Old world monkeys have not developed the prehensile tails, however, they do extend them horizontally when they run along branches, as a balancing aid. The failure of the African monkeys to use a prehensile tail meant that if they did grow larger, they would find an arboreal life increasingly awkward and consequently spend more time on the ground. This is clearly evident by the lack of ground living New World monkeys, whereas in the Old World there are many. The primate's tail seems of less value for terrestrial life and there has been a tendency to reduce and even lose the tail. The mandrill (Mandrillus sphinx) and drill (Mandrillus leucophaeus), have tails that are reduced to a tiny stump.