HOWELL AND GERSH-WATER CONSERVATION BY DIPODOMYS 
 
fecal pellets as they are deposited and nibble these to pieces, so that it
is im- 
possible to determine the amount excreted. The bladder seems never to be

completely emptied in micturition, and urine is voided only a drop or two
at 
a time, but at fairly frequent intervals. It will not, apparently, consume
its 
own urine as this leaves the urethra but will lick it from any surface that
may 
be moist. And it will become considerably excited when it smells succulent

food, such as apple. Whether activity actually decreases when the animal
is 
on dry diet we are unable to say, but it appears to sleep more soundly. 
When placed on an air-dried diet an adult white rat will survive an average

of 5 days (Richter, MS), although an occasional individual may last a day
or 
two longer. If moisture in some form be given before the ninety-sixth hour

the animal will usually recover, but if withheld beyond this time it will
usually 
die-a finding which is suggestive of the thesis that after the fourth day
of a 
dry diet its system has frequently suffered some irreparable injury. At the

fourth day it is obviously in some distress, with increasingly faltering
gait 
and lethargy, so that it becomes less excited at the smell of moisture. It
too 
nibbles its feces, and in addition will nibble its toes; Dipodomys has not
been 
observed to do this. 
METABOLIC WATER 
Basal metabolism is the rate of oxidation of the tissues when an animal is
at rest. 
It is increased by any sort of expenditure of energy. As in all oxidation
the temperature 
(of the body) indicates the rate, but this is greatly augmented by activity.
One of the 
products of oxidation is water, and this is referred to as metabolic water
in animals. 
It is not formed from food directly, except from such slight putrefactive
action as may 
take place in the intestines, but from oxidation in the body tissues. The
quality of the 
food consumed can not affect it except indirectly in that carbohydrates or
fats help 
build different substances in tissue. The digestive ability has nothing to
do with it, 
and a desert mammal can secure no more metabolic water from its tissues than
can an 
aquatic mammal, the body temperature of the two being equal. 
The oxidation of proteins, fats, and carbohydrates, says Babcock (1912),
produces a 
quantity of metabolic water nine times the weight of the hydrogen present.
In table 1 
is given the composition of 100 grams of air-dried rolled oats, as stated
by Atwater (1906), 
and the maximum amount of metabolic water derivable therefore by any and
all sorts 
of mammals. 
Factors intimately connected with the rate of metabolism include rate of
respiration, 
body temperature, and activity. It is obvious that the increase of any one
of these, 
to secure more metabolic water, also increases the rate of water loss from
the body, and 
a sort of vicious circle would then ensue. On the whole it would seem that
an animal 
on a dry diet gets along better with a slow rate of living, and hence lower
metabolism, 
than one that has to sacrifice much water by loss from the body concomitant
to high 
metabolism. Hence, metabolic water is a critical factor to be accepted thankfully,

like free water in the food, but its increase proves too expensive an undertaking
to be 
practicable. 
The elimination of the waste products derived from protein appears to cause
the most 
difficulty for a mammal on a dry diet, for these are poisonous and can not
be utilized. 
In the case of insects, reptiles, and birds this is done by the excretion
of semisolid uric 
acid, but in mammals the end product is urea. When the end product is uric
acid ap-