Spread in many regions of the world, monkeys are animals that often climb trees. However, there are ones that stay on the ground also. Baboons belong to this type. There are 260 known living species of them today. They eat leaves, fruits, flowers, insects and small animals. They are considered intelligent animals. Generally, they have tails but there are also ones that do not have tails.
The smallest monkeys are only 5-6 inches long with their tails and they weigh only around 4 to 5 oz. Pygmy Marmoset is one such monkey. The largest ones may be 3.3 ft in length and they may weigh as much as 77 pounds. Mandrill belongs to this group. Some of these animals have tails that help them hold onto trees and branches.
The tails of moneys appear to have adopted to suit their environments. Most of these primates that live in South American forests have prehensile tales which could be used to grasp objects. But those live in Africa and South Asia don't have such tails. The thick forests of South America make them necessary as the monkeys often will live on top of trees. In the not so thick forests of South East Asia and Africa the need doesn't arise as they often are on the ground.
When you consider the anatomy of animals you will notice that tails are only possible with vertebrates. But even scorpions have tale like structures. However, only vertebrates have prehensile tails. These tails have special areas that have no hair to make it easier to grasp objects. They are called the friction pads.
Monkeys often have relationship with humans. Some are raised as pets. There are others used for experiments. In certain agricultural regions they become pests damaging crops. In certain places monkey gather where tourists visit. Such animals are also considered pests. There have been occasions where tourists are attacked by them.
When it comes to experiments leading to drugs it is the African Green monkey that is being used most often. They are easy to handle and they have better resemblance to humans. They have faster reproductive cycles enabling them to bring test results quicker. While some of these animals are bred in captivity others are caught in the wild. In a few experiments these animals have been sent to space for testing purposes. In some countries flesh of these animals is eaten. In some African countries it is sold as bush meat.