a light 
individual of Litoria rubella (toe-web 
chromatophore index = 2) and a dark 
individual (chromatophore index = 5). 
DISCUSSION 
Tree frogs, being generally arboreal, are 
potentially able to bask although there are few 
specific reports of basking behaviour. Litoria 
caerulea has been reported to bask (Brattstrom 
1970) as have Litoria chloris (Buttemer 1990) and 
some other hylids ( H. labiatus and Hyla cinerea; 
279 
Valdivieso and Tamsitt 1974; Freed 1980; Snyder 
and Hammerson 1993); other frogs also bask (Tyler 
et al. 1983; see Hutchison and Dupre 1992). Being 
exposed to solar insolation and wind in 
conspicuous basking sites would potentially 
desiccate and overheat frogs, particularly smaller 
species such as Litoria rubella. 
Most amphibians, which evaporate water from 
their skin as if it were a free water surface, would 
rapidly dehydrate when basking in exposed, 
arboreal conditions (see Shoemaker et al. 1992; 
Hutchison and Dupre 1992). Many such frogs 
adopt a water-conserving posture when exposed 
to desiccating conditions; the ventral skin is firmly 
adpressed to the substrate, the limbs are held 
tightly under the body, and the head is lowered to 
protect the gular region (see Stille 1958; Pough et 
al. 1983). Litoria rubella routinely adopts such a 
water-conserving posture when perched, and 
during measurement of evaporative water loss. 
A number of arboreal, basking frogs have a 
reduced rate of evaporative water loss. For 
example, the Southern African rhacophorid tree 
frog Chiromantis xerampelitia has a very low rate of 
EWL (Loveridge 1970), as does the South American 
tree frog Phyllomedusa xerampelina (Shoemaker et al. 
1972) and Southern African hyperoliid reed frogs 
(Withers et al. 1982a,b). Consequently, it is not 
surprising to note that the Australian tree frog 
Litoria rubella has a reduced EWL (Table 1), as do 
some other Litoria spp (Withers et al. 1984; 
Buttemer 1990), at least when they are in the water- 
conserving posture with the ventral skin concealed. 
However, the resistance of Litoria spp is 
intermediate (10-40 sec cm 1 ) between that of 
typical anuran amphibians (=1) and the "water¬ 
proof" frogs (>100). When L. rubella were not in a 
water-conserving posture, their skin resistance to 
evaporative water loss was only about 1.6 sec cm' 1 , 
which is similar to that expected for "non¬ 
waterproof" frogs of this size. 
Basking in exposed sites presumably elevates 
body temperature, which confers thermoregulatory 
advantages (see Hutchison and Dupre 1992). A 
number of basking frogs (which have a reduced 
rate of evaporative water loss) blanch to a pale or 
white colour when basking, presumably to reduce 
the absorption of incident solar radiation and 
prevent overheating). The tree frogs Chiromantis 
xerampelina and Hyperolius spp are bright white 
when basking in sunlight (Stewart 1967; Passmore 
and Carruthers 1979; Kobelt and Linsenmair 1986; 
Shoemaker et al. 1989). The canyon frog Hyla 
arenicolor blanches when basking at high ambient 
temperature (Snyder and Hammerson 1993). The 
tree frog Hyla cinerea blanches at high ambient 
temperature (King et al. 1994). Litoria chloris basks, 
after feeding (Buttemer 1990). L. rubella also basks 
in sites exposed to solar insolation, and blanches to 
