Howler 2011). This allows the howler monkey to

Howler
monkeys (Alouatta sp.), are typically
known for being one of the most vocal animals in the kingdom, but they are also
one of the few species of monkeys to move entirely quadrupedally (Cant, J.
G.,1986; Smithsonian, 2017). The Alouatta
sp. possesses highly adapted hind legs and prehensile tails which aid in
this movement and help offset the hindering hyoid bone, which restricts the
howler monkey’s front limb movement despite allowing them to have one of the
loudest cries in the kingdom (Smithsonian, 2017). When walking across branches,
howler monkeys move on all four limbs by employing their front arms as an extra
set as legs (Youlato, D., & Guillot, D., 2015). However,
when moving between branches, Alouatta sp.,
will also use their prehensile tail to support themselves while using their
four limbs to manoeuvre themselves (Schön, M. A., 1968). In addition to this,
when jumping to neighbouring branches, Alouatta
sp. use their hind legs to launch themselves towards the branch while using
their arms to catch themselves (Schön, M. A.,
1968). This
contrasts to the spider monkey (Ateles sp.),
which only moves quadrupedally half of the time, and Old World monkeys such as the
Aotidae family, which do not have
prehensile tails. (AnimalSake, 2018; Cant, J. G.,1986).

The
prehensile tail and specialized hind legs of the howler monkey involves the use
of the skeletal, muscular, and nervous systems in allowing for arboreal locomotion.
The prehensile tail, which functions as an extra limb, employs the use of the
nervous system to grip branches and other structures during movement, which then
allows the muscular system to support the weight of the Alouatta sp. (National Geographic, 2011). This allows the howler
monkey to hang from overhead structures without the use of their arms, which is
useful for feeding as their diet consists mostly of leaves (Smithsonian, 2017; Youlato, D., & Guillot, D., 2015).
Their tails also lack hair on their underside, which supports their ability to
grip (Smithsonian, 2017). Skeletally, the tail of the howler monkey can grow to
exceed the length of their body, which aids locomotion as they can hang further
down from the branch (Smithsonian, 2017).

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This
differs from primates such as the orangutan (Pongo genus), which are primates which lack a tail, but compensate
for the loss of the prehensile tail by using their arms to swing between
branches (Thorpe, S., et al., 2007). Skeletally, the orangutan does not possess
a tail, however, the Pongo genus does
have elongated arms to aid in swinging between branches. Furthermore, the
nervous system of the orangutan allows it to grasp using its arms, whereas the
howler monkey uses its tail to grip at branches (Smithsonian, 2017). The
muscular system of the Pongo genus
allows the orangutan to support its weight while it moves between branches,
much like how the prehensile tail of the howler monkey allows it to hang from
branches while eating (Thorpe, S., et al. 2007).

The Alouatta sp.
have
specially adapted hind limbs which aid their prehensile tail for agile walking
and climbing. The skeletal system contains various small adaptations which aids
in this. For example, the position of the foramen magnum, which is the space in
the skull that the spinal cord passes through, is at a backwards angle which
allows the howler monkey to move quadrupedally (Fiorenza,
L., & Bruner, E. 2017; Youlato, D., & Guillot, D., 2015). The hind
legs of the howler monkey are also elongated compared to their height, making
it easier for them to jump to neighbouring branches as needed (Smithsonian,
2017; Youlato, D., & Guillot, D. 2015). Their feet contain five toes each and
are arranged in a way which can be used to grip branches as they walk
quadrupedally (Smithsonian, 2017). This contrasts to the tufted chapuchin
monkey (Sapujus apella), which has
shorter hind legs to allow for a change in posture when eating (Conceição, R.
T., 2014).

The
nervous system of the howler monkey also helps their arboreal locomotion. For
one, the nervous system allows the hind feet of the howler monkey to grip at
branches while it walks (Smithsonian, 2017). This makes the primate more
balanced as it moves. This contrasts to other primates that are bipedal and
walk on two feet-such as Homo sapiens.
Humans are exclusively bipedal, and their feet are unable to grip the ground
like Alouatta sp. can (Alexander, R.
M. 2004).

The
muscular system of the Alouatta sp.
provides their hind legs with enough strength to allow for the primate to jump
towards other branches (Schön, M. A., 1968). It also allows them to grip the
branches below them with their toes as they walk along a single branch (Smithsonian,
2017). This contrasts to the spider monkey (Ateles
sp.), which unlike the howler monkey, does not use its hind legs to propel
itself for movement, and instead uses its arms and tail for momentum (Grand, T.
I., 1968).

The
hind legs of howler monkeys have developed many specializations to aid in their
arboreal locomotion. Their ability to grip the ground with their feet, as well
as their lengthened back legs and the position of their foramen magnum, allows
the primate to walk quadrupedally across branches as well as to leap to
neighbouring ones (Fiorenza, L., & Bruner, E. 2017; Smithsonian, 2017; Youlato,
D., & Guillot, D. 2015). Howler monkeys also use their prehensile tails to hang
from overhead branches while feeding or moving between branches (Smithsonian,
2017; Youlato, D., & Guillot, D., 2015). These adaptations help negate their
limited available arm movement due to their hyoid bone, as well as allowing the
Alouatta sp. to move easily through trees
(Smithsonian, 2017).