" Siamangs—large black members of the gibbon family—swing high up in the tallest trees of the Asian Jungle. Every morning, the male and female burst into spectacular duets. Their song begins with a few loud whoops, which gradually build into ever louder, more elaborate sequences. Amplified by balloonlike throat sacs, the sound carries far and wide. I have heard them in Indonesia, where the whole forest echoed with their sound. The siamangs listen to one another during breaks. Whereas most territorial animals need only to know where their boundaries run and how strong and healthy their neighbors are, siamangs face the added complexity that territories are jointly defended by pairs. This means that pair-bonds matter. Troubled pairs will be weak defenders, while bonded pairs will be strong ones. Since the song of a pair reflects their marriage, the more beautiful it is, the more their neighbors realize not to mess with them. A close-harmony duet communicates not only “stay out!” but also “we’re one!” If a pair duets poorly, on the other hand, uttering discordant vocalizations that interrupt one another, neighbors hear an opportunity to move in and exploit the pair’s troubled relationship. "

― Frans de Waal , Are We Smart Enough to Know How Smart Animals Are?