Echolocation in bats is generally seen as a sort of natural sonar, in which the bats use ultrasonic clicks to navigate the night sky and find prey. But it may also be a rudimentary language, transmitting greetings and social information.
Many animals do, in a sense, possess a basic form of language that they can use for communication, with everything from primates to whales to bees showing some basic ability to transmit information through sounds. But this is the first time researchers have definitively shown that bats use echolocation to speak to each other as much as they do to find their way around.
A team of German scientists working in Panama tested a group of local bats by playing audio recordings of various bats. These recordings fell into one of three groups: familiar bats from the same species, unfamiliar bats from the same species, or bats from another species entirely. The bats responded to the echolocation calls in complex ways, including apparently preparing to release massive amounts of their pheromones.
Interestingly, it was bats that the test subjects didn't know that attracted the most attention, at least if the recorded bat was from the same species. They pretty much ignored the recordings from the other bat species. It would be interesting to speculate that this is because the different species were actually speaking different languages, but there's no real way to know that. In any event, whenever the bats did respond vocally, they made a unique noise that carried with it an identifiable acoustical signature that no other bat could reproduce. The researchers think this is the bat equivalent of saying, "Hello, it's me." So bats really do possess their own language, albeit on ultrasonic frequencies humans can't hear.
Of course, there are some crucial differences between human language and that of our animal counterparts. For a start, humans can use language to communicate abstract concepts divorced from the speaker's current position in time and space. Animal language, on the other hand, is forever locked in the here and now.
It's also possible to break down human statements in two ways, by either its individual sounds (phonologically) or its individual meanings (morphologically). This is known as double articulation, but animal languages don't possess this feature, which again limits the ability of animals to express anything more than the simplest of information.. Animal language is also almost entirely an instinctual ability, whereas human language is learned culturally.
Still, using echolocation to communicate greetings is a reasonably advanced form of animal language, and it's possible further study will reveal greater complexity in how bats communicate. Indeed, the researchers are already fairly sure the bats aren't just greeting each other. They're also relaying social information, which might explain why they responded more strongly to unknown bats - they were trying to make it clear what their social place was relative to the other bat.