The answer is actually rather humorous. It is because the sloth is so slow that it can survive. The sloth’s extreme sluggishness works to its advantage. Its laziness aids in its camouflage. It may be the only animal (including humans) in nature whose inclination to avoid activity is its greatest trait.
The primitive, slow mammal sleeps 18 out of 24 hours a day. Due to this, it expends very little energy. Because of expending bare amounts of energy, it does not have to eat as much to survive. It eats volumes less than any of its more active cousins in the animal world. Add to that, it is a complete vegetarian. The leaves it consumes are very abundant. These factors allow the sloth to sleep the day away. When it must eat, it moves very little, due to its food being such close proximity to it.
The sloth also has bad grooming habits. It barely takes care of itself, leading to its dirty, slovenly appearance. Though, once again, this works to the animal’s advantage. Its thick, coarse hair is a breeding ground, a veritable ecosystem by itself. Because it is rarely tended to, two species of blue-green algae grow on it. This growth turns the sloth into the greenish color the sloth is known for, making it almost invisible in the trees. This renders the sloth near undetectable to its predators, especially in combination to its slow, almost nonexistent movement patterns.
When the sloth sleeps, it lays its head down on its chest. When in this position, the sloth strongly resembles a bunch of partially dead, dried leaves or even a wasp nest. When threatened, the sloth curls up into a ball. Its thick fur combined with tough skin makes it difficult to injure. Its predators, animals like jaguars, ocelots, and other carnivorous mammals, all find it difficult to take the sloth down. This camouflage combined with its tough defenses also serve to protect it from airborne threats like happy eagles.
The sloth also has a good healing factor. It recovers relatively rapidly from most wounds it endures. It helps that many injuries it receives are shallow, surface injuries that do not take as long to heal. Its wounds also heal rather cleanly, despite its uncleanliness, vastly reducing its chances of infection. The sloth survives many injuries that would kill other animals.
Another advantage the sloth has are its claws. They are long and sickle shaped. They enable the sloth to hang from the branches of the trees it lives on. They give the sloth incredible grip to the branch it is hanging from. A sloth attached a tree is almost impossible to pry free. Its claws are not comprised of muscle, much like other claws. In order to pry a sloth free, the branch must either be broken, or the claws themselves snapped or somehow separated from the body or dislodged from the tree. Sloths often keep hanging from a branch after they die. Many remain there until their bodies have decomposed, leaving only a skeleton behind.
The sloth is perfectly designed for the environment it lives in. Where other types of animals, such as monkeys, rely on their swiftness and agility to survive, the sloth subsists through simply being too tough and inconvenient to kill. Its lethargy and laziness all serve to keep it alive. While it does not conform to the general image of “survival of the fittest”, it is nevertheless a great example of an animal perfectly suited and fit for its environment.
Sloths in Culture:
One quite famous example comes to mind. The children’s movie series Ice Age features a sloth as one of the main characters. The lovable, goofy, rather clumsy and slow sloth name Sid was an instant fan favorite. The movie series had five installations and is adored by kids and adults worldwide today.