SWIMMING AND WALKING 139 
Johnson says the propelling organs are the hind feet. 
He has never seen the tail used as a scull, and believes that 
it is principally used as a rudder. When the animal is 
swimming straight away the hind feet strike out together, 
and when it is turning the strokes may alternate. 
Beavers which I watched closely when they were swim- 
ming by daylight in Yellowstone Park of course used their 
hind feet for propelling, but not exclusively. Though the 
tail was usually carried motionless behind, I sometimes 
saw it used with a sculling motion. As at such times the 
animal went straight ahead the tail was not then being used 
for steering, though I did see it used for that purpose on 
other occasions. Sometimes the swimmers turned without 
using the tail for a rudder, evidently doing it by means of 
the hind feet. 
The fore feet are not used at all in swimming, being then 
folded against the body. 
A beaver can remain under water for a considerable 
period. Bailey says commonly four or five minutes at a 
time, but that they can swim half a mile or more without 
coming to the surface, and have been timed for 15 minutes 
under water in traps. When moving about under the ice in 
winter they may get air from bubbles under the ice or from 
open holes. Mills says they can swim two hundred yards 
under water without coming to the surface, and can remain 
under water five to ten minutes. On one occasion one 
stayed under water more than eleven minutes. Dugmore 
says (p. 77) eight to nine minutes, and (on p. 13) six to seven 
minutes. 
The above-mentioned Yellowstone beavers did but little 
slapping with the tail. They often dived without apparent 
reason, usually quietly, but sometimes noisily. Even small 
sticks thrown into the water near the animals did not trouble 
them. Something which did cause one to slap its tail was a 
