This sub-family embraces only one genus, which is very distinct from all
the others. The Howlers are the largest of the South American Apes, and are
characterised by their thick unwieldy body, their pyramidal head, and small
facial angle, owing to their long, somewhat Dog-faced muzzle. The angle of
the lower jaw is very large and massive, and their chief characteristic is
the conspicuous thickening of the throat, owing to the great enlargement of
the hyoid bones--which are widely inflated and cavernous--to form the
curious vocal organ which the males of these animals possess, and by which
their voice can be so augmented as to be heard at a distance of several
miles.[11] The skull is truncated behind {190}in the male (less so in the
female) for the reception of the vocal apparatus. Their incisor teeth are
small and equal, the canines are prominent and have an oblique ridge across
the crown from the outer front, to the inner hind, cusp, and the upper
molars are large. The tail is powerful and prehensile, naked towards the
tip, where it is tactile and very sensitive. The thumb is movable, the face
is naked, and the chin bearded. Some have short, and some have long, fur
over their bodies, but it is generally more plentiful about the head. In
appearance they are the most unattractive and repulsive of the American
Monkeys. Their intelligence is also of a very low order.
The roof of the brain-case is depressed; the plane of the opening for the
passage of the spinal-cord from the brain is almost perpendicular to that
of the base of the skull; the condyles for the articulation of the neck are
situated as far back as possible. Sir William Flower, in his valuable
monograph on the brain of _Mycetes_, has shown that the frontal lobes are
small and the cerebral hemispheres only just cover the cerebellum. In
regard to its grooves and convolutions, the main brain (_cerebrum_) of
_Mycetes_ can be distinguished from that of all other Monkeys. The whole
organ is small as compared with the size of the animal; it wants the
roundness and fulness of that of the Spider-Monkeys (_Ateles_) and of the
Capuchins (_Cebus_). Its surface markings are comparatively few and simple,
and depart remarkably from the ordinary type seen in the order. In the Old
World Apes there is a striking similarity in the character of the surface
markings of their cerebral hemispheres. There is a slight ascensive
development from _Cercopithecus_ towards _Hylobates_; and further
complications overlying the same primitive type--such as large
proportionate {191}size, and complexity of convolutions--are observed in
the Chimpanzee and Gorilla, leading up to the brain of Man. Among the New
World genera there is a much greater divergence. Among the Capuchins
(_Cebus_), and among them only, there is a precise repetition of the Old
World type; but in the genus _Mycetes_ we have modifications in which there
is no parallel among the Catarrhine (or Old World) series. There is an
absence in its brain of signs of serial elevation; and it exhibits a great
dissimilarity to all, even the lowest of the Old World forms, and to those
American Monkeys, which in brain-character closely resemble Old World Apes.
It shows an affinity in some of its more striking characters to such low
forms of New World Apes as _Nyctipithecus_. The low type of brain is in
keeping, as Sir William Flower further observes, with their surly and
untameable disposition, and with the observation that their intelligence is
of a very different order from that of their neighbours, the Spider-Monkeys
and Capuchins of higher cerebral organisation.
"When Howlers are seen in the forest," remarks Mr. Bates, "there are
generally three or four of them mounted on the topmost branches of a tree.
It does not appear that their harrowing roar is emitted from sudden alarm;
at least, it was not so in captive individuals. It is probable, however,
that the noise serves to intimidate their enemies." The muscular power
employed in giving vent to their cavernous roar appears to be small. Their
food consists chiefly of fruits and leaves.
In colour the Howlers vary very much. The young of both sexes often differ
from their parents, and the females from the males, and there is also great
individual variation.
The geographical distribution of some of the species is very restricted,
several of them being confined to a special district {192}of the Amazon,
into which no other species intrudes. They are found, however, from Eastern
Guatemala to Paraguay.