S1, E2: They have the most complex social lives of any animal group on the planet. From dusky leaf monkeys who compete to babysit an infant to gibbons who learn dare-devil treetop acrobatics with their playmates, friends and family are at the heart of primate life. In the Amazon we meet a female spider monkey elder, who leads her troop to a unique food resource: aquatic plants brought within reach on raging floodwaters.