Prairie Dogs: America's Meerkats - Language is a short video talking about the language that prairie dogs in the southwestern United States speak. Through a series of short, chirping vocalizations, prairie dogs convey useful information about an approaching predator that seems to convey an avoidance pattern. They can distinguish not only the predator but the size, shape and the dominant color; this has been tested to the point where the same human with different shirts will be described differently. All of this is in a short pitched call a tenth of a second long, repeated several times. Other vocalizations don't have an accompanying behavior so it's not clear what they mean. It's a neat bit of nature and, as the video speculates at the end, this is probably only the beginning of our ability to decode meaning in the calls of animals.
This kind of thing is useful to think about in a game that has spells like Speak with Animals. In the video we learn that prairie dog language has rough semantic equivalents to nouns and adjectives, but not to verbs. I'm thinking that when speaking with animals, a few simple verb-like things can be conveyed such as "run", "hide", "attack", "eat" and maybe for some types of animals "take" but the possibilities should be fairly limited. And of course it would vary for some types of animal, so some might understand "climb" or "push" but others maybe not.
Of course what makes it interesting is that you have to actually describe the thing you're talking about, to some extent. Now, a giant rat would probably understand "orc" since he's seen it; using Speak with Animals to describe an orc to a ferret might be more challenging. What's interesting in the prairie dog video is that they never confuse dogs with coyotes, and they do invent words for shapes, so the cleric using Speak with Animals may actually wind up coining some new terms in his target's language!
Extrapolating a bit from the existence of the spell, it may be that people in the D&D world have actually done investigation, perhaps with teams of researchers, one conversing with an animal using Speak with Animals and the other listening without aid of the spell, transcribing the conversations, and using them as a sort of Rosetta Stone to decode the animal languages. So for instance, a druid may be genuinely able to speak with the black bears in his forest, or magic-users could send messages by speaking with ravens (who we already know are highly intelligent).
In game terms, this could allow clever characters to learn the language of an animal companion such as a ferret, which would then serve as a sort of scout around the dungeon in the vein of Beastmaster. A Magic-User being able to speak with his familiar cat, or a paladin with his warhorse, etc - the list of uses for this sort of thing is not a short one. Of course the limitations are real, so for instance the ferret or cat shouldn't be able to count, but knowing whether it's an ogre or a group of kobolds in the next room is certainly knowledge worth having. And it makes the fantasy world just a bit richer.