ANIMAL SURVIVAL IN HOT DESERTS—EDNEY 413 
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ye) O©Woodlouse O0:0002 kg. (105%) 
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7 Ov Mouse 0-021 kg. (21:5 %) 
2 © Rat O1 kg. (12-8%) 
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2 Rabbit 2:0 kg. (4-77 %e) 
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2 Dog 16 kg.(2:38%) © 
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~ Man 70 kg. (147%) Od Donkey 95 kg. (133%) 
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Ey Camel 800 kg. (0:77%) © 
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4 3 2 i fo) TREES 3 
Log Body weight 
Ficure 1.—The relation between body weight and the amount of water which must evap- 
orate per hour from the body to preserve a constant temperature in desert conditions. 
(Date in part from Schmidt-Nielsen, 1954.) 
ular we want to know whether or not each source of loss is inevitable, 
and whether it is harmful or beneficial. 
Loss of water associated with nitrogen excretion is the only one 
that has no possible compensation. The heat loss involved in evap- 
oration is in this case of no use to the animal, for evaporation occurs 
at a distance from the body. 
Water loss from the respiratory surfaces is inevitable. It may 
be reduced to a minimum if the respiratory surfaces are tucked away 
within the body and are capable of occlusion, as they are in insects and 
spiders; but where the respiratory membranes are external, as in wood- 
lice, or where there is a ventilated respiratory system, as in verte- 
brates, the loss of water is considerable. Most land vertebrates are 
compelled to ventilate the respiratory membranes on account of their 
size, and in mammals and birds because of the high metabolic rate 
necessary to support a constant high temperature. But this source 
of loss may have its compensations, for evaporation of water occurs 
at the surface of the respiratory membranes, and therefore with- 
draws heat from the body itself. 
A small loss of water by transpiration from the general body sur- 
face is also inevitable, for no integument is entirely impermeable to 
water. In small animals surface transpiration must be reduced as 
far as possible if the animals are to exist in dry habitat. If trans- 
