"Not only does the chimpanzee often break the silence of the
forest when all other voices are hushed, but he frequently answers
the sounds of other animals, as if in mockery or defiance. ...
Although diurnal in habit, the chimpanzees often make the night
reverberate with the sounds of their terrific screaming, which I
have known them to continue at times for more than an hour, with
scarcely a moment's pause,--not one voice but many, and within
the area of a square mile or so I have distinguished as many as
seven alternating adult male voices.

"The gorilla is more silent and stoical than the chimpanzee, but
he is far from being mute. He appears to be devoid of all
risibility, but he is often very noisy. Although diurnal in habit,
he talks less frequently during the day than at night, but his
silence is a natural consequence of his stealth and cunning. There
are times, however, when he ignores all danger of betraying his
whereabouts or his movements, and gives vent to a deluge of
speech. At night his screams and shouts are terrific."

The gibbons (including the siamang) have tremendous voices, with
numerous variations, and they love to use them. My acquaintance
with them began in Borneo, in the dense and dark coastal forest
that there forms their home. I remember their cries as vividly as
if I had heard them again this morning. While feeding, or quietly
enjoying the morning sun, the gray gibbon (_Hylobates
concolor_) emits in leisurely succession a low staccato,
whistle-like cry, like "Hoot! Hoot! Hoot!" which one can easily
counterfeit by whistling. This is varied by another whistle cry of
three notes, thus: "Who-ee-hoo! Who-ee-hoo!" also to be duplicated
by whistling. In hunting for specimens of that gibbon, for