If olfactory (scent) signs are the most common form of communication among animals, acoustic signals must surely be the next in line, as animals of all kinds rely to a great extent on their hearing ability in order to succeed and survive.
Among mammals, generally speaking, small animals squeak and large ones rumble. The reason for this is that the smaller the animal's head, the higher the frequency of sound it can receive and transmit.
In order to determine which direction a sound is coming from, the brain must distinguish which ear it reaches first, and the time difference between them. The closer the animal's ears are together, the shorter the wavelength, and therefore the higher the frequency, will be necessary for good directional hearing.
However, just to show that nothing in nature can be taken for granted, some small-skulled species of birds can communicate with ultra-low sounds. In the case of mammals it must also be remembered that a large head and body allow space for bigger vocal equipment - so a roaring gerbil would seem quite out of place.
The acoustic communication signal most frequently heard by humans is, of course, bird song. On a still summer evening the territorial song of the male blackbird will carry a considerable distance, while the chiffchaff's distinctive call is something of a classic backing track to summers in the English countryside.
If we were to listen carefully we might also hear the chirping of crickets and grasshoppers together with the rather similar sounds made by shrews, but many of the acoustic signals transmitted by animals go unheard or unnoticed by humans.
The largest of all animals, the whales, appear to be experts in the art of communication by sound, with each whale of the same species having its own repertoire of 'songs' which it repeats at intervals. Smaller whales such as the dolphins and porpoises also have a wide range of sounds, although these tend to be the clicking kind rather than 'songs'. These sounds alter quite noticeably when dolphins are hunting prey - being much more rapid and excited.
Very small creatures must also communicate with others of their kind and a good example of this is the acoustic signal of the death-watch beetle. These wood-eaters got their name because their taps and clicks on the rafters of houses were often heard by people sitting up all night to care for a dying relative.
Animals who live in a dense habitat such as wood transmit information by a form of Morse code achieved by banging their heads against the roof of their tunnel. This means of communication is clearly heard by humans.
Frogs too are frequently heard by humans, and frogs and toads have developed their vocal contact to a fine art by using bags of air as resonators. These may be situated in the mouth, throat or on the side of the head and they are acoustically most effective. A species of American tree frog provides an excellent example of the complex nature of communication among animals.
Although this frog gives a two-part call which humans hear in full the male frogs hear only the first part of the call which warns them of the presence of a male intruder, while the females hear only the second part which informs them of the presence of a potential mate!
Grasshoppers and crickets create sound by 'fiddling' - a process which consists of rubbing the hind legs over the ribs of the forewings. Male gorillas will beat their chest with cupped hands to produce a sound which carries some distance. This action is thought to be a form of communication although it could also be a means of releasing tension.
Elephants communicate by a series of rumbling noises made in their throats and trunks, although a cow elephant calling her calf will simply slap her ears against the side of her head. Trumpeting is restricted to moments of extreme excitement or danger.
Kangaroos, hares and rabbits will thump their hind legs on the ground as a warning signal. The rattlesnake gives a distinctly audible and sinister sounding warning by vibrating its bell-shaped tail segments while other snakes, lizards and crocodilians will hiss loudly to warn off intruders.
The study of acoustic communication among animals is producing new discoveries each year - leaving us to marvel at the complexity of nature and the way in which life on Earth has developed.
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