Predictions From the Ranging Hypothesis for the Evolution of Long Distance Signals in Birds

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Abstract
The Ranging Hypothesis (RH) (MORTON, 1982) proposed a form of distance assessment (ranging) based upon perception of signal degradation using memorized signals as a yardstick to distance. The predictions of the RH include distance assessment mechanisms, DAMs; it is proposed that these have opened a new evolutionary process illustrated by the complicated songs and singing behaviour in the oscine passerines ("songbirds"). The RH identifies sources of selection favouring learning, multiple or single song types, song structural complexity not accounted for by species isolating mechanism ideas, and emphasizes the ecological basis for the evolution of long distance communication. New importance is given to the acoustic physical structure of songs. The RH encompasses and contrasts song evolution in warm climate regions with those in cold temperate climates. Three interrelated stages of long distance signal evolution are presented: detectability, threat, and disrupt. A singer/listener role dichotomy in selective pressures is described and the results discussed. Listeners developed distance assessment mechanisms (DAMs) resulting in an evolutionary arms race between listeners and singers. Singers developed methods to use DAMs to their best interest (threat and disrupt). Song learning in passerines developed in response to this arms race to enhance disruption, a situation most prevalent in cold temperate zone regions. The acoustic determinants of effective song distance are described and discussed in relation to the evolution of signal structures. Finally, the RH is discussed in relation to some previous hypotheses on song function and evolution. The Ranging Hypothesis (RH) (MORTON, 1982) proposed a form of distance assessment (ranging) based upon perception of signal degradation using memorized signals as a yardstick to distance. The predictions of the RH include distance assessment mechanisms, DAMs; it is proposed that these have opened a new evolutionary process illustrated by the complicated songs and singing behaviour in the oscine passerines ("songbirds"). The RH identifies sources of selection favouring learning, multiple or single song types, song structural complexity not accounted for by species isolating mechanism ideas, and emphasizes the ecological basis for the evolution of long distance communication. New importance is given to the acoustic physical structure of songs. The RH encompasses and contrasts song evolution in warm climate regions with those in cold temperate climates. Three interrelated stages of long distance signal evolution are presented: detectability, threat, and disrupt. A singer/listener role dichotomy in selective pressures is described and the results discussed. Listeners developed distance assessment mechanisms (DAMs) resulting in an evolutionary arms race between listeners and singers. Singers developed methods to use DAMs to their best interest (threat and disrupt). Song learning in passerines developed in response to this arms race to enhance disruption, a situation most prevalent in cold temperate zone regions. The acoustic determinants of effective song distance are described and discussed in relation to the evolution of signal structures. Finally, the RH is discussed in relation to some previous hypotheses on song function and evolution.