These are Howler Monkeys. See the baby leaning on the papa? What a cute family, but they make a really amazing sound. At night and in early morning you can hear the howling, sounding like dogs howling at the moon. Very loud. You can hear them howl on Wikipedia - who maintains that their howls can be heard for 20 miles! They hang out and run around the tree tops, using their tails to grab onto branches.