Water loss from the respiratory tract depends on the rate of oxygen con -
sumption and the ability to extract this gas from the inspired air. The oxygen consumption of reptiles depends on their activity and bod y temperature. In addition,
the basal metabolic rate seems to vary considerably in diffe rent reptiles. An inspection of the rate s of oxygen con sumption of the different reptiles in T able 5.2
indicates a range from 0.26 mllg day in the desert tortoise to 2.6 mllg day in the
iguana. Such differences are partly reflected in the respective rates of water loss
from the respiratory tract of such animals. The desert tortoise thus loses water
through this channel at the rate of 0.4 mg/g day while the iguana loses 3.4 mg/ g
day. The relationship between oxygen consumption and respiratory water loss is,
however, not an exact one as there is considerable variation in the ability of the
different animals to extract oxygen from the inspired air. Usually this corresponds
to a reduction of oxygen from the atmospheric volume of 20.9 % to only 17 to 20%.
The chuckawalla, Sauromalus obesus, can on occasion breathe sporadically at
the rate of only 2 or 3 breaths an hour and reduce the oxygen concentration in
its lungs to 5% of the volume in the atmosphere (SCHMIDT-NIELSEN, CRAWFORD,
and BENTLEY, 1966). Such behaviour would reduce respiratory water loss, but
whether this occurs normally as a response to promote water conservation is uncertain. The thyroid hormones control the rate of ox ygen consumption in birds
and mammals but this does not normally seem to be so in reptiles, though in certain
circumstances, as when the temperature is elevated, the y ma y be involved (LYNN,
MCCORMICK, and GREGOREK, 1965).
Evaporative water losses in reptiles are probably temporally regulated, in conjunction with the body temperature, by changes in beha viour. Reptiles ma y be
diurnal or nocturnal in their habits and avoid extreme temperatures by seeking th e
shelter of cool refuge s. WARBURG (1965a) found goannas, Trachysaurus rugosus,
in burrows 2 to 3 metres deep. The temperature in these hole s never exceeded 28 0,
even though the external air temperature was as high as 40
0 and that of the surface
soil 50
0
•
b) Urinary and Faecal Water Loss
Like in birds, it is difficult to distinguish clearly between urinary and faecal water
losses in reptiles, as both products pass through a cloaca prior to their expulsion.
The proportion of the animals' total water loss that is lost through such channels
varies, and in terrestrial situations this is partly dependent on the temperature,
which influences the rate of evaporation more than other water losses . At a temperature of 23 0 (in dry air) the crocodilian, Caiman sclerops, loses about 20% of
its dail y total water deficit through the cloaca (BENTLEY and SCHMIDT-NIELSEN,
1965 and 1966). The lizard, Trachysaurus , which lives in semi-arid areas, loses 40 %
of its overall water loss in this way at 23 0 but only about 25 % at 35
0
• At 30
0 Uta
besperis, a lacertilian from southern California, onl y loses 5% of its total dail y water
decrement in the urine and faeces (CLAUSSEN, 1967). While in aquatic situations
evaporation is expected to be small , a net water loss may still occur osmotically
across the skin if the environmental fluid is hyperosmotic, like the sea. Additional
water ma y also be excreted through the cephalic 'salt ' glands in such species. When
the estuarine turtle, Malaclemys centrata, is kept in sea-water, it loses about equal
141
sumption and the ability to extract this gas from the inspired air. The oxygen consumption of reptiles depends on their activity and bod y temperature. In addition,
the basal metabolic rate seems to vary considerably in diffe rent reptiles. An inspection of the rate s of oxygen con sumption of the different reptiles in T able 5.2
indicates a range from 0.26 mllg day in the desert tortoise to 2.6 mllg day in the
iguana. Such differences are partly reflected in the respective rates of water loss
from the respiratory tract of such animals. The desert tortoise thus loses water
through this channel at the rate of 0.4 mg/g day while the iguana loses 3.4 mg/ g
day. The relationship between oxygen consumption and respiratory water loss is,
however, not an exact one as there is considerable variation in the ability of the
different animals to extract oxygen from the inspired air. Usually this corresponds
to a reduction of oxygen from the atmospheric volume of 20.9 % to only 17 to 20%.
The chuckawalla, Sauromalus obesus, can on occasion breathe sporadically at
the rate of only 2 or 3 breaths an hour and reduce the oxygen concentration in
its lungs to 5% of the volume in the atmosphere (SCHMIDT-NIELSEN, CRAWFORD,
and BENTLEY, 1966). Such behaviour would reduce respiratory water loss, but
whether this occurs normally as a response to promote water conservation is uncertain. The thyroid hormones control the rate of ox ygen consumption in birds
and mammals but this does not normally seem to be so in reptiles, though in certain
circumstances, as when the temperature is elevated, the y ma y be involved (LYNN,
MCCORMICK, and GREGOREK, 1965).
Evaporative water losses in reptiles are probably temporally regulated, in conjunction with the body temperature, by changes in beha viour. Reptiles ma y be
diurnal or nocturnal in their habits and avoid extreme temperatures by seeking th e
shelter of cool refuge s. WARBURG (1965a) found goannas, Trachysaurus rugosus,
in burrows 2 to 3 metres deep. The temperature in these hole s never exceeded 28 0,
even though the external air temperature was as high as 40
0 and that of the surface
soil 50
0
•
b) Urinary and Faecal Water Loss
Like in birds, it is difficult to distinguish clearly between urinary and faecal water
losses in reptiles, as both products pass through a cloaca prior to their expulsion.
The proportion of the animals' total water loss that is lost through such channels
varies, and in terrestrial situations this is partly dependent on the temperature,
which influences the rate of evaporation more than other water losses . At a temperature of 23 0 (in dry air) the crocodilian, Caiman sclerops, loses about 20% of
its dail y total water deficit through the cloaca (BENTLEY and SCHMIDT-NIELSEN,
1965 and 1966). The lizard, Trachysaurus , which lives in semi-arid areas, loses 40 %
of its overall water loss in this way at 23 0 but only about 25 % at 35
0
• At 30
0 Uta
besperis, a lacertilian from southern California, onl y loses 5% of its total dail y water
decrement in the urine and faeces (CLAUSSEN, 1967). While in aquatic situations
evaporation is expected to be small , a net water loss may still occur osmotically
across the skin if the environmental fluid is hyperosmotic, like the sea. Additional
water ma y also be excreted through the cephalic 'salt ' glands in such species. When
the estuarine turtle, Malaclemys centrata, is kept in sea-water, it loses about equal
141