ANIMAL BEHAVIOR—ERNEST P. WALKER 289 
In the wild they would probably not go farther than a few feet in 
their own nest tree. It is not until they are about 90 days old that 
they are ready to assume full activity. Golden hamsters, on the 
other hand, develop very rapidly, and by the thirty-fifth day, although 
not full size, they are apparently on their own in all respects. 
Swans (Cygnus, Chenopis, and Olor) are remarkably good parents 
and keep close watch that their young are protected from enemies. 
The female generally stays in the background and keeps the young 
with her while the male goes out to meet the intruder. If the danger 
is imminent he will approach with a direct rush and make vicious bites 
with his beak and will strike severe blows with his wings. If the 
danger is not imminent but he still feels an intruder might do harm 
he frequently approaches indirectly, that is, comes up to one side of 
the enemy apparently as though to catch it off guard. On one 
occasion while I was sitting very quietly on a rock in the swan yard 
trying to get a picture, the male swan persistently worked around to 
one side of me and when I did not move he finally grabbed my arm 
with his beak and tried to strike me with his wings. As soon as I 
started to move away he was satisfied. In this instance I had been 
trusting to my lack of motion to allay his suspicions, but my efforts 
to “‘freeze’”’ were not successful. One of the poses of the male black 
Australian swan (Chenopis atrata) threatening an intruder is shown 
in plate 15, figure 1. 
SLEEP 
Generally we think of sleep as a very simple state of inactivity 
which is similar in all animals, but actually the attitudes and types of 
sleep of various animals differ considerably. I think it likely that 
most, if not all, of the mammals that live in burrows well beneath the 
surface of the ground sleep very soundly, as they are comparatively 
free from danger while in their dens. Naturally, my observations of 
this fact have been limited and to my knowledge it has not been 
carefully studied; however, the few creatures of the burrowing type 
that I have been able to study all seem to sleep very soundly. A 
golden hamster can be picked up and handled gently within 30 seconds 
after it has ceased activity and has thrown itself down to go to sleep. 
I have similarly handled pocket mice when they were asleep in their 
nests and have found that they were quite difficult to arouse, which is 
in marked contrast to the great alertness of many other forms. 
Animals that live above ground are, of course, subject to a wide 
variety of hazards when they are asleep and must therefore, as it were, 
“sleep with one eye open.” This is particularly true of rabbits, most 
birds that sleep in the open and, no doubt, most other creatures that 
are in similar exposed locations. My pet flying squirrels selected a 
laundry bag hanging on a bathroom door for their nest and appear 
