I was sorting some papers today & came across some notes I wrote for a lecture about animal communication. And I thought they'd make a good subject for a blog.
There are several different modes of communication available to animals: chemical, tactile, acoustic, & visual. But have you ever wondered why a given species uses some of these, but not others? In some cases the answer's fairly straightforward - if animals live in close contact then touch is likely to play a key role in communication. In others...
Chemical communication can work over long distances, and over very short ones. Ants, for example, use pheromones to communicate within the nest, as well as in laying down trails for others in the colony to follow. (Which reminds me - I must go & stand the cat bowls in water; the local ants have wised up to the fact that Ginny & Merry-aka-Thunderpaws leave left-overs.) Pheromones are very effective over long distances and at very low concentrations: think of female moths, wafting their pheromones into the breeze as an advertisement of their readiness to mate. They can carry only relatively simple messages ('yoo-hoo, I'm here! & waiting!') because they can't be pulsed or modulated in the way that other signal modalities can. And they're highly species-specific.
Acoustic signals can also be long-range - the epitome of this would be the songs of humpback whales. They can, however, be absorbed by the environment: in a forest habitat, high-frequency calls attenuate more than low-frequency calls, and many forest birds tend to produce calls in the same range of frequencies. Sounds fade out quickly once generated (if they didn't, we'd be awash in a sea of echoes), & this means that they can be used in complex sequences to transmit complex information.
The same is true of visual signals - feathers, skin colouration, firefly flashes. These, particularly those involving the use of colour, are often species-specific. But their use - with the exception of bioluminescent signals - is generally limited to daylight, and their use is restricted by the need to have a line-of-sight between displayer & viewer. On the other hand, because visual signals can fade out so quickly, they too can carry quite complex information. To me the epitome of this is cuttlefish, with their ability not only to change colour extremely rapidly, but also to send different messages to individuals to their left and right!
So anyway, you can probably see from this that some signals are going to work better than others in a given environment. For example, both chemical and auditory modes will work well in a situation where you can't see other individuals of your species - at night, or in caves, or in dense forest. And as I said above, in that dense forest some sounds will work better than others, in the sense that they'll travel further: sounds with a frequency of 1-3.5khz attenuate the least. So you can get convergent evolution of song where they all tend to use the same set of sound frequencies, albeit in different combinations.
PS on the cephalopod ability to change colour quickly - PZ has an excellent post on the physiology of this process (alas! the video he links to is no longer available :-( )