416 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
MAMMALS 
There is by now a good deal of information about mammals which 
live in deserts, reviewed by K. and B. Schmidt-Nielsen (1954). In 
these animals the effect of size on water economy becomes at once 
apparent. Small species such as the African jerboa or the American 
kangaroo rat can avoid extremes of temperature and dryness by living 
in burrows. They do so and emerge to feed at night only. On the 
other hand, being mammals they must lose water both in respiration 
and in excretion. Yet these animals can exist indefinitely on perfectly 
dry food and no water. The answer lies in certain quantitative 
changes in their water physiology, not in the invention of any qualita- 
tively new process. They conserve the water of metabolism better 
than other mammals in several ways. First, they do not sweat, and 
their rate of transpiration is less than half that of the ordinary white 
rat. It seems likely that the commonly observed absence of sweat 
glands in small mammals receives a general explanation here: it re- 
sults from the necessity imposed by their relatively large surface to 
conserve water. 
Jerboas lose in evaporation from the respiratory surfaces only half 
of the water lost by man per unit oxygen uptake, perhaps by exhaling 
air at a lower temperature so that it requires less water to saturate 
it. They excrete only very small quantities of very concentrated 
urine, and they lose very little water with the feces, which are de- 
posited practically dry. 
These small animals do not use water to maintain a constant tem- 
perature in the desert for the good reason that in air at 40° C. they 
would have to lose 20 percent of their body weight per hour. But 
there is an emergency procedure, for if the body temperature ap- 
proaches the lethal level (about 42° C.) copious salivation occurs, 
which wets the fur of the chin and throat and thus reduces the body 
temperature. This can only be effective for a short time, however, 
because the loss of water is great and soon leads to desiccation of the 
tissues. 
With a large mammal such as a camel it is a very different matter. 
These animals cannot escape the heat of the day; they must either 
tolerate it, or use water to prevent a rise in body temperature. Here, 
therefore, we find a shift in the balance of physiological mechanisms, 
again only quantitatively, but in relation to the possibilities and 
limitations imposed by size. 
The camel does not store water. It exists for periods of a fortnight 
or more on dry food alone by tolerating a much greater depletion in 
body weight than most other mammals can. Thus a camel tolerates 
a loss of water equal to nearly a quarter of its body weight (100 kg. 
out of 450 kg.) as compared with something like 12 percent in man. 
