The World's Fair

The paper is pretty curious and is essentially a study that tries to determine whether shrew twittering is done for (1) just communicating to each other or (2) more for echolation (as in helping to navigate through terrain) purposes. They do this by comparing how often these shrews twitter in different "substrate densities" (fancy talk for how dense the greenage is, or in this case, how thick is the hay in the cage), as well as under the influences of different smells (heavy hay smell or lighter hay smells - all of this culminating in the following statement:

The finding that substrate density affected call rate in both species supports the echo-orientation-hypothesis and suggests that shrews use the echoes and reverberations of their calls for probing habitat routing or type.

Or in other words: Shrews seem to twitter for navigation purposes.

Here's another "twitter" paper. This one involving marmosets.

Now this paper is also quite interesting to read through, as it tries to determine whether these lovely creatures "phee" or "twitter," and whether these vocalizations are dependent on the animal's age. Just for clarity's sake, here is how the study defines a "phee" and a "twitter":

"Phee calls, used for long distance contact [Oliveira & Ades, 2004], are high frequency calls, ranging from 5.5 to 10.5 kHz, and they are emitted with high amplitude and in multi-syllable bouts [Norcross & Newman, 1993]. These calls are emitted when a marmoset is visually isolated from its group as, for example, when foraging in dense forest [Stevenson & Rylands, 1988]. By contrast, twitter calls contain several short duration notes, ranging from 6 kHz up to 26 kHz and of relative low amplitude [Epple, 1968], and these calls are generally used for close contact [Hook- Costigan & Rogers, 1998]."

Which makes you realize that the founders of Twitter clearly are not up to speed on their marmoset research (otherwise, we'd be pheeing as oppose to twittering). Better still, right there in bold is your dose of geekery for today: an actual scientific definition of what a "twitter" is and it involves mentions of kiloHertzs and amplitude (awesome).

Anyway, this paper basically demonstrates that you can tell roughly how old a marmoset is by the types of vocalizations it makes. In fact, there appears to be a strong trend between maturity and multi-phee responses. That's right, even marmosets get more verbose as they get older. Also, I especially loved this figure:

It basically says that the younger you are (that is, if you're a marmoset), the more likely you'll twitter (in response to a phee anyway).

And finally, to end this discussion, let me just show this paper:

Hmmm... Looks like there is a genetic basis to all of this twitter craziness (or at least in French there is).

References:
Why do shrews twitter? Communication or simple echo-based orientation. Biol Lett. 2009 Oct 23;5(5):593-6

Contact calls of common marmosets (Callithrix jacchus): influence of age of caller on antiphonal calling and other vocal responses. Am J Primatol. 2009 Feb;71(2):165-70.

[The FoxP2 gene makes humans speak and birds twitter] Med Sci (Paris). 2008 Nov;24(11):906-7.