Signals and Their Evolution

Abstract

That certain elements of behaviour are inherited is beyond question. Provided that some variation in these elements exists between individuals of the same species, then natural selection will operate and behaviour will evolve. Ethologists have been highly successful, using the comparative approach, in tracing the phylogenetic development of certain of these inherited elements. An example of this kind of study is Lorenz’s (1941) work on courtship behaviour in the Anatidae. “Mock preening” by the drake has evolved along several lines from the original form. In the mallard Anas platyrhynchos, the display resembles normal preening, the bill being drawn along the underside of the partly-lifted wing producing a loud “Rrrrrr” sound. At the same time a bright blue feather, the speculum, is revealed by the lifting and separation of the wing feathers. The mandarin Aix galericulata mock-preens with an exaggerated wing movement, where the wing is raised like a sail to display the large red tertiary feathers. At the same time a bright orange secondary is touched by the bill, but no sound accompanies this display. The shelduck Tadorna tadorna has elaborated the acoustic components, producing a low rumbling sound with a powerful bill stroke along the shafts of the wing quills. The “fixed action pattern” of courtship preening thus appears to show the same kind of radiation that numerous anatomical features show. However, the basic tenet of evolutionary theory is that natural selection operates gradually on small differences within interbreeding populations and it is, perhaps, anomalous that Lorenz chose to emphasize the invariant species-specific nature of the fixed action patterns that he studied. Fixed action patterns are recognizable from individual to individual, but this does not mean that there is no variation. Measurement of such variation is difficult because, unlike morphological features, behaviour will not be identical from performance to performance by the same individual. Nevertheless, to a greater or lesser extent, variation in behaviour between individuals of the same species will be there.