2000 Volume 49 Issue 2 Pages 69-78,99
Communication is a substitute for fighting in contests over resources. Communication functions to manage the behavior of other animals without risking direct encounters. Females use communication to assess male qualities as mates, thus sexual selection has influenced communication greatly.
The origin of vocal communication is still exhibited in the first land animals, the amphibians. Frogs and toads differ from birds and mammals in that they continue to grow larger after reaching sexual maturity. Larger individuals are able to produce lower pitched calls and they are better fighters than smaller ones. In amphibians, lower pitched calls are both more threatening to other males and attractive to females. Importantly, the physical structure of vocalizations is directly related to the function of these sounds. The relationship between function and a vocalization's physical form is not arbitrary, as is the case with human words.
How is the relation between body size and pitch of calls manifested in birds? The body size/sound pitch relationship is more symbolic and best describes the motivation of the calling bird. Birds use low, harsh, vocalizations when aggressive and high, tonal ones when appeasing or fearful. This relationship is described by the motivation-structural rules model, which relates size symbolism to motivation and is derived from the primitive relation between body size and fighting ability. The motivation-structural rules model is useful in producing hypotheses to test the relation between the physical form of vocalizations and their function.
Vocalizations used for long distance communication, such as most bird song, is a different matter. Here, motivation is usually not as important as sounding close to others. I develop ranging theory to describe how birds assess their distance from one another. The avian ability to resolve very short time intervals between sounds is used to perceive degradation. Degradation is any change in the song as it travels from the singer, such as from reverberation, changes in frequency or amplitude components above those due to spherical spreading of the sound. By comparing a song in their own memory with one that they hear, they are able to judge the distance over which the sound traveled. Ranging theory helps to explain why song learning evolved in some groups as well as the function of dialects, song repertoires, and song complexity.