Primates are a group of mammals that includes 356 species. Primates include prosimians, monkeys and apes. Primates inhabit tropical regions throughout Central America, South America, Africa, Madagascar and east and southeast Asia. They form complex social units and many species possess adaptations suited for life in the trees, including dextrous hands and a prehensile tail. There is no single trait that defines primates, but the group shares a general set of features such as a shortened rostrum, eye sockets that are encircled by bone and forward-facing eyes that equip them with binocular vision and depth perception.
Primates have five fingers on each hand, fingernails and an opposable thumb. The bones in their forearms and hind limbs are unfused. The forearm consists of two bones, a radius and ulna. The lower portion of their hind limb also consists of two bones, the tibia and fibula. This structure may be another adaptation suited to an arboreal lifestyle.
Many primates form complex social groups. Monkeys live in troops that consist of several females and one or more males. Gorillas gather in modest groups of between 2 and 20 individuals. Chimpanzees form large communities of 20 to 100 individuals. Despite the tendency to be social creatures, a few primate groups such as orangutans, some lemurs and galagos remain solitary.
Primates are extremely diverse in size and form. The smallest primates are the mouse lemurs which weigh about 1¼ ounces. The largest primates are gorillas which can weigh in excess of 440 pounds.