POCKET GOPHERS—GRINNELL 345 
apprised of goings-on outside the walls of its tunnel, it does need to 
be aware of conditions in front and behind. We find that it moves 
in its cylinder nearly as well in backward direction as forward. 
Since, as seems apparent, the general question of the pocket 
gopher’s occurrence over wide territory must take into account its 
very special mode of gaining a livelihood, it will be useful in our 
discussion to inquire further as to its digging proclivities and the 
structures correlated with these. Comparison of the pocket gopher 
as an extreme type of digger with, say, the California ground 
squirrel shows significant differences. While the brain case has in 
the two animals relatively about the same capacity, the skull of a 
gopher is four times as heavy as that of a ground squirrel, total 
weights of the two animals being considered. As indicated above, 
the skull and teeth of the pocket gopher, together with the muscles 
which operate them, comprise the chief engine of digging. This 
engine operates in powerful fashion in cutting away the earth, so as 
to make possible the rapid extension of the gopher’s underground 
system of passageways. The adequate housing of the heavy incisor 
teeth and the need of meeting the severe stresses during the action of 
the muscles which operate the jaws have resulted in the great 
thickening and ridging of the bones of the skull. We find that the 
fore feet are larger than the hind feet, a reversal of the ratio in 
animals which can run with agility, and the fore feet are provided 
with long, laterally compressed, curved claws. The forearm and 
shoulder are heavily muscled, and thus the actions of the jaws and 
teeth are supplemented in loosening and particularly in transporting 
the soil. 
So far as is known, no pocket gopher goes into dormancy at any 
season; none either aestivates or hibernates. The source of food upon 
which the pocket gopher can depend year in and year out and which 
it can seek in safety is comprised in the underground stems and root 
stalks of various grasses and herbs. These it gets almost altogether 
by digging its way to them; it gathers food only as it can advance 
under cover. While it is true that gophers do pull into the tem- 
porarily open mouths of burrows, stems and leaves of above-ground 
plants, these latter, I am led to believe, constitute only a minor frac- 
tion of the total annual food supply of the animal. The only de- 
pendable food source, continuing throughout the year, is comprised 
in underground stems and roots. And this is an exceedingly im- 
portant consideration in our present study, for the general geo- 
graphic limitation of 7homomys, to North America west of the one- 
hundredth meridian coincides with the territory where sharp alter- 
nation of dry and wet seasons is characteristic of the climate. 
Linked up with this climatic peculiarity there is undoubtedly in the 
Southwest relatively greater abundance of plants with nutritious 
