Final answer:
New World monkeys can use their tails because they are adapted to an arboreal lifestyle, allowing them to move through the forest canopy and grasp branches.
Step-by-step explanation:
The reason why only New World monkeys can use their tails is due to their arboreal lifestyle. New World monkeys are exclusively found in the New World (South America) and are adapted to life in the trees. Their prehensile tails serve as an extra limb, allowing them to grasp branches and move easily through the forest canopy.
On the other hand, Old World monkeys, which are found in Africa and Asia, have non-prehensile tails that are often reduced in size. These monkeys have adapted to both arboreal and ground-dwelling lifestyles, and their tails are not used for grasping.
It is important to note that not all New World monkeys have prehensile tails. Some species, like tamarins and marmosets, have non-prehensile tails but are still considered New World monkeys.