Spider Monkey at the Auckland Zoo (New Zealand)
Spider monkeys have prehensile tails, which means tails can grip to branches, acting as an extra hand. The tail easily supports the monkey's weight - freeing up all four limbs for feeding. Thumbs would get in the way of swinging so they have developed without them. Spider monkeys live in highly flexible groups, or troops, ranging from six to forty individuals, with females normally outnumbering males two to one. The larger group will break up into sub-groups, which move independently to one another within the same wider territory area. Within each sub-group, females are recognised as leaders, moving in front of the group when in search of food, and deciding the route to be taken. Not all females are leaders, and only the leading females possess the knowledge of food source locations.