Boom boom, Krak-oo
All animal species are able to communicate with each other, but the extent of their language is typically limited to "me hungry," "le tired," and "shit, run away, it's a cheetah." Their communication systems are very simple - they can't create variations in syntax like humans can. For example, when we say "Don't go over there, I heard there's a tiger on the loose," we're demonstrating a possibility: There may or may not be a tiger in that general direction, but we're still able to warn you of that possibility. In animal speak, that sentence would translate directly to "tiger," because, up until now, researchers hadn't found evidence that animals, specifically a species of monkey, can express themselves in anything other than the most basic of terms.
Klaus Zuberbühler and his colleagues have been working closely with a group of Campbell's monkeys in the Tai National Park of the Ivory Coast. While studying the various calls of the monkeys, Zuberbühler's team found that the animals made a "krak" sound, which indicated that a leopard is approaching. However, the monkeys also demonstrated variations of this basic holler. By adding a simple "oo" to the end to create "krak-oo," the tree-dwelling primates communicated that they had heard a predator, but not seen it, or that they had heard similar calls of alarm from another species of monkey that dwell nearby.
The researchers catalogued many variations of the new monkey grammar, including "Boom boom," which - no, doesn't refer to the act of coitus, but instead means that the branch you're standing on is not sturdy, so perhaps you should move your flea-grooming business elsewhere. "Hok hok hok" is also an alert, but it refers to crowned eagles, which prefer to strike from above.
This is the first time that meaningful syntax has been demonstrated in a nonhuman primate species, and it's an amazing development. We're always drawing the line between what makes them animals and us humans, but that line gets blurrier as we learn more about our Simian brethren.