ments are probably mediated principally by the nervous system, though in some
reptiles melanocyte stimulating hormone mediates colour change (WARING, 1963).
Evaporation of water from the respiratory tract and skin increases when the temperature of the body and the environment rises, but this is physically obligatory
and cannot be considered as part of a regulatory response to facilitate cooling.
The principal avenues for the exchange of water and solutes between the reptile
and its environment are basically the same as in mammals and birds. The ph ysiological significance and magnitude of the exchanges through the different channels
however, differ. The main osmoregulatory organs are the kidneys, as in all vertebrates, but in certain reptiles the action of these organs may be augmented.by
cephalic 'salt' glands, and the cloaca and urinary bladder.
1. Water Exchanges
a) Skin and Respiratory Tract
In a terrestrial environment reptiles lose water by evaporation from the external
integument, and the lungs and pulmonary passages. This loss increases when body
temperature and environmental temperature increase; thus the bobtail goanna,
Trachysaurus rugosus, when kept in dry air at 25
0
, loses water by evaporation at
the rate of 0.7 g/100 g day while at 37.5
0 it is 2.9 g/100 g day (WARBURG, 1965a).
This water loss is small, relative to that in birds, mammals and the Amphibia, and
makes little contribution to thermal cooling; the temperature of the goannas when
equilibrated to such-conditions only being one or two degrees less than that of the
environment.
The increased evaporation of water at high environmental temperatures is due
to the decrease in the saturation deficit for water in the surrounding air, and an
increase in the bod y temperature of the animal. The metabolic rate of reptiles increases two to three times for every 10
0 rise in body temp~rature(BENEDICT, 1932),
so that there is an increased rate of gas exchange in the lungs that further facilitates
water loss .
Different species of reptiles have been observed to lose water by evaporation
at various rates (BOGERT and COWLES, 1947). The reasons for such differences were
not initially clear as they could involve several factors such as: the rate of oxygen
consumption, the ratio of surface area to body weight or the particular properties
of the skin of the different species . This was not further investigated until SCHMIDTNIELSEN in 1964 (a) suggested, after examining the fragmentary information that
was then available, that cutaneous evaporation in reptiles may be greater and more
variable than was hitherto considered likely. When cutaneous and pulmonary water
losses were measured separately in a variety of species of reptiles (at 23 0) the movement through the integument was indeed found to contribute from 66 to 87% of
the total evaporative loss (see Table 5.2; BENTLEY and SCHMIDT-NIELSEN, 1966;
SCHMIDT-NIELSEN and BENTLEY, 1966). At higher temperatures (35
0 and 40
0
) pulmonary water loss was increased relative to that from the skin but was still about
50% of the total. Similar results have been described in a variety of lizards (see
CLAUSSEN, 1967). The rates of water loss across the skin vary considerably in different reptile species, in a manner that suggests that animals normally occupying
138
reptiles melanocyte stimulating hormone mediates colour change (WARING, 1963).
Evaporation of water from the respiratory tract and skin increases when the temperature of the body and the environment rises, but this is physically obligatory
and cannot be considered as part of a regulatory response to facilitate cooling.
The principal avenues for the exchange of water and solutes between the reptile
and its environment are basically the same as in mammals and birds. The ph ysiological significance and magnitude of the exchanges through the different channels
however, differ. The main osmoregulatory organs are the kidneys, as in all vertebrates, but in certain reptiles the action of these organs may be augmented.by
cephalic 'salt' glands, and the cloaca and urinary bladder.
1. Water Exchanges
a) Skin and Respiratory Tract
In a terrestrial environment reptiles lose water by evaporation from the external
integument, and the lungs and pulmonary passages. This loss increases when body
temperature and environmental temperature increase; thus the bobtail goanna,
Trachysaurus rugosus, when kept in dry air at 25
0
, loses water by evaporation at
the rate of 0.7 g/100 g day while at 37.5
0 it is 2.9 g/100 g day (WARBURG, 1965a).
This water loss is small, relative to that in birds, mammals and the Amphibia, and
makes little contribution to thermal cooling; the temperature of the goannas when
equilibrated to such-conditions only being one or two degrees less than that of the
environment.
The increased evaporation of water at high environmental temperatures is due
to the decrease in the saturation deficit for water in the surrounding air, and an
increase in the bod y temperature of the animal. The metabolic rate of reptiles increases two to three times for every 10
0 rise in body temp~rature(BENEDICT, 1932),
so that there is an increased rate of gas exchange in the lungs that further facilitates
water loss .
Different species of reptiles have been observed to lose water by evaporation
at various rates (BOGERT and COWLES, 1947). The reasons for such differences were
not initially clear as they could involve several factors such as: the rate of oxygen
consumption, the ratio of surface area to body weight or the particular properties
of the skin of the different species . This was not further investigated until SCHMIDTNIELSEN in 1964 (a) suggested, after examining the fragmentary information that
was then available, that cutaneous evaporation in reptiles may be greater and more
variable than was hitherto considered likely. When cutaneous and pulmonary water
losses were measured separately in a variety of species of reptiles (at 23 0) the movement through the integument was indeed found to contribute from 66 to 87% of
the total evaporative loss (see Table 5.2; BENTLEY and SCHMIDT-NIELSEN, 1966;
SCHMIDT-NIELSEN and BENTLEY, 1966). At higher temperatures (35
0 and 40
0
) pulmonary water loss was increased relative to that from the skin but was still about
50% of the total. Similar results have been described in a variety of lizards (see
CLAUSSEN, 1967). The rates of water loss across the skin vary considerably in different reptile species, in a manner that suggests that animals normally occupying
138